A Technical Report for Water Qualitative and Quantitative Sensors including Technical Sensor Drawings

and Signal Conditioning Methods

Table of Contents

pH ........................................................................................................................................................ 2

ORP ..................................................................................................................................................... 9

Chlorine............................................................................................................................................. 14

Ion-Selective Electrodes (ISE) ........................................................................................................... 18

Nitrates ............................................................................................................................................. 18

Dissolved Oxygen .............................................................................................................................. 20

Conductivity ...................................................................................................................................... 23

Turbidity ............................................................................................................................................ 28

Temperature ..................................................................................................................................... 34

Flow .................................................................................................................................................. 38

Water Electrodes Cleaning Mechanisms .......................................................................................... 39

References ........................................................................................................................................ 42

pH

Introduction/Definition

The pH is defined as a negative decimal logarithm of the hydrogen ion activity in a solution and indicates the degree of acidity or alkalinity of the solution.

log[ ]H

pH a += −

Where 𝑎𝐻+ is the activity of hydrogen ions in units of mol/L (molar concentration)

The concept of pH was first introduced by Danish chemist Søren Peder Lauritz Sørensen at the Carlsberg Laboratory in 1909.

pH tests are the most commonly performed measurements in water analysis. Pure water is said to be neutral, with a pH close to 7.0 at 25 °C (77 °F). Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline. Neutral water, H2O is made up of on equal number or hydrogen H+ ions and hydroxide OH- ions. Water molecules dissociate in hydrogen (H+) and hydroxide (OH-) ions,

2H O H OH+ −+

but the number of ions formed is very small. Water (in the absence of other chemicals) at 25°C contains 10-7 mol/l of hydrogen ions and 10-7 mol/l of hydroxide ions, where the concentration (mol/l) of hydrogen ions [H+] multiplied by the concentration (mol/l) of hydroxide ions [OH-] is constant:

14[ ].[ ] 10Kw H OH+ − −= =

Kw is the dissociation constant for water and it depends on temperature. Acids in water increase the [H+] and, because the product [H+] [OH-] must be constant, acids decrease the [OH-]. Bases increase [OH-] and decrease [H+].

Measurement Methods

Colorimetric Methods: These methods cannot easily used for on-line measurements. A sample is picked up and a reagent (or pH paper) is added in it which changes color of the sample. The pH is then estimated by comparison with a color chart. Colorimetric analysis is based on the principle that many substances react with each other and form a product whose color is determined by the concentration of the substance to be measured. When a substance is exposed to a beam of light of intensity Io a portion of the radiation is absorbed by the substance's molecules, and a radiation of intensity I lower than Io is emitted. A photoelectric cell (photodiode or phototransistor) collects the radiation emitted by the sample and converts it into an electric current, producing a potential in the mV range. A microcontroller determines the concentration of the sample that corresponds to the mV value and to display it on an LCD. This type of instrumentation is called colorimeter (photometer), however colorimetric-type pH sensors are subjected to higher cost and maintenance and also, the color dyes are sensitive to oxidants. Ion Selective Field Effect Transistors: ISFET electrodes - using “Ion-selective field effect transistors”. One variant of ISFETs is the pHFET [23] which is a hydrogen ion sensitive metal-oxide-semiconductor FET (MOSFET). This silicon “chip” combines a pH-responsive membrane much like that of the glass electrode with the amplification of a field-effect transistor. The integral amplification and small size have led to the development of inexpensive, battery-powered, pocket-sized pH measurement systems. These devices have found unique and expanding niches,

including the food industry where the measurement of pH using breakable glass electrodes presents an unacceptable safety hazard, the measurement of the pH of gels, pastes, and slurries, and for the measurement of strongly alkaline solutions where conventional glass bulbs respond to the sodium ions and give an erroneously low reading. The basic operation of this device is presented in the following schematic which shows a cross-section of a pH-sensitive ISFET.

Figure 1: pH measurement using an ion-selective field effect transistor, including an amplifier

circuit for constant drain current operation.

The pHFET differs from a MOSFET in that the metal gate of the MOSFET is replaced by a pH-responsive membrane material such as silicon nitride, aluminum oxide, or tantalum oxide, which contacts the sample solution directly. As with the glass electrode, electrical contact is made to the sample through a reversible reference electrode. A suitable voltage applied to the reference electrode (relative to the silicon substrate) will charge the capacitor formed by the solution, insulating layers, and silicon substrate, and create mobile charge in the channel region. A potential simultaneously applied between the drain and source electrodes will result in current flow.

Electrochemical Cells (Glass Membrane Electrodes)[1,2,3,22]: This is the most well established method and is based on the construction of a galvanic cell. The pH is measured using a setup with two electrodes: the indicator electrode and the reference electrode. These two electrodes are often combined into one (combined pH electrode). When the two electrodes are immersed in a solution, a small galvanic cell is established. The potential developed is dependent on both electrodes. Ideal measuring conditions exist when only the potential of the indicator electrode changes in response to varying pH of the sample, while the potential of the reference electrode remains constant. The relation between measured potential E (mV), pH and temperature (K) can be expressed by the Nernst equation in the following way:

' 'ln 2.303ind ref T TH

RT RTE E E E a E pHF F+= − = + = +

where E = Measured voltage (mV) Eind = Voltage of indicator electrode (mV) Eref = Voltage of reference electrode (mV) E'T = Temperature dependent constant (mV) R = Gas Constant (8.3144 J/K) T = Absolute Temperature (K) F = Faraday's constant (95484.56 C/mol)

Therefore at a given (constant) temperature the potential of a solution depends on the pH only.

This equation can be seen as the standard formula for straight lines Y = a + b X, where a is the offset and b is the slope of the line. In this case, the offset is '

TE and the slope is the temperature dependent factor. At 25°C the slope gives -59.18 mV/pH. Electrode construction and operation The pH is determined by measuring the potential of an electrochemical cell. In most cases the three probes (indicator electrode, reference electrode and temperature probe) are combined in one electrode. Figure below shows a simplified diagram of a pH sensor.

Figure 2: combined pH glass electrode

When the pH probe is in contact with a solution a potential forms between the pH probe and the reference probe as shown in figure below. The meter measures the potential and converts it, using the calibration curve parameters, into a pH value.

Figure 3: Development of a potential (mV) between pH probe and reference probe.

The heart of the electrode is a thin pH-sensitive glass; this glass membrane is sealed to the electrode and contains a solution of potassium chloride KCl at pH 7. A silver wire plated with silver chloride contacts the solution. The Ag/AgCl combination in contact with the filling solution sets an internal reference potential.

Figure 4: Working principle of a pH glass membrane

The above figure shows, the outside surface of the glass membrane is in contact with the sample being measured, and the inside surface contacts the filling solution. A complex mechanism at each glass liquid interface defines the potential of the pH glass electrode. While the inner pH glass / filling solution potential is constant, the outside potentials varies based on the [H+] in the sample. This equilibrium depends also on temperature. The reference electrode is a silver wire coated with silver chloride in contact with a defined electrolyte solution. In many reference electrodes a gel is used instead of a liquid as the internal filling. These gels also contain KCl to maintain the reference potential and add sufficient conductivity. The tube containing the reference element and solutions/gel is in contact with the sample to measure through a junction (diaphragm). This liquid junction provides the electrical connection between the reference electrode and the sample being measured. It is essential to maintain a free flow of ions through the junction otherwise the reference electrode will not respond properly to pH changes in the sample.

Instrumentation and Analog Signal Conditioning

Due to the high electrical resistance of the pH glass electrode, the electrical mV voltage produced between the reference electrode and the glass electrode must be buffered/amplified using an operational amplifier before the A/D converter interface. Usually a voltage follower or a non-inverting amplifier is used. The glass electrode typically has an inner resistance, Rg, of the order of 108Ω and thus the amplifier’s input resistance, Ri, must be considerably higher (in order to ensure that the voltage supplied by the electrode is not reduced due to current flow towards the instrument). A value of 1012Ω is required. For the same reason it is also important that the amplifier does not send any current through the glass electrode as this will give an error potential and could even disturb the electrode. The so-called bias current, Ib, should therefore be below 10-

12A. When Ri >> Rg, Ib = 10-12 and Rg = 108Ω the error introduced can be calculated according to Ohm's Law: Verror = Ib .Rg = 0.1 mV. The op-amp selected should provide offset null trimming to zero the input offset voltage or input bias currents. Below we illustrate some analog signal conditioning circuits that aim to interface the pH electrode to the Analog to Digital converter interface. The first stage is the impedance matching and amplification circuit (buffer pH sensor high impedance signal). This circuit uses a low noise and low input bias op-amp (OPA129, TL072, etc.) to amplify the pH Probe’s high impedance signal using a non-inverting amplifier. The DC gain is defined by R2 and R1 and is given by G=Vout/Vin=(1+R2/R1)=20, however an additional variable resistor (potentiometer) can be used in series with R2 to adjust the pH meter slope response. The op-amp should have a dual supply voltage (e.g. V+=5V and V-=-5V), such dual supply can be

provide ether using two voltage sources (batteries) connected in line or using specific voltage inverter microchips.

Figure 5: An impedance matching and amplification circuit of a pH Probe

The output of this stage is a voltage that is positive for pH <7 and negative for pH >7. Therefore usually another stage is used to invert the voltage output and adjust the DC offset; both of these tasks are accomplished using another op-amp in inverting mode. The output of the second stage is given by Vout=-(R2/R1)Vin+ (1+R2/R1)Vbias. By adjusting Vbias thought a potentiometer the output of this circuit will be a positive voltage that increases linearly with pH and therefore appropriate to be interface in an A/D converter. This stage is presented in figure below. The Capacitor C1 was added to the circuit diagram for nullifying AC noise signals. Additionally a passive RC low pass filter can be used to further minimize noise.

Figure 6: An inverting amplifier with DC offset adjustment.

To attain reliable and consistent results, the amplifier and other circuits must have a small temperature coefficient, i.e. the influence of temperature variations must be under control. In addition proper grounding and or guarding should be applied to alleviate a lot of problems related to noisy electrode signals as well as electrode cables should not run parallel to power lines as they may pick up noise. Calibration

An ideal pH sensor should output zero mV at pH 7 and should have a 59.2mV response per pH unit (response is positive + for pH <7 and negative for - pH >7). The pH sensor is calibrated using standard buffer solutions of known pH. A two point calibration procedure is usually used, where a pair of standards is chosen to bracket the pH range of interest. The commonly used buffers have pH values of 4.01, 7.00 and 10.00. First the system's (electrode and meter together) Zero Point is

adjusted using a pH 7.00 buffer and then a second buffer is used to get the pH value of the second buffer and identify the slope. The slope can be given by 2 1 2 1( ) / ( )b b b bS V V pH pH= − − mV/pH Note that the two-point calibration is necessary to ensure electrode works properly since a broken electrode can give acceptable pH7 output in calibration mode. Therefore the response of the electrode is measured in each buffer solution and a calibration function is determined by linear interpolation. Temperature Compensation The potential developed across the pH indicator glass electrode is temperature dependent. When measuring pH using a pH electrode the temperature error from the electrode varies based on the Nernst Equation as 0.03pH/10C/unit of pH away from pH7. As shown in the table below, the error due to temperature is a function of both temperature and the pH being measured. Note that there is no error at pH 7 and 25° C. Temperature compensation can be achieved manually or automatically. Manual temperature compensation is usually achieved by entering the temperature of the fluid being measured into the instruments menu and then the instrument will display a "Temperature Compensated" pH reading. This means that the pH value is corrected to the value expected at 25° C. Automatic temperature compensation requires input from a temperature sensor and constantly sends a compensated pH signal to the display. Automatic temperature compensation is useful for measuring pH in systems with wide variations in temperature. Some pH sensors equipped with automatic temperature compensation (ATC) use a platinum resistance detector (RTD) sensor to directly measure the temperature of the sample. The instrumentation within the meter then corrects the calibration function such that the millivolt reading is correctly interpreted as the pH of the sample at the measurement temperature.

pH vs. Temperature Error Chart pH 2 pH 3 pH 4 pH 5 pH 6 pH 7 pH 8 pH 9 pH

10 pH 11

pH 12

5° .30 .24 .18 .12 .06 0 .06 .12 .18 .24 .30 15° .15 .12 .09 .06 .03 0 .03 .06 .09 .12 .15 25° 0 0 0 0 0 0 0 0 0 0 0 35° .15 .12 .09 .06 .03 0 .03 .06 .09 .12 .15 45° .30 .24 .18 .12 .06 0 .06 .12 .18 .24 .30 55° .45 .36 .27 .18 .09 0 .09 .18 .27 .36 .45 65° .60 .48 .36 .24 .12 0 .12 .24 .36 .48 .60 75° .75 .60 .45 .30 .15 0 .15 .30 .45 .60 .75 85° .90 .72 .54 .36 .18 0 .18 .36 .54 .72 .90

Table 1: The pH measurement error due to temperature and pH value. Values in light blue are less than 0.1 error and may not require temperature compensation. Values in gray are temperature

and pH in which there is no error in pH from temperature.

The importance of pH measurement

The pH measurement is important in almost all uses of water. In particular, pH balance is important in maintaining desirable aquatic ecological conditions in natural waters. pH is also maintained at various levels for efficient operation of water and wastewater treatment systems such as coagulation, disinfecting, softening and anaerobic decomposition of wastes. The pH of

most natural waters lies between 6.5 and 8. No health effects are associated with pH, unless dealing with extreme cases of alkaline or acidic conditions in potable water. In addition, pH measurements are required for the compensation of readings received by other types of water sensors e.g. free chlorine sensors. The following figure illustrates some typical pH values of common materials

Figure 7: Some typical pH values of common materials

The lifetime of a pH electrode depends on several factors including storage conditions, correct maintenance and the type of sample measured. Under normal laboratory conditions, for aqueous samples, the average lifetime is between 12 and 18 months, supposing of course that the electrode is kept clean and kept hydrated during storage. If the probe is used with dirty samples (e.g. stirred solutions with particles), is subjected to mechanical abrasion or used at high temperature or high pressure, the lifetime may be only a few weeks. In hot alkaline solutions, pH probes can be damaged after only a few hours. Regular maintenance helps pH probes keep working efficiently for several years.

ORP Introduction/Definition

ORP is a measurement of Oxidation Reduction Potential mV (or Redox Potential) most commonly used to measure the effectiveness of water disinfection sanitizers. In other words, ORP is a measure of the oxidizing or reducing power (ability) of a water solution. This means that ORP is not a measurement of concentration (total) of all oxidizing (or reducing) species, but a measure of the total oxidizing capability (electrochemical potential) that the solution has. The unit of measurement for ORP is mV.

Oxidation and reduction always occur together. Oxidation refers to any chemical action in which electrons are transferred between atoms. The atom that loses an electron is said to be oxidized and the atom that gains an electron is said to be reduced.

Chemicals like chlorine, sodium hypochlorite, bromine and ozone are all strong oxidizers. Their ability to oxidize or to steal electrons from other substances is that makes them good sanitizers. The sanitizing action is caused by the alteration of the chemical makeup of unwanted organisms. Oxidizers literally burn off germs, bacteria and other organic material in water leaving as a by-product a few harmless chemicals. As the measured value of ORP increases, the solution has more potential to oxidize and thus to destroy any organisms and to do this destruction more quickly.

Measurement Method

An ORP measuring electrode is identical to a pH measuring electrode except a noble metal is used instead of a glass electrode as the measuring element. Noble metals are used because they will not enter into the chemical reaction taking place. Most commonly, the measuring electrode is made of platinum but other noble metals such as gold or silver can be used.

An ORP sensor produces very tiny voltages generated when a metal is placed in water in the presence of oxidizing and reducing agents. This voltage is measured between the measuring electrode (platinum electrode which is the positive pole), and a reference electrode (which is the negative pole), with the water in between. The measuring electrode (+) of the probe, is usually made of platinum and the reference electrode (-), usually made of silver surrounded by an electrolyte solution. The difference in voltage between the two electrodes is actually the output of an ORP sensor. As an oxidizer is added to the water, it steals electrons from the surface of the platinum measuring electrode, causing the electrode to become more and more positively charged. As you continue to add oxidizer to the water, the electrode generates a higher and higher positive voltage.

Electrode construction and operation

The ORP electrode [1,2] is a combination style electrode comprised of a pure platinum metal measuring half-cell and a reference half-cell (identical to the one used in the pH electrode) to which the platinum half-cell is referenced. The reference-cell is a Ag/AgCl (silver/silver chloride) wire in 3.5M KCl saturated with AgCl (silver chloride). A second junction for protection of the reference wire is common in industrial electrodes and is referred to as a "Double Junction". A combination ORP electrode works the same as a combination pH electrode. The measuring electrode generates a millivolt output based on the oxidizing or reducing reactions taking place while the reference electrode generates a constant millivolt output. The working range of an ORP electrode is +/- 2000mV. The figure below shows a simplified diagram of an ORP sensor.

Figure 8: A schematic of an ORP electrode

Instrumentation and Analog Signal Conditioning

The same analog signal conditioning circuits (preamplifier) is used as in the pH sensor. Although the ORP sensor is a fairly low impedance device (10-100kΩ), the preamplifier ensures that no current is drawn through the platinum electrode and therefore the measurement is not invalidated by "polarization" effects (i.e. electrode potential shifting when current is drawn). Calibration

ORP sensor calibration is similar to that of a pH sensor and therefore the ORP sensor is calibrated using standard buffer solutions of known ORP mV. Two point calibration procedure is usually used and also pH buffer solutions can be used (as pH 7.0 buffer has 87 mV ORP and pH 4.0 buffer has and 264 mV ORP). As with pH sensor, the response of the ORP electrode is measured in each buffer solution and a calibration function is determined by linear interpolation.

Temperature Compensation

Temperature compensation is not used for ORP measurements. The correction factors are system and chemical dependent and are not easily determined. Usually ORP sensor readings are fluctuating as much as 25mV and thus the output of the sensor is needed to be averaged to develop a smooth ORP signal.

ORP Vs. various oxidizing agents

Correlating the ORP to an exact concentration of chlorine in water is often desired, but is not so straightforward. There are two causes for this. First, the activity of the chlorine in the water is greatly affected by the pH. This is caused by a conversion of the form of chlorine in the water from hypochlorous acid (HOCI) to hypochlorous ion (OCI-) as the pH increases. Unfortunately, the latter, hypochlorous ion is 80 to 300 times less effective than the hypochlorous acid. The lower oxidizing activity of this form of the chlorine results in a lower oxidation potential, or ORP. It is therefore critical to know the exact pH (or better yet, control the pH) in order to make a correlation to PPM chlorine. At pH 7.5, HOCl and OCl– are at equilibrium in water (50:50). Lowering the pH raises the percentage of HOCl, and ORP increases (larger mV readings) to reflect this shift in oxidative potential. In other words, an ORP of 700 mV at pH 6.5 has the same “killing” potential as an ORP value of 700 mV at pH 8.5. It would require a much higher dose of hypochlorite to achieve this constant ORP at pH 8.5, since HOCl would represent only 15 percent

of the total free chlorine. The figure below shows the relationship between pH, ORP and ppm chlorine for various conditions frequently used in disinfection.

Figure9: Free Chlorine concentration (ppm) Vs ORP (mV) and pH values

Table 2: Free Chlorine concentration (ppm) as a function of ORP (mV) and pH

In addition, based on the Nernst equation, the half-cell oxidation-reduction reactions can be represented as:

lno oxcell

red

aRTE EnF a

= +

Where EORP=Ecell-EAgCl,

Eo = the potential under standard conditions of unit activity referred to the Standard Hydrogen Electrode R = Gas Constant (8.3144 J/mol.K) T = Absolute Temperature (K) F = Faraday's constant (95484.56 C/mol) n = number of electrons exchanged in the reaction The aox and ared are the concentrations of the oxidizing agent and its reduced form (or the reducing agent and its oxidized form). In the case when chlorine is used as an oxidizing agent, the reduced form of chlorine is the unreactive chloride (salt). Since the ORP is proportional to the ratio of these concentrations a greater concentration of chlorides in the solution will also cause the ORP to decrease. It should be reinforced however, that only ORP represents the true oxidizing capability of the solution, while a measurement of ppm chlorine may represent chlorine present in a very inactive oxidation state.

Measurement of ozone in water with ORP is much more straightforward than the measurement of chlorine. The primary reason for this is that the ozone does not change its form in the water and therefore is not affected by pH. The relationship between ORP and ozone is shown in figure below.

Figure 10: Ozone concentration (ppm) Vs ORP (mV) values

The importance of ORP measurement

As previously mention above ORP is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. Chemicals like chlorine, bromine, and ozone are all oxidizers. It is their ability to oxidize (in a lose sense, to "steal" electrons from other substances) that makes them good water sanitizers / disinfectants, because in altering the chemical makeup of unwanted plants and animals, they kill or de-activate them. Then they "burn up" the remains, leaving a few harmless chemicals as the by-product. Of course, in the process of oxidizing, all of these oxidizers are reduced - so they lose their ability to further oxidize things. They may combine with other substances in the water, or their electrical charge may simply be "used up." To make sure that the chemical process continues to the very end, you must have a high enough concentration of oxidizer in the water to do the whole job. Essentially, ORP is a cost effective and simple method monitor in real-time the sanitizer effectiveness (real-time monitoring of Free Chlorine, which is another measure of disinfectant effectiveness, is more challenging and costly than measurement of ORP). Scientific and field studies in Germany and elsewhere have shown that ORP readings are better indication of bactericidal properties of chlorine than PPM Free Chlorine values. In 1971, the World Health Organization adopted an ORP standard for drinking water disinfection of 650 millivolts. That is, the WHO [10] stated that when Oxidation-Reduction Potential (ORP) in a body of water measures 650/1000 (about 2/3) of a volt, the sanitizer in the water is active enough to

destroy harmful organisms almost instantaneously. In Germany, which has about the strictest water-quality standards in the world, an ORP level of 750 millivolts was established by the Deutsche Institut fur Normung (DIN) Standard 19643 [6], as the minimum standard for public pools in 1982 and DIN Standard 19644 for public spas in 1984. France and most other European countries have since adopted these standards.

ORP has proven to be a reliable and cost effective method of evaluating water sanitation. Research has shown that at 650-700 mV of ORP, bacteria such as E. coli and Salmonella are killed on contact or within a few seconds. Tougher organisms such as listeria, yeasts and molds may require 750 mV or higher in order to be killed. The table below illustrates the relationship between ORP and bacterial levels measured from commercial spa water (Oregon, 1985)[7].

Table 3: Relationship between ORP and bacterial levels measured from commercial spa water.

Chlorine

Chlorine, dissolved in water, is one of the most effective and economical germ-killers for the treatment of water to make it potable or safe to drink. Chlorine's powerful disinfectant qualities come from its ability to bond with and destroy the outer surfaces of bacteria and viruses. Drinking water chlorination is one of the most widely used methods to safeguard drinking water supplies. In U.S. approximately 80% of the municipal water systems are disinfecting with chlorine.

Chlorine can be added to water as chlorine gas Cl2, an aqueous sodium hypochlorite solution NaOCl, or solid calcium hypochlorite Ca(OCl)2. When chlorine is added to water in any of these forms, it creates hypochlorous acid HOCl. Hypochlorous acid is very effective for killing germs but is a weak acid that dissociates into hydrogen ion (H+) and a hypochlorite ion (OCl-) according to the following equation: HOCl OCl H− ++

.

In general water is not completely clean and therefore when chlorine is added in water, a part of chlorine react with organic material in the water and is not available for disinfection, this is called the chlorine demand of the water. The remaining chlorine concentration after the chlorine demand is called total chlorine. A part of total chlorine is reacted with nitrates or ammonia and is unavailable for disinfection; this part is termed as combined chlorine (or chloramines). The rest of chlorine is term as total free chlorine, which is the chlorine available for disinfection to inactivate disease-causing organisms. Total Free Chlorine refers to the sum of hypochlorous acid, HOCl, and hypochlorite ion, OCl- . Both kill micro-organisms and bacteria by attacking the lipids in the cell walls and destroying the enzymes and structures inside the cell, rendering them oxidized and harmless. The difference between HOCl and OCl- is the speed at which they oxidize. Hypochlorous acid is a very effective better disinfectant (100x) compare to OCl-, and is often referred to as active (or free available) chlorine. Hypochlorous acid is able to oxidize the organisms in several seconds, while the hypochlorite ion may take up to 30 minutes. The proportion of hypochlorous acid and hypochlorite ion depends primarily on pH and also slightly affected by temperature. Therefore the disinfecting effect is strongly relative to the pH of water. As the pH increases, the ratio of hypochlorous acid to hypochlorite ion decreases. Below a pH of 7.5, hypochlorous acid is the dominant species. Above a pH of 7.5, hypochlorite ion is the dominant species. Figure below illustrates the relationship between the chemical forms of chlorine and pH at 20° C. The graph indicates a significant change in the ratio of hypochlorous acid to hypochlorite between pH 6 and 9, within the typical pH range for drinking water treatment. The steepest portion of the curve is between pH 7 and 8. This is significant because HOCl is a stronger disinfectant than OCl–.

Figure 11: Percentage (%) of Chlorine Concentration vs. pH

Free Chlorine levels in drinking-tap water

In water distribution networks (pipes) the aim is to deliver effective disinfection at the endpoints (i.e., water taps) of the system. According to WHO [10], the residual concentration of free chlorine must be greater than or equal to 0.5 mg/liter after at least 30 minutes contact time at pH less than 8.0. This definition is only appropriate when users drink water directly from the flowing tap. A free chlorine level of 0.5 mg/liter of free chlorine will contain a sufficient level of residual to maintain the quality of water through the distribution network, but is probably inadequate to maintain the quality of the water when this water is stored in the home in a container or bottle for 24 hours.

Therefore, free chlorine levels should maintain as follows: At 30 minutes, after the addition of sodium hypochlorite (or when a sample is taken from a home tap directly connected to the distribution network) there should be no more than 2.0 mg/l of free chlorine residual present (this ensures the water does not have an unpleasant taste or odour). At 24 hours, containers that are used by families to store water should at least maintain a minimum of 0.2 mg/l of free chlorine residual present (this ensures microbiologically clean water). Otherwise the addition of sodium hypochlorite is recommended.

In general, typical levels of free chlorine in drinking water are 0.2 - 2.0 mg/L to ensure microbiologically clean water, although regulatory limits allow levels as high as 4.0 mg/L.

Free Chlorine levels in swimming pools

Recommended minimum free available chlorine at residential pools is 1-3 ppm and at residential spas is 3-5 ppm. The levels of HOCl and OCl- vary with the pool's pH level. Ideally, the level of pH in the pool should be between 7.2 and 7.8 (7.4 is the pH of human tears). Higher pH drastically reduces the sanitizing power of the chlorine while lower pH causes bather discomfort, especially to the eyes. Chlorine also reacting with urea in urine from bathers can create nitrogen trichloride, which has an effect similar to teargas. In general, irritating effects as well as the well-known 'chlorine smell' in swimming pools is ascribed to chloramines (NH2Cl, NHCl2, NCl3) formed by chemical reactions between ammonia-derivatives and chlorine. Once the HOCl and OCl- have completed cleaning the pool, they either combine with another chemical, such as ammonia, or are broken down into single atoms. Both of these processes render the chlorine harmless. Sunlight speeds these processes up. Therefore one has to keep adding chlorine to the pool as it breaks down.

Measurement Methods

There are three main standard methods to measure free chlorine residual in drinking water:

1 Colorimetric methods: Colorimetric detection is a method based on the addition of reagents (as described in pH measurement methods). A water sample is taken, a reagent is added (powder or tablet of DPD, N,N-Diethyl-p-Phenylenediamine, chemical) and based on the color intensity of the solution, the free chlorine levels are determined [8].

2. Amperometric Probes: Amperometry is an electrochemical technique that measures the change in current resulting from chemical reactions taking place as a function of the concentration of a substance. A typical amperometric sensor consists of two dissimilar electrodes – an anode and a cathode (i.e. silver/platinum or silver/gold). Generally the following reduction- oxidation reaction is taking place in a chlorine amperometric probe:

Cathode (measuring electrode): HOCl + H+ + 2e -> Cl- + H2O (reduction of hypochlorous acid)

Anode (reference electrode): Cl- + Me -> MeCl + e (oxidation of chloride ions)

The anode may be split into two parts, a reference and a counter electrode making the measurement more stable. Such systems are called three-electrode sensors with measuring (or working) (WE) and counter electrodes (CE) of pure gold and a Ag/AgCl reference electrode (RE).

Figure 12: Illustration of Amperometric free chlorine probe

When a precise potential is applied to the measuring electrode only hypochlorous acid is electrochemically reduced. HOCl is reduced to chloride ion at the gold cathode. At the same time, the silver anode is oxidized to form silver chloride (AgCl). The release of electrons at the cathode and acceptance at the anode creates a current flow, which under constant conditions, is proportional to the free chlorine concentration in the medium outside the sensor. In other words, the voltage between measuring and reference electrode is constant and the current flow is a direct measure for the concentration of free Chlorine. The resulting low current output is then conditioned to 4-20mA current by the sensor's onboard electronic circuitry.

A simplified schematic of a potentiostat circuit is shown below, where the reference electrode is connected to the inverting input of an op-amp. The measuring electrode (WE) is connected to the ground and the potential of the reference electrode is kept constant (Ei). The current that flows thought Rm is proportional to free Chlorine concentration and can be found by measuring the voltage on Rm and hence I=Vm\Rm. Another approach of a potentiostat circuit is to measure the current output of the measure electrode (WE) using a current to voltage converter.

Figure 13: A simplified potentiostat circuit for three-electrode sensors

Typically electrodes are covered with a membrane (where hypochlorous acid (HOCl) diffuses through it), providing for better selectivity of the analysis. In the case of no membrane, the system is called bare-electrode amperometric and in the case of no applied voltage, the system is called galvanic. From a technical standpoint, many of the electrochemical methods that fall under the amperometric measurement category, including bare-electrode and galvanic systems, are sometimes wrongly referred to as polarographic.

Compensation

Amperometric free chlorine sensors directly measure only HOCl, not OCl- or Cl2, therefore the accuracy of their measurements is subjected in changing chlorine concentration, pH, temperature, sample flow, and pressure. In addition, amperometric sensors are more prone to fouling with the due to their specific contraction and this will result in increased cleaning and calibration frequency.

A pH of 7.0 to 8.0 is typically the normal operating range for most drinking water facilities. The HOCl concentration is much lower versus the OCl- in this range. As amperometric free chlorine sensors directly measure only HOCl any change in pH within this range will substantially affect the accuracy of the sensor. Therefore in order to reliably measure total free chlorine an additional pH sensor is used to mathematically compensate the reading based on pH value, however, as previously mentioned reliable measurement of pH requires temperature compensation and therefore an additional temperature sensor.

Due to the flow and pressure sensitivity, the amperometric free sensor must be installed into a flow cell to allow sensor operate at a constant flow rate and also prevent air bubble formation on the membrane. This enables the obtaining accurate free chlorine readings. Sensors directly installed in-pipe with no flow cell to condition the sample provide erratic responses as the readings are both flow and pressure (due to membrane) dependent.

Figure 14: A free chlorine measurement system (courtesy Dr. A. Kuntze GmbH)

For the above discussion, it is clear that the direct measurement of free chlorine using amperometric sensors in a complicated and expensive method which requires a flow cell, several compensations as well as frequent cleaning and calibration procedures. The price of bare free chorine sensors starts from 1000 euros.

3. ORP/pH method: The measurement provided by an ORP sensor is proportional to the concentration of free available chlorine. Using a polynomial formula, one can calculate the total free chlorine value. This calculation requires the pH and temperature measurement of the sample. This method does not measure free residual chlorine directly and therefore any oxidant present in water sample will be read as an increase in free residual chlorine. However, it is a reliable and cost effective method to evaluating drinking water disinfection.

Ion-Selective Electrodes (ISE) Several ions dissolved in drinking water, like Nitrate, Chloride, Ammonium, etc, could be measured by Ion-Selective Electrodes (ISE). The ion-selective measurement theory and equipment are similar to those of pH. An ion-selective electrode combines a sensor covered in an ion-specific membrane with a reference electrode. The sensor measures the activity of a specific ion by relating it to electric potential using the Nernst equation. The electric potential is equivalent to the logarithm of the ionic activity when temperature and pH are constant. The electric potential across the membrane is compared to the reference electrode, and the net charge is calculated. The net charge is directly proportional to the concentration of the ion being measured and displayed on a meter in mg/L. Membranes isolate the electrode from the water sample. More importantly, it is the surface upon which the charge develops. Hydrogen ion responsive glass was the earliest membrane used for ISEs.

Nitrates

Among many ions, nitrate (NO3-) is considered one of the chemicals of greatest health concern.

Nitrates can reach surface and groundwater due to agricultural activity (inorganic nitrogenous fertilizers and manure), wastewater disposal and from the oxidation of nitrogenous waste products (e.g. ammonia release from septic systems)[4]. High levels of nitrate are toxic both to humans and animals. The United States Environmental Protection Agency (EPA) regulates that a nitrate concentration above 10 ppm is unsafe for human consumption. Nitrate in concentrations of more than 30 ppm has been known to inhibit growth and impair the immune system. Methemoglobinemia, also known as infant cyanosis and blue-baby syndrome, develops after ingesting water with excessive levels of nitrate. Nitrate reacts in the intestinal tracts of an infant and changes hemoglobin into methemoglobin. Because hemoglobin is important for transporting oxygen throughout the body, methemoglobinemia can cause the infant to suffocate slowly. It is called blue-baby syndrome because of the bluish color in the baby’s extremities that results from the oxygen deprivation. Finally, stomach and gastrointestinal cancer are also linked to consumption of nitrate contaminated water.

There are three methods for measuring nitrate concentrations in water.

Colorimetric method: Using cadmium as a reagent it is possible to measure the concentration of nitrates based on the intensity of the resulting red solution.

Ion-Selective Electrodes (ISE) sensors: Using a nitrate ISE sensor one can measure the concentration of nitrates in water. The membrane is usually comprised of Organic Ion Exchangers and Chelating Agent Membranes integrated into the PVC materials. Selectively, they bind specific ions from an aqueous state and are exchanged across the membrane that creates the electric potential. Fluctuations in temperature or pH, or high concentrations of bicarbonate, chloride, or other undesired ions, during measurement can result in inaccurate nitrate readings. Moreover, membrane usually becomes dirty from particles in the water resulting in sluggish and erroneous sensor response. For this reason, ion-selective electrodes are most commonly used for lab rather than field (in-line) measurements.

UV Spectrophotometric Method: An alternative to using color-changing reagents or ISE probe is to use an ultraviolet spectrophotometer. UV spectrophotometers measure light at wavelengths smaller than those that are visible to the human eye. Nitrate absorbs ultraviolet light. A UV spectrometer measures the absorbance of a water sample by transmitting UV light in the range of 200 to 380 nm through the sample. The spectrophotometer can then calculate the concentration of nitrate present in the sample because the nitrate concentration will be directly proportional to the measured absorbance.

UV spectrophotometers must compensate for the turbidity of the water. Suspended substances can cloud the water and affect the measured absorbance of a sample. The UV spectrophotometer estimates and compensates for turbidity by using mathematical equations based on particle diameter and other parameters. Calibration is necessary to find the regressions needed to compensate for these parameters properly. The UV light source, UV spectrophotometer, sampling probe, and processing computer can all be housed within a single case and deployed in the field. UV spectrophotometers are capable of sampling at determined intervals and can provide real-time nitrate concentration measurements. However the cost of such systems is tens of thousands of euros and therefore not appropriate for home use.

Dissolved Oxygen

Introduction/Definition

Dissolved oxygen (D.O.) is the amount of oxygen gas dissolved in a given quantity of water at a given temperature and atmospheric pressure. As such, it should not be confused with combined oxygen as found in the water molecule, H2O. It is usually expressed as a concentration in parts per million or mg/l. It can also be expressed as percent saturation, where saturation is the maximum amount of oxygen that can theoretically be dissolved in water at a given altitude and temperature. A high DO level in water supply is good because it makes drinking water taste better. However, high DO levels speed up corrosion in water pipes. Dissolved oxygen (D.O.) levels are used as a general indicator of water quality mainly in industrial waste water and environmental monitoring. In water quality applications, such as aquaculture (including fish farming) and waste water treatment, the level of DO must be kept high. For aquaculture, if the DO level falls too low the fish will suffocate. In sewage treatment, bacteria decompose the solids. If the DO level is too low, the bacteria will die and decomposition ceases; if the DO level is too high, energy is wasted in the aeration of the water. With industrial applications including boilers, the make-up water must have low DO levels to prevent corrosion and boiler scale build-up which inhibits heat transfer.

Measurement Methods

Although dissolved oxygen (DO) is usually displayed as mg/L or ppm, DO sensors do not measure the actual amount of oxygen in water, but instead measure partial pressure of oxygen in water. Oxygen pressure is dependent on both salinity and temperature.

There are two fundamental techniques for measuring DO— galvanic and polarographic (amperometric). Both probes use an electrode system where the DO reacts with the cathode to produce a current. If the electrode materials are selected so that the difference in potential is -0.5V or greater between the cathode and anode, an external potential is not required and the system is called galvanic. If an external voltage is applied, the system is called polarographic.

Galvanic probes are more stable and more accurate at low dissolved oxygen levels than polarographic probes and often operate several months without electrolyte or membrane replacement, resulting in lower maintenance cost. In the other hand, polarographic probes need to be recharged every several weeks of heavy use.

Recently, an optical (luminescent) dissolved oxygen sensor is available [24] enabling significant reduction in maintenance compared to electrochemical DO sensors. Optical sensing of oxygen is based on the measurement of the red fluorescence of a dye/indicator illuminated with a modulated blue light. The presence of oxygen results in a phase shift of the red fluorescence light when compared to the modulated blue light source; hence by measuring this phase shift, it is possible to make a direct measurement of the oxygen concentration in the sample. However, the cost of luminescent dissolved oxygen sensor is significantly higher (around 1000 EUR) compared to that of electrochemical DO sensors.

Electrode construction and operation

Galvanic DO sensors consist of two electrodes: an anode and cathode which are both immersed in electrolyte (inside the sensor body). An oxygen permeable membrane separates the anode and cathode from the water being measured. Oxygen diffuses across the membrane. It interacts with the probe internals to produce an electrical current. Commonly, potassium hydroxide (KOH) is used as electrolyte. The cathode must be a noble metal (such as silver or gold) for the cathode potential to reduce molecular oxygen when the cell circuit is closed. The anode should be a base metal (iron, lead, cadmium, copper, zinc, or silver) with good stability. The chemical reactions

within the cell are as follows: O2 + 2H2O + 4e- → 4OH-, Anode: 2Pb → 2Pb2+ + 4e- and Overall: O2 + 2H2O + 2Pb → 2Pb(OH)2

Instrumentation and Analog Signal Conditioning

Higher pressure allows more oxygen to diffuse across the membrane and more current to be produced. The actual output from the sensor is a voltage V=I.R in millivolts. This is achieved by passing the current I across a thermistor R (a resistor that changes output with temperature). The thermistor corrects for membrane permeability errors due to temperature change. In other words, increasing permeability at higher temperature allows more oxygen to diffuse into the sensor, even though the oxygen pressure has not changed. This would give falsely high DO if the thermistor were not used. Typically DO sensors have very low impedance and therefore specific voltage followers are not required. The DO galvanic sensors usually have 0.00 mV output at 0 ppm and 50 mV output at 10 ppm (depends upon the model sensor and the membrane that is used) and linear response between there points. The sensor can be interface to an A/D converter through a simple non-inverting amplifier.

Compensations

DO Sensors need several compensation procedures to acquire accurate and reliable measurements. Generally the DO sensor output needs to be compensated with the temperature of the water sample, the atmospheric pressure (or the pressure of the water at the point of measurement) which affects the saturation of oxygen and the conductivity (salinity) of the sample. In addition the output of a DO sensor is affected by the characteristics of the DO probe membrane. For instance, membrane thickness determines the output level and the speed of response to change in DO levels.

The electrode pair permits current to flow in direct proportion to the amount of oxygen entering the system. The magnitude of the current gives us a direct measure of the amount of oxygen entering the probe. Because all of the oxygen entering the probe is chemically consumed, the partial pressure of oxygen in the electrolyte is zero. Therefore, a partial pressure gradient exists across the membrane and the rate of oxygen entering the probe is a function of the partial pressure of oxygen in the air or water being measured. Since the partial pressure of dissolved oxygen is a function of temperature of the sample, the probe must be calibrated at the sample temperature or the probe’s meter must automatically compensate for varying sample temperature. Note that this thermal effect is different from the thermal response of the membrane discussed above. The reading of a DO probe must be corrected for the amount of salt in the sample because the salt in solution will reduce the actual concentration of oxygen (solubility).

It is important to note that all DO Probes require some flow to provide accurate readings, the membrane/sample interface should have a few cm/sec flow of the sample for precision performance. Without flow at the interface, the surrounding oxygen will be consumed and the local reading dropped. The output of the probe increases (up to a point) with relative movement between the probe and sample. Therefore, DO measurements also depend on water flow to some extent.

The amount of oxygen that a given volume of water can hold is a function of the atmospheric pressure at the water-air interface, the temperature of the water, and the amount of other dissolved substances (such as salts or other gases) in the water. Recall seeing bubbles in a pot of water just before it starts to boil. These bubbles are the air which was dissolved in the water at room temperature. When the water boils, the dissolved oxygen is ejected—warmer water

contains less DO. When other substances, such as salts, are dissolved in a unit volume of water, there is less room for oxygen to dissolve—oxygen is less soluble than most salts

The saturated dissolved oxygen concentration (mg/L) as a function of temperature and salinity is approximated with the following exponential equation:

( ) ( ) ( ) ( ) ( ) ( ) ( ). . / . / . / . / . . / . /− = − + − + − − − + 5 7 2 10 3 11 4 2 3 2ln C 139 34 1 5757 x 10 T 6 6432 x 10 T 1 2438 x 10 T 8 6219 x 10 T S 1 7674 x 10 10 754 T 2 1407 x 10 T

where T = Temperature in degree Kelvin, S = Salinity in parts per thousand (ppt) and C = Concentration of DO in mg/L

As the pressure of the air above the water is increased, more oxygen will be dissolved in the water. This increases the concentration of the dissolved oxygen. According to Henry’s law (P=k.C), the solubility of a gas in a liquid is directly proportional to the pressure of that gas above the liquid.

Calibration

As with other electrochemical sensor, two point calibration procedure is usually used and DO response is assumed to be linear between these points. The one of the points on the line is the zero point of the sensor. At the zero point, the sensor signal obtained in the absence of oxygen lies below the resolution of the sensor. This point is called the zero-current point of the sensor. The second point of the calibration line can be set as required. Its position is based on the fact that the partial pressure of oxygen in liquid and air is equal. At any given temperature and barometric pressure the partial pressure of oxygen in water-saturated air (100% humidity) is exactly the same as it is in air-saturated water. Thus a sensor can be calibrated in water-saturated air, using the 20.9% oxygen available in air as the full-scale standard. Both temperature and barometric pressure affect the partial pressure of oxygen in air saturated water vapour. This calibration technique will give a 100% saturation reading as a function of the ambient temperature and pressure.

As described above calibration routines for dissolved oxygen probes use a two point linear calibration where one point is at zero mg/L oxygen and the second point is at saturation. The zero measurement is not zero volts due to the conductivity of the electrolyte between the electrodes as well as any errors in the analog signal conditioning circuit. Finally, it should be noted that the sensor membrane is sensitive to contamination which in turn results in lower readings when measuring or lesser slopes when calibrating because a portion of the membrane surface is not available for the diffusion of oxygen.

The importance of DO measurement

The aforementioned indicated that the accurate DO measurements are complicated due to the compensation parameters which will increase dramatically the overall system cost and complexity resulting either faulty DO readings or increasing the systems maintenance (cleaning DO membrane). Additionally, DO values are not directly related to potable water quality coming from water distribution systems, except for indicating water taste or future corrosions of metallic water pipes or tanks). Therefore, it is decided not to include this DO sensor in the water system.

Conductivity Introduction/Definition

Conductivity is a measure of a solution's ability to conduct electric current. An instrument measures conductivity by placing two plates of conductive material with known area (A) and distance (L) apart in a sample. Then a voltage (V) (a sine wave AC signal of low amplitude in the low kHz region) is applied and the resulting current (I) is measured. Conductivity in water is affected by the presence of inorganic dissolved solids (metals, salts, etc.) such as chloride, nitrate, sulphate, and phosphate anions (ions that carry a negative charge) or sodium, magnesium, calcium and iron cations (ions that carry a positive charge).

Conductance (G) is defined as the reciprocal of the resistance (R) and is determined from the voltage and current values according to Ohm’s Law:

G =1/R=I/V

The basic unit of measurement for conductance is Siemens (S). Since cell geometry affects conductance values, conductivity (or specific conductance) is expressed in Siemens per centimeter (S.cm-1) or mho.cm-1 or Ω-1.cm-1 to compensate for variations in electrode dimensions. For most solutions this measurement unit is much too large and either μS/cm or mS/cm are used instead. Electrical conductivity can be used as an index of the total dissolved solids (TDS) in a water sample measured in PPM (which is the same as mg/l). Correlating PPM to μS/cm can be difficult, as water can be makeup of different salt concentrations and dissolved metals, which can alter the conversion factor. However, if it is needed to convert to PPM, the following conversion factor can be used for drinking water samples, 1 ppm = 0.64 x μS/cm.

Since the charge on the ions in solution facilitates the conductance of electrical current, the conductivity of a solution is in general proportional to its ion concentration. Note that solutions may not show a direct correlation to concentration as different salts in water have a different ability to conduct electricity because of the differences in charge and size/weight and mobility of the different ions. In general, higher ionic concentration yields higher conductivity. The conductivity of water from various sources is given below: Absolute pure water=0.055µS/cm, Distilled water=0.5 to 1µS/cm, most drinking water=500 to 800µS/cm up maximum of 1055µS/cm. Sea water is about 55mS/cm.

Measurement Methods

There are basically two types of sensors for measuring water conductivity.

Electrode Conductivity Sensors (conductive sensors): The basic conductivity probe (cell) is comprised of two conductive surfaces separated by a given distance in a body. The body material can be anything from PVC, CPVC, PVDF, TEFLON, PEEK, or even stainless steel. The measuring surfaces (usually pin configuration) are typically constructed of graphite, stainless steel, titanium, or platinum. The basic criteria for determining which is best are based on cost and performance requirements.

Conductivity cells usually have standardized specific cell constants (K) that are used in the determination of conductivity C (S/cm) as follows

C=G x K

Where G is the conductance (S) and K is the cell constant (cm-1) define the volume between the electrodes. Cell constant K is directly proportional to the distance (L) separating the two conductive plates and inversely proportional to their surface area (A) and thus K = L/A. The measurement principle of a conductivity cell is illustrated in figure below

Figure 15: Illustration of a conductivity cell probe

Electrode conductivity sensors should be cleaned frequently as scratches and abrasions on the surface of the pins increase the surface area which alters the cell constant causing calibration and measurement difficulties. Graphite being a soft material is most susceptible. Cleaning should be done with chemicals and soft non-abrasive cloths. Sanding is not recommended. HCL is an excellent material to dissolve many coatings.

The basic two-pin conductivity cell is all we have discussed to this point. There is four-pin technology that tries to better control the field surrounding the conductivity sensor to improve reproducibility and minimize errors due to fouling.

Toroidal-Inductive Conductivity Sensors (non-contacting sensors): Another type of technology is the non-contacting (Toroidal) cell, which uses a magnetic field to sense conductivity. This sensor does not depend upon surface area like the sensor described above.

The toroidal sensor uses two magnetic cores, and they are wrapped with copper wire and placed into the “donut” on the end of the black arm of the sensor. An AC voltage is applied across the wires of one of the cores called the transmitting (drive) coil which in turn generates a magnetic alternating field that induces an electric current in the liquid sample. The ions present in the liquid enable a current flow that increases with increasing ion concentration. The ionic concentration is then proportional to the conductivity. The current in the liquid generates a magnetic alternating field in the receiving (pick-up) coil. The resulting current induced in the receiving coil is measured and used to determine the conductivity value of the solution. An illustration of the operating principle of a toroidal sensor is presented in the figure below. Toroidal conductivity sensors can be calibrated to permit excellent measurements over a very wide range of operation. Low range can be something close to 500 μS and the upper range can go to 1 Siemen or even higher.

Figure 16: Illustration of a toroidal-inductive conductivity sensor

The main advantage of toroidal conductivity is that the toroidal coils are not in contact with the solution. They are either encased in a polymeric material or are external to a flow through cell. This makes the sensor virtually immune to fouling and thus reduces maintenance cost. As long as the hole in the center of the “donut” is kept open, the sensor will provide accurate readings. Cleaning is very easy by wiping the sensor with a wet rag, and open up the hole by inserting a pencil or similar object. Calibration is maintained for extended periods even if the sensor is dirty. One of the main disadvantages of toroidal conductivity sensors is that they lack the sensitivity of contacting sensors, which makes them not appropriate for pure water applications. In addition, toroidal sensors are also typically larger than contacting sensors, and the solution current induced by the toroid occupies a volume around the sensor. Hence, toroidal sensors need to be mounted in a larger pipe.

Instrumentation and Analog Signal Conditioning

Reliable conductivity measurement of a drinking water sample using a conductive sensor requires three main sub circuits. A sine wave oscillator, an amplification circuit and an AC to DC converter (demodulator).

Typically the AC signal used to excite conductivity sensors is in the form of a sine wave. AC signal is used instead of DC to prevent ionization of the electrodes. Generally, the applied sinusoidal excitation frequency is proportional to the conductivity of the sample, e.g. low frequencies are applied at low conductivities. For drinking water applications 5-10kHz is most appropriate. Alternately, a square wave (bipolar pulse) excitation produced by analog switches (MAX303) using a pulse signal from a microcontroller can also be used [25]. A very common oscillator circuit used to generate a low-distortion sinusoidal wave at low to moderate frequencies is the Wien-bridge oscillator. With the component values listed in the figure, this oscillator can cover a frequency range of 1 to 5 kHz. Oscillation is not always occurred and therefore a careful adjustment of the potentiometer is needed to generate the oscillation.

Figure 17: A sinusoidal Wien-bridge oscillator.

The conductance of the sample can be measured using a current to voltage converter as shown in the figure below and thus the conductance value is given by G=1/R=-Vout/(Vexc.Rf), where Vexc is the voltage of the sinusoidal excitation signal, Rf and Vout is the feedback resistance and voltage output of the current to voltage converter.

Figure 18: A current to voltage converter for conductivity sensor.

The output of current to voltage converter can be further amplified using an inverting amplifier and then demodulated (AC to DC conversation) before being feed to the A/D converter. Demodulation is needed to find the peak of the signal received for the conductivity probe after the amplification. Several demodulation (or peak detection) circuits exist, the simplest is the one that uses a network of capacitors and diodes (preferably fast low-forward-voltage Schottky diodes) as shown below.

Figure 19: A demodulator-peak detector circuit

This circuit works as follows, during the negative half cycle of the ac sinusoidal signal D1 is forward-biased, and C1 charges to V0 while D2 is open-circuited, and then during the positive half

cycle,D1 becomes an open circuit and D2 becomes a short circuit, and some of the charge in C1 flows into C2. The process continues until enough charge is pumped into C2 to raise its voltage to 2V0. The discharge time constant should be selected to be small enough for fast response (conductivity measurement) and large enough compared to the excitation sine wave period.

It is noted that another option is the use of active rectifiers (using op-amp). Finally the output can be interfaced into A/D converter through a voltage follower (buffer).

Calibration

Conductivity sensor calibration is similar to that of other water sensors and therefore the conductivity sensor is calibrated using standard buffer solutions of known conductance.

Temperature Compensation

All conductivity measurements are temperature dependent. The degree to which temperature affects conductivity varies from solution to solution and can be compensated for using a temperature sensor (e.g. thermistor) in the conductivity electrode. An increase in temperature makes the ions in the water move faster and thus the conductivity is increased (the warmer the water, the higher the conductivity). Conductivity levels in drinking water falsely increase approximately 2% per °C (and in more pure waters may increase up to 4 or 5% per °C). Therefore, in moderate conductivity solutions (drinking water), temperature correction can be based on linear equation involving a temperature coefficient θ (%/°C) and the sample conductivity at the given temperature as shown by equation below

100( ) ( )

100 .( )refref

C T C TT Tθ

=+ −

Where C(Tref) is the conductivity at Tref, C(T) is the sample conductivity at T, Tref is the reference temperature, T is the sample temperature and θ is the temperature coefficient (e.g. 2%/°C for drinking water)

In addition several measurement errors can introduce due to the sensor cable. For instance, in high conductivity measurements (low resistance< 50Ω) the cable resistance induces error and in low conductivity measurements (below 4μS) the cable capacitance must be compensated.

The importance of conductivity measurement

The presence of minerals in drinking water (hardness) is preferred not only because of the health benefits, but also the flavor. On the other hand, soft water tastes salty and is sometimes not suitable for drinking. However, excess water hardness often causes deposits in pipes and in household appliances, which can, in time, harbor microbial communities. In addition, deposits reduce flow through pipes and is a poor conductor of heat. Eventually, pipes can become completely clogged. Finally, hard water reduces soap's ability to lather, whether in the shower, sink, dishwasher or washing machine, and reacts with soap to form a sticky scum. This problem is usually solved using water softeners.

Turbidity Introduction/Definition

Turbidity is the measurement of scattered light that results from the interaction of incident light with suspended and undissolved material in a water sample and it is an important water quality indicator. Turbidity is defined by the International Standards Organization (ISO) as the reduction of transparency of a liquid caused by the presence of undissolved matter. Turbidity can be interpreted as a measure of the relative clarity of water and often indicates the presence of dispersed, suspended solids; particles not in true solution such as silt, clay, algae and other microorganisms; organic matter and other minute particles. Solids in drinking water can support growth of harmful microorganisms and reduce the effectiveness of disinfection processes (i.e. chlorination, UV irradiation) resulting in increased health risks. In almost all water supplies, high levels of suspended matter are unacceptable for aesthetic reasons and can interfere with chemical and biological tests.

Excessive turbidity, or cloudiness, in potable water is aesthetically unappealing, and may also represent a health concern. Turbidity can provide food and shelter for pathogens. If not removed, turbidity can promote regrowth of pathogens in water distribution systems, leading to waterborne disease out-breaks, which have caused significant cases of gastroenteritis throughout the world. Suspended solids (the particles of turbidity) provide “shelter” for microbes by reducing their exposure to disinfectants. Further, waters with high turbidity from organic sources may give rise to a substantial chlorine demand. This could result in reductions in the free chlorine residual in distribution systems as protection against possible recontamination. Drinking water can serve as a transmission vehicle for a variety of hazardous agents (E. coli,, Legionella, arsenic, etc). Contaminants in drinking water can produce adverse effects in humans due to multiple routes (ingestion, inhalation and dermal) of exposure. In the EU, the quality of water intended for human consumption is governed by the Drinking Water Directive (DWD), Council Directive 98/83/EC. According to DWD [9] a set of microbiological and chemical parameters must be monitored and tested regularly and their values cannot exceed a predetermined threshold. Such parameters include Turbidity, Conductivity, E. Coli, pH, odour, taste etc. Moreover WHO [10] recommends the surveillance of household water storage systems as this water is more vulnerable to contamination. Regarding WHO [10] and EU DWD [9] drinking water turbidity value must not exceeding 1,0 NTU (nephelometric turbidity units) in the water ex-treatment works. The appearance of water with a turbidity of less than 5 NTU is usually acceptable to consumers.

Studies of water distribution systems have shown related findings with respect to turbidity and microorganisms [12,14]. Haas et al [11] noted that increased values of pH, temperature and turbidity were associated with increased concentrations of microorganisms.

Measurement Methods

Turbidity is measured using the techniques of turbidimetry or nephelometry and is expressed in arbitrary units (Nephelometric Turbidity Unit, NTU). The direct relationship between turbidity data and suspended solids concentration depends on many factors, including particle size distribution, particle shape and surface condition, refractive index of the scattering particles and wavelength of the light. There are three basic designs of turbidity meters [15, 16]:

– the nephelometer, which measures directly the intensity of light scattered by the sample. The light intensity is directly proportional to the amount of matter suspended in the light path. The sensor is mounted at an angle (usually 90o ) to the traversing beam to record scattered light. Nephelometers usually provide greater precision and sensitivity than turbidimeters and are normally used for samples of low turbidity containing small particles.

– the turbidimeter, sometimes called absorption meter, which measures the intensity of the beam after it has passed through the sample. Suspended matter in the light path causes scattering and

absorption of some light energy. The transmitted light is measured, in relation to initial beam intensity. Turbidimeters are more appropriate for relatively turbit samples in which the scattering particles are large in relation to the light wavelength used.

– the ratio turbidimeter, which measures both transmitted and scattered light intensities. For this purpose, transmitted light and 90o -scattered light are measured simultaneously with two different light sensors, which produce two voltages, V0 and V90 , respectively. Changes in the light absorption of the process medium, e.g. because of coloring, have the same influence on both light sensors. Thus, the signal ratio remains unchanged and is given by

90out

1 0 2 90

VV =V Vc c+

Where c1 and c2 are calibration coefficients. This feature has a number of advantages, including the elimination of the effect of coloring on readings and the increase of the long-term stability of the instrument (due to reducing drift of light source intensity). This design appears to be more appropriate for liquids either strongly colored or of variable color concentration, and for samples of high turbidity.

Figure 20: Illustration of light sensors setup for the ratio turbidimeter.

Continuous turbidity monitoring has become increasingly popular, mainly because the alternative practice of sampling and sedimentation analysis or filtration-and-weighing procedures are time-consuming and error-prone. Turbidity sensors probes may also be the only viable means of assessing suspended sediment changes in circumstances where conditions are harsh and access is limited. Generally, turbidity values can serve as a simple and convenient measure of the concentration of suspended solids in water-supply installations. Commercially available turbidity meters have relatively high price due to several reasons including analog signal conditioning circuits and filtering due to ambient light interference as well as due to expensive cleaning mechanisms (brushes). Turbidity systems that are compliant with ISO7027 use infrared LED radiation ( of 860nm wavelength) and measure the 90o Light Scatter (Nephelometric).

Sensor construction and operation There are many different types of turbidity sensors depending on the light source used (LED, laser diode) the emitted wavelength, the detectors number and setup and the housing of the light detectors (e.g. photodiodes could receive scattering light directly or through optical fibers). Commercially available sensors are usually bulky and both the light source and the photodetectors are usually located inside the sensor housing to minimize interference with the water. Turbidity process sensors to be directly housed in water pipes are vary rare and usually sensors are have a sample cell which increases both size and cost. In the simpler form a nephelometric turbidity system comprises of the two main components: 1) A light source and light source driver circuit 2) An optical sensor (photodiode) and the signal conditioning circuit. The light source (e.g. laser diode) emits light, which is transmitted through an

optical gap to the water sample. The photodiode receive the scattered light and its signal is amplified/filtered and sampled with an Analog-to-Digital converter, then a microcontroller is used for further averaging of several samples and for the conversation of digital sensor values to nephelometric turbidity unit (NTUs). Instrumentation and Analog Signal Conditioning The diode laser source is preferred because it improves the signal to noise ratio (SNR) due to its high directionality and relatively high light intensity. However, the received signal is very sensitive to small changes in turbidity and instantaneous readings may vary considerably as particle density changes or large particles move. Therefore the output of the sensor is needed to be averaged to develop a smooth turbidity signal. The photodiode’s spectral sensitivity should be selected to fit with that of the light source. The photodiode converts the scattered light directly into electrical current and then a high-gain, low-noise CMOS (Complementary metal-oxide-semiconductor) transimpedance amplifier (see Figure below) converts photocurrent (ISEN) to voltage output VOUT= ISEN R.

Figure 21: A circuit schematic of a transimpedance amplifier (current to voltage converter)

As light impinges on the photo diode, charge is generated, causing a current to flow in the reverse bias direction of the photodetector. If a CMOS op amp is used, the high input impedance of the op amp causes the current from the detector to go through the path of lower resistance R. Additionally, the op amp input bias current error is low because it is CMOS (typically < 200 pA). The non-inverting input of the op amp is referenced to ground, which keeps the entire circuit biased to ground. The voltage across the detector is nearly zero and equal to the offset voltage of the amplifier. With this configuration, current that appears across the resistor R is primarily a result of the light excitation (irradiance) on the photodiode. The capacitor C is sometimes needed to stabilize the amplifier when the photodiode sensor has a large capacitance or operated at high

frequencies. The capacitance can be estimated by c/ 2totC C Rfπ= where Ctot is the total input capacitance (photodiode junction capacitance + op amp input capacitance) and fc is the unity gain frequency of the op amp. Finally the signal should be passed through a low pass filter, to reduce noise, before being sampled by A/D converter. Calibration

Turbidity sensor calibration is similar to that of other water sensors and therefore the turbidity sensor is calibrated using standard buffer (formazin) solutions of known turbidity. In addition, common practice uses indirect methods for the sensor calibration in order to avoid the use of the

D

C

R

Vout

carcinogen and expensive formazin solutions. In other words, a number of samples are created and the turbidity of each sample is measured both by the turbidity sensor under calibration and by a laboratory turbidimeter used as reference. Then the relationship between turbidity (in NTU) and the voltage output (in V ) of the turbidity sensor is extracted. Interferences The measurement of turbidity in drinking water applications is subjected to interferences mainly due to stray light and bubbles in the sample water. Other turbidity interferences are listed in [18]. A positive bias is usually reported from the previous interferences (slightly higher measurements than the actual turbidity. A usual solution of the stray light interference is the rationing (ratio turbidimeter). This method also improves problems related to color interferences and optical intensity drifts of the light sources. As the bubble interference is concerned the best way to decrease the interferences is to let the sample stand or having constant flow for several minutes to allow bubbles to vacate or perform large integration periods. Contamination also results in a positive bias (reported measurement is higher than actual turbidity) because coatings on both light source and detectors reduce the amount of light energy to be transferred to electric signal. The particle sizes can either provide positive or negative bias (wavelength dependent). Large particles scatter long wavelengths of light more readily than will small particles. Small particles scatter short wavelengths of light more efficiently than long wavelengths. Theory of light scattering Very simply, the optical property expressed as turbidity is the interaction between light and suspended particles in water. A directed beam of light remains relatively undisturbed when transmitted through absolutely pure water, but even the molecules in a pure fluid will scatter light to a certain degree. Therefore, no solution will have zero turbidity. In samples containing suspended solids, the manner in which the sample interferes with light transmittance is related to the size, shape and composition of the particles in the solution and to the wavelength (color) of the incident light. A minute particle interacts with incident light by absorbing the light energy and then, as a point light sources itself, reradiating the light energy in all directions. This omnidirectional reradiation constitutes the "scattering" of the incident light. The spatial distribution of scattered light depends on the ratio of particle size to wavelength of incident light [13]. Particles much smaller than the wavelength of incident light exhibit a fairly symmetrical scattering distribution with approximately equal amounts of light scattered both forward and backward. As particle sizes increase in relation to wavelength, light scattered from different points of the sample particle create interference patterns that are additive in the forward direction. This constructive interference results in forward-scattered light of a higher intensity than light scattered in other directions (see figure below).

(a) In small particles (smaller than 1/10 the wavelength of light) scattering is symmetric

(b) In large particles (approximately 1/4 the wavelength of light) scattering concentrated in

forward direction

(c) In larger particles (larger than the wavelength of light) scattering in extremely concentrated in forward direction. Also maxima and minima of scattering intensity are developed at wider angles

Figure. 22. Angular patterns of scattered intensity from particles of three sizes. (a) small

particles, (b) large particles, (c) larger particles.(Source:[13]) In addition, smaller particles scatter shorter (blue) wavelengths more intensely while having little effect on longer (red) wavelengths. For example blue light tends to be scattered by the oxygen and nitrogen molecules in the atmosphere giving the blue sky we see!. Conversely, larger particles scatter long wavelengths more readily than they scatter short wavelengths of light. Particle shape and refractive index also affect scatter distribution and intensity. Spherical particles exhibit a larger forward-to-back scatter ratio than coiled or rod-shaped particles. The refractive index of a particle is a measure of how it redirects light passing through it from another medium such as the suspending fluid. The particle's refractive index must be different than the refractive index of the sample fluid in order for scattering to occur. As the difference between the refractive indices of suspended particle and suspending fluid increases, scattering become more intense. The color of suspended solids and sample fluid are significant in scattered light detection. A colored substance absorbs light energy in certain bands of the visible spectrum, changing the character of both transmitted light and scattered light and preventing a certain portion of the scattered light from reaching the detection system. Light scattering intensities as particle concentration increases. But as scattered light strikes more and more particles, multiple scattering occurs and absorption of light increases. When particulate concentration exceeds a certain point, detectable levels of both scattered and transmitted light drop rapidly, marking the upper limit of measurable turbidity. Decreasing the path length of light through the sample reduces the number of particles between the light source and the light detector and extends the upper limit of turbidity measurement. The importance of turbidity measurement

The WHO Guidelines for Drinking Water Quality state: “No health based guideline for turbidity has been proposed; ideally, however median turbidity should be below 0.1 NTU for effective disinfection, and changes in turbidity are an important process control parameter.” Turbidity is a measure of the relative clarity of the water. Turbidity values higher than 5NTU (i.e. nephelometric turbidity units) can be visible to the naked eye, especially when water is in large volumes such as in, white sinks or baths. Turbidity in the water is caused by suspended and colloidal matter, such as clay, silt, finely divided organic and inorganic matter, plankton and other microscopic organisms. Each of the constituents of turbidity in any given sample of water can affect the level of disinfection and inactivation of pathogens in many ways. In many cases the strength or the turbidity value can determine what type of treatment is need and to what level. In water sources that are subject to fecal pollution, we can expect a correlation between turbidity and fecal indicators or pathogens. Turbidity can be a significant source of chemical disinfectant demand when the type of turbidity is organically composed. When high amounts of chemicals disinfectants are used, the outcome of this would be a lot of disinfection by-products that can cause long term health effects. Therefore, the type of turbidity is more important than the NTU

measurement. In relatively unpolluted sources of water, turbidity is not a good sign of the presence of pathogens, telling us that pathogens do not always come along with turbidity, so it is rather challenging to identify a direct mathematical descriptor that will correlate turbidity to pathogens.

Temperature The measurement of temperature is important in many water monitoring applications mainly because the measurements of many water quality sensors depend on temperature and therefore temperature compensation is needed to obtain accurate results. The most popular temperature sensors used today are the Thermocouple, Resistive Temperature Detector (RTD), Thermistor, and the Integrated Silicon Based (semiconductor) sensors. The sensor’s temperature range, ruggedness and sensitivity are just a few characteristics that are used to determine whether or not a device will satisfy the requirements of the application.

Of these technologies, the most widely used in water monitoring applications is the platinum RTD sensors because is the most accurate, linear and stable over time and temperature. A constant current must be passed through the RTD, the same as with thermistors, and the change of voltage with temperature is measured. Materials for RTDs can be gold, silver, copper or platinum. Platinum, however, has become the most-used metal for RTDs. A thin film of platinum or a thin platinum wire is deposited on a flat ceramic material and sealed. Platinum has a nearly linear temperature versus resistance relationship. The operating temperature range of RTD’s is from –220°C to 850°C. In some cases also thermistors are used. A thermistor is a thermally sensitive resistor. This is a semiconductor composed of metallic oxides such as manganese, nickel, cobalt, copper, iron, and titanium. There are two basic types of thermistors — negative temperature coefficient (NTC) and positive temperature coefficient (PTC). NTC thermistors are much more commonly used than PTC thermistors. The resistance of NTC thermistors decreases with increasing temperature. NTC Thermistors have non-linear resistance temperature characteristic and give a relatively large output (change of resistance) for a small temperature change. The table below summarizes the main characteristics of these four temperature sensors.

Table 4: Characteristics of temperature sensing technologies

The most stable, linear and repeatable RTD is made of platinum metal. The temperature coefficient of the RTD element is positive and almost constant. Typical RTD elements are specified with 0°C values of 50, 100, 200, 500, 1000 or 2000Ω. The RTD element requires a constant current excitation. If the magnitude of the current source is too high, the element will dissipate power and start to self-heat. Consequently, care should be taken to insure that less than 1 mA of current is used to excite the RTD element. An approximation to the platinum RTD resistance change over temperature can be calculated by using the constant a = 0.00385Ω/Ω/°C. This constant is easily used to estimate the absolute resistance of the RTD at temperatures between -100°C and +200°C.

Therefore the RTD resistance is given by

R(T) ≈ R0 (1 + T × α)

Where:

R(T) = the RTD element’s resistance at T (Ω),

R0 = the RTD element’s resistance at 0°C (Ω),

T = the RTD element’s temperature (°C),

α = 0.00385 Ω/Ω/°C

If a higher accuracy temperature measurement is required, or a greater temperature range is measured, the standard polynomial formula can be used (Calendar-Van Dusen Equation) or the temperature can be found using a look-up table.

Instrumentation and Analog Signal Conditioning

The simplest method to interface and RTD is shown in the circuit schematic below. A precision current source is used providing constant current of 1mA. The current should be small (<1mA) to avoid self-heating.

Figure 23: RTD temperature sensing using current excitation

For best linearity, the RTD sensing element requires a stable current reference for excitation. This can be implemented in a number of ways, one of which is the floating current source shown in the figure below. In this circuit, a voltage reference, along with two operational amplifiers, is used to generate a floating 1 mA current source for an RTD-type sensor; however, it can be tuned to any current. With this configuration, the voltage of VREF is reduced via the first resistor R1=25kΩ by the voltage VR1. The voltage applied to the non-inverting input of the top op amp is VREF- VR1. This voltage is gained to the amplifier’s output (Vout=[1+R1/R1]Vin) by two to equal 2(VREF- VR1). Meanwhile, the output for the bottom op amp is presented with the voltage VREF-2VR1. Subtracting the voltage at the output of the top amplifier from the non-inverting input of the bottom amplifier gives: 2(VREF- VR1)-(VREF-2VR1) which equals VREF. Therefore the transfer function of the circuit is: IOUT=VREF/RL . This current is ratio-metric to the voltage reference. The same voltage reference should be used in other portions of the circuit, such as the analog-to-digital (A/D) converter reference.

Figure 24: A floating current source using two op amps and a precision voltage reference.

The signal received using the above circuit should be gained and filtered using a low pass filter (LPF) to remove noise and eliminate unwanted signals prior to the input of an A/D converter. A simple low pass filter can be implemented using an RC circuit as shown below.

Figure 25: A simple low-pass RC filter

A LPF prevents high frequencies are from reaching the output. When the frequency of the input signal to an RC low-pass filter reaches what is called the cutoff frequency, given by fC=1/(2πRC) the output voltage drops to 1/√2 the input voltage (equivalent to the output power dropping to half the input power).

A more advance LPF can be constructed from op amps, resistors and capacitors, such filters are called active filters and they are capable of handling very low frequency signals (approaching 0 Hz) and also they can provide voltage gain if needed (unlike passive filters where low cut off frequencies require exceedingly large capacitance). Figure below shows a 2nd order, low pass filter created with an op-amp, R1, C1A, C1B, R2 and C2. It is designed to have a Bessel response and a bandwidth of 10 Hz. The DC gain is defined by R3 and R4 and is given by G=1+R3/R4=7.47 V/V. It reduces noise and prevents aliasing of higher frequency signals. This type of filter is also referred to as an anti-aliasing filter because is used to eliminate circuit noise in the frequency band above half of Nyquist of the sampling system. In this manner, these high-frequency noises, that would typically alias back into the signal path, are removed.

Figure 26: An active low-pass Sallen-Key Filter with Gain

This filter uses a Sallen-Key topology specially designed for high gain. The capacitor divider formed by C1A and C1B improve this filter’s sensitivity to component variations; the filter can be unproduceable without this improvement. R5 isolates op-amp’s output from the capacitive load formed by the series connection of C1A and C1B; it also improves performance at higher frequencies. The voltage at op-amp’s output can now be interfaced to an A/D converter to digitized the voltage which is proportional to the temperature

Therefore, a current generator excites the RTD sensor, then an op amp is used to gain and filter the signal and then an A/D converter converts the voltage across the RTD to digital code to be further proceeded using a microcontroller.

In some cases RTD sensor wires (cabling) may be several ohms especially when wires are long and this additional resistance can contribute a significant error temperature measurement. The classical method of avoiding this problem has been the use of a wheatstone bridge. The bridge output voltage is an indirect indication of the RTD resistance. The bridge is constructed from the RTD with additional three resistors that have a zero temperature coefficient. A more advance bridge can also be developed using an op-amp (active bridge). Finally another lead compensation

circuit is shown in the figure below. This configuration cancels errors due to wire resistance and wire resistance drift over temperature. However, it worth to mention that it is not expected to face the wire compensation problem as the RTD sensor is expected to have a very short interfacing wire (50cm) to the analog signal conditioning circuits.

Figure 27: A wire resistance cancellation circuit used in 3-wire RTDs.

Another instrumentation method to interface an RTD sensor for precise measurements can be implemented using a high performance Delta-Sigma A/D converter and two resistors. This method is usually referred as ratiometric and can achieve up to 0.1°C accuracy.

The traditional instrumentation method described previously requires multiple resistors, capacitors and few operation amplifiers and special adjustments for low noise and stable system. The Delta-Sigma ADC solution (see figure below) provides a plug-and-play solution with minimum adjustment. The RTD is directly connected to the ADC and a single low tolerance resistor is used to bias the RTD from the ADC reference voltage and accurately measure temperature ratiometrically. A low drop out linear regulator (LDO) is used to provide a reference voltage. This solution uses a common reference voltage to bias the RTD and the ADC which provides a ratio-metric relation between the ADC resolution and the RTD temperature resolution. Only one biasing resistor, RA, is needed to set the measurement resolution ratio (shown in equation below).

12RTD A n

codeR Rcode−

= −

Where Code is the ADC output code, RA is the biasing resistor and n is the ADC number of bits (e.g. 22 bits with sign Delta Sigma ADC)

Figure 28: RTD Instrumentation Circuit using a high performance Delta-Sigma ADC

The key feature of a ratiometric measurement technique is that the temperature accuracy does not depend on an accurate reference voltage. The ADC reference voltage varies with respect to change in RTD resistance due to the voltage divider relation. This measurement maintains constant resolution and eliminates the need for a constant biasing current source or a voltage source, which can be costly. Resistance RA and RB must be sufficiently large to minimize error due to self-heat while providing adequate measurement resolution.

Flow The measurement of water flow is important to the system under development mainly due to two reasons. The first is that most water sensors provide more accurate results in the presence of flow and the second is that measurements can be taken only when flow exists and thus the water quality is evaluated in real time when the water is used. In addition, flow measurements in household applications can be utilized to measure water consumption or to suspect a pipe leakage.

A common type of flow sensor is the turbine sensor. The turbine sensor looks like a little fan or propeller, with the axis of the fan aligned with the direction of flow and the blades of the fan covering the entire cross-section of the flow path. Because the fan axis is in-line with the material flow, it can be difficult to measure the axial rotation directly, so turbine sensors often have windows that allow the blades to be observed from the side. By counting the number of times a blade passes a fixed point on the window in a given period of time and knowing both the density of the material and the cross-sectional area of the flow path, the sensor can determine the volume of fluid that has passed through the sensor. At first, monitoring the blade rotation might seem to be a daunting task, given the electrical isolation of the blade from the rest of the system. Fortunately, in many cases we can simply illuminate the window with a source that emits light at a frequency which the blade (but not the flowing material nor the sensor window) reflects well and then measure the reflected signal to get an output that rises and falls as the blade passes the point of illumination. Alternatively, Hall Effect turbine flow sensors are equipped with magnets on the rotor and a Hall-Effect sensor is used to detect the rotation of the rotor. A schematic of a turbine flow sensor is presented below. The characteristics of the turbine determine both the lower and the upper limits of flow that can be measured, since too low a flow will prevent the turbine from moving while too high a flow can physically deform the blades. Typical turbine-flow sensor families support measurement ranges of 0.5-200 liters per minute.

Figure 29: A schematic of a turbine flow sensor

Turbine flow sensors generate a pulse signal whose frequency is proportional to the flow. This signal is used to drive one of the microcontrollers Timer/Counter modules. This counter module counts the number of transitions (pulses) using the external pin as the counters clock and combined with a second timer module that has a resolution of the system-clock period (or using a real time clock interrupt), it is possible to measure the number of pulses per second. In other words, at each one second an interrupt occurs, the current counter value is stored and then counter is cleared to start the next count. The value of counter is therefore read once per second and represents the actual Pulses/Sec. Based on the specifications of the specific Turbine flow sensor it is possible to measure the water flow based on the Pulses/Sec value.

Water Electrodes Cleaning Mechanisms In line water sensors eliminate the transportation lag and sample deterioration problems associated with offline analysis; however they illustrate the need for efficient probe cleaning mechanisms to maintain reliable measurements. Several cleaning mechanisms have been proposed for the removal of various coatings. Probe sensors, either solid or membrane, require periodic cleaning. This can be done manually, by withdrawing the probe through an isolating valve; or automatically, using automatic probe-cleaning devices such as pressurized liquid jets, and mechanical (wipers or shutters), or ultrasonic cleaning and scraping devices. Conventionally, four types of automatic cleaners are ultrasonic, brush, water-jet, and chemical. These methods concentrate on the removal of coatings from the measurement bulb. Particles and material clogged in the porous reference junction are generally difficult to dislodge. The impedance of plugged reference junctions can get so high that it approaches an open circuit. For instance pH reading exceeds the scale indicating that probes need replacing or manual-chemical cleaning. In addition, recently several alternative cost effective methods have been proposed that can either actively remove fouling (e.g. electrolysis) or passively prevent fouling. For instance, copper-mesh prevents biofouling due to microorganisms [19, 21] and flat measuring surface probes eliminate deposits that can foul the electrodes. Cleaning mechanisms is an important cost parameter which can consume as high as 50% of the operational budgets.

Figure 30: Copper-mesh screens block organisms and provide biofouling protection

Figure 31: A self-cleaning electrode with flat measuring surface.

Flat surface electrodes The most cost effective self-cleaning method used for in-line water sensors is based on the mechanical package and design of the probe. These probes have flat measuring surface and gel-filled double junction reference half-cell. This design provides an extra barrier against reference side contamination. In the center of the measuring surface is a flat glass surface indicator electrode. This surface is surrounded by the flat porous reference junction. The large area of this porous junction has thousands of pores that provide excellent sample contact. This flat sensing surface virtually eliminates deposits that can foul the electrode and significantly reduces necessary maintenance. Also, it prolongs electrode life and virtually eliminates breakage. This simple, but effective system has no moving parts and requires no power. When the electrode's flat measuring surface is exposed to turbulent flow, the resulting scrubbing action provides a self-cleaning effect in most applications. For the typical spherical electrode, the downstream side is shielded from the flow; coating forms on this dead flow area, causing sluggish and drifting signals.

Figure 32: Self-Cleaning Operation of Flat Electrode Vs Spherical Electrode

Flat surface electrodes get adequate cleaning action at velocities of 1 to 2 fps. The addition of filters shifts the maintenance from the electrode to the filter. Usually, the filter must be changed more often than the electrode to be cleaned. This method is recommended in drinking water applications as is the most cost effective method and requires medium range water flows that are usually supported in the process (even in household applications, velocities of 1-2fps are easily supported by home water pressure boosting pumps).

Figure 33: A bottom view of a pH Flat Surface Electrode (courtesy SensoreX Corp).

Water-Jet Cleaners The water-jet cleaner directs a high-velocity water jet to the measurement bulb. The reading of the loop becomes erratic during washing. Therefore, the cycle timer that starts the jet should also freeze the pH reading and switch the pH controller to manual during the wash cycle and for 2 min or more after the wash period for electrode recovery. The water jet works well in removing materials that are easily dissolved in water. This method is recommended in drinking water applications but requires additional pressure pumps and valves to control the water-jet. Chemical Cleaners The chemical probe cleaning method uses a chemical jet, such as a dilute acid or base that is compatible with the process. A base is typically used for resins and an acid for crystalline precipitations (carbonates) and amorphous precipitations (hydroxides). A dilute hydrochloric acid solution is frequently used. Chemical cleaning tends to be the most effective method, but acid and base cleaners chemically attack the glass. In addition, cleaning cycles that are too frequent or too long can cause premature failure of the glass electrode. As with the water jet, the cycle timer must hold the last pH reading and suspend control action during the wash cycle. In water treatment applications, electrodes are often manually cleaned by soaking them in a dilute hydrochloric acid solution for several hours. Soaking electrodes for 1 min in a dilute solution of hydrofluoric acid in a nonglass container can reactivate electrodes that are sluggish or have too small a span or efficiency. The reactivation occurs by the hydrofluoric acid dissolving part of the aged gel layer. The electrode should then be soaked overnight in its normal storage solution (typically a 4-pH buffer). In drinking water applications manual chemical cleaning is an efficient method however requires maintenance personnel. Automatic chemical cleaning is not recommended due to health risks as well as chemical maintenance and control equipment cost.

Brush Cleaners The brush cleaner removes coatings by rotating a soft brush around the measurement bulb. The brush does not reach the reference junction. It has an adjustable height and a replaceable brush and can be electrically or pneumatically driven. Soft brushes are used for glass, and ceramic disks are used for antimony electrodes. Sticky materials can clog the brush and smear the bulb. Brush-wiper cleaners provide good cleaning results in drinking water application however wipers require accurate moving parts which cost multiple times the cost of the water probe. Ultrasonic Transduces Ultrasonic cleaners use ultrasonic waves to vibrate the liquid near electrode surfaces. Effectiveness depends on the vibration energy and fluid velocity past the electrodes. Heavy-duty electrodes are needed to withstand the ultrasonic energy. The ultrasonic cleaner works well in processes where fine particles and easily supersaturated sediments are formed or in suspension. It can move loose and light particles and oil deposits. Ultrasonic cleaners are sometimes not effective in applications where the coatings are difficult to remove. In addition ultrasonic transduces require large excitation energy and the method provide poor cleaning results in drinking water applications. Automatic Electrochemical Cleaning (Electrolysis) Electrolysis of water is the decomposition of water H2O into oxygen O2 and hydrogen gas H2. A DC electrical current source is connected to two electrodes (typically made from some inert metal such as platinum, gold or stainless steel) which are placed in the water. Hydrogen will appear at the cathode (the negatively charged electrode, where electrons enter the water), and oxygen will appear at the anode (the positively charged electrode). Electrolysis of pure water requires excess energy, however in drinking water applications electrolysis occurs due to the salts (electrolytes) diluted in drinking water and the limited self-ionization of water. The electrochemical cleaning (electrolysis) acts threefold: the generated gases hydrogen and oxygen blast away even persistent coatings, additionally oxygen oxidizes organic compounds and hydrogen reduces rust and manganese oxide and likewise destroys organic coatings. The produced gas volumes are small and unused gas molecules recombine automatically to the water they stem from. The use of electrolysis as an automatic sensor cleaning method has been patented in 2005 by Dr. A. Kuntze GmbH and is available for free chlorine and conductivity sensors. The self-cleaning procedure is as follows: A cleaning cycle lasts approx. 20 second and performed usually once a day. During the cleaning the measuring value is locked for five minutes to give the electrode time to polarize. Therefore by keeping the sensors clean from the beginning of the installation, there is no need for recalibration after the cleaning. This method is promising for drinking water applications as strongly reduces calibration and cleaning maintenance demands, however is applicable to limited types of sensors and requires additional (though) simple controllers to perform electrolysis.

Figure 34: Electrochemical automatic sensor cleaning (ASR) by electrolysis of water (courtesy Dr. A. Kuntze GmbH)

References 1. SensoreX Corp Technical

articles, http://www.sensorex.com/support/categories/category/technical_articles 2. Georg Fischer Signet Systems Technical articles, http://www.gfsignet.com/go/Support/Tech-

Tips 3. NexSens Technology Inc Technical articles, http://nexsens.com/knowledgebase/ 4. Fondriest Environmental, What is

Nitrate, http://www.fondriest.com/fileshare/pdf/Nitrate.pdf 5. Pulse Instruments Technical articles on ORP, http://www.pulseinstruments.net/orpmain.aspx 6. T. Suslow, Oxidation-Reduction Potential (ORP) for Water Disinfection Monitoring, Technical

Report, Agriculture and Natural Resources, University of California, 2004 7. Hybrid Turkeys, Oxidation Reduction Potential (ORP): A New Tool for Evaluating Water

Sanitation, 2010. 8. Jim Huntley and Dr. Vadim Malkov, Hach Company, Amperometric Probes or DPD Analyzers:

Which Is Best For On-Line Chlorine Monitoring? In Waterworld 2009 9. European Communities (Drinking Water)(No.2) Regulations 2007 10. World Health Organization Guidelines for drinking-water quality, third edition,2004 11. C.N.Haas, M.A. Meyer, and M.S. Paller, Microbial alterations in water distribution systems

and their relationship to physical-chemical characteristics, Journal of American Water Works Association , 1983

12. M.W LeChevallier, and W.D. Norton Examining Relationships Between Particle Counts and Giardia, Cryptosporidium, and Turbidity, Journal of American Water Works Association, 1992

13. Brumberger et al. Light Scattering, Science and Technology ,1968, page 38. 14. Lee SH, Levy DA, Craun GF, Beach MJ, and Calderon RL. Surveillance for water-borne disease

outbreaks-United States, 1999{2000, In CDC Surveillance Summaries, November 22, 2002. 15. D.M Lawler Turbidimetry and Nephelometry In Encyclopedia of Analytical Science, 1995,

Academic Press Ltd, UK. 16. Basic turbidimeter design and concepts. EPA. Turbidity Guidance Manual 1999

http://www.epa.gov/safewater/mdbp/pdf/turbidity/chap11.pdf 17. HACH Company. Portable Turbidimeter Model 2100P, Instrument and Procedure Manual. 18. M. Sadar Making Sense of Turbidity Measurements - Advantages in Establishing Traceability

Between Measurements and Technology. In 2004 National Monitoring Conference, Chattanooga, TN, USA.

19. YSI Inc, Resource library, http://www.ysi.com/resource-library.php 20. Aine Whelan and Fiona Regan, Antifouling strategies for marine and riverine sensors, Journal

of Environmental Monitoring., 2006 21. Alliance for Coastal Technologies. “Biofouling Prevention Technologies for Coastal

Sensors/Sensor Platforms”. Workshop Proceedings, 2003. No. ACT-03-05. 22. David H.F. Liu and Bela G. Liptak, Environmental Engineers' Handbook, Second Edition

Chapter 7. Wastewater Treatment 23. John G, Webster, The Measurement, Instrumentation and Sensors Handbook, 1999 24. Dunand, F. et al., Efficiency and benefits of a luminescent oxygen sensor to monitor dissolved

oxygen in power plants, API 2008. 25. Sun Lingfang et al, Design of Electrical Conductivity Instrument Based on Pulse-Width

Adjustable Bi-directional Pulsed Voltage," Computer Science and Information Engineering, World Congress on Computer Science and Information Engineering 2009