WHAT IS CONDUCTIVITYAND HOW IS ITMEASURED?A Technical Handbook For Industry

By Joe Novak

Hach Company, 2003All rights reserved.

ContentsCHAPTER 1 INTRODUCTION

The Properties of Water ______________________________________________3Ion Product Constant of Water _________________________________________3

CHAPTER 2 BASIC THEORY OF CONDUCTIVITYWhat is Conductivity? ________________________________________________4Why is Conductivity of Interest? _______________________________________4Electrical Conductivity in Liquids ______________________________________4Polarization ________________________________________________________5Mhos versus Siemens _________________________________________________5The Conductivity Spectrum ____________________________________________5TDS Relationship to Conductivity and Resistivity__________________________6Conductivity/Concentration Relationships ________________________________6High Purity Water Measurements _______________________________________7pH versus Conductivity Measurements___________________________________7Effects of Temperature On Solution Conductivity__________________________8Temperature Compensation ____________________________________________8Non-linear Temperature Compensation __________________________________8Resistance Measurement of Conductivity_________________________________8

CHAPTER 3 CONDUCTIVITY SENSORSContacting Style Sensors _____________________________________________10Electrode Materials __________________________________________________11Non-contacting (Electrodeless) Style Sensors ____________________________11When To Use Contacting Versus Electrodeless Style Sensors________________12Sensor Installation __________________________________________________12Analyzers__________________________________________________________14Analyzer Installation ________________________________________________14Interconnect (Extension) Cable ________________________________________15Calibration_________________________________________________________15Dry Method Calibration ______________________________________________15Wet Method Calibration______________________________________________15Maintenance Schedule/Cleaning _______________________________________15

CHAPTER 4 COMMON APPLICATIONSDemineralizers/Deionizers ____________________________________________18Heat Exchanger Leakage _____________________________________________18Distilled Water _____________________________________________________18Chemical Peeling ___________________________________________________18Boiler Operation ____________________________________________________18Cooling Towers _____________________________________________________19Rinsing ___________________________________________________________19Semiconductors_____________________________________________________19Metals ____________________________________________________________19Acid Manufacture___________________________________________________20Scrubbing Towers ___________________________________________________20Pulp and Paper Making ______________________________________________20Desalinization ______________________________________________________20High Purity Water___________________________________________________20Reverse Osmosis ____________________________________________________20

BIBLIOGRAPHY _____________________________________________21

GLOSSARY ________________________________________________22

2

The Properties of WaterWater is the most common substance and the most impor-tant. In vapor, liquid, or solid form it covers more than sev-enty percent of the Earths surface and is a majorcomponent of the atmosphere. Water is also an essentialrequirement for all forms of life, most living things arelargely made up of water. Human beings, for example, areabout two-thirds water.

Pure water is a clear, colorless, and odorless liquid that ismade up of one oxygen and two hydrogen atoms. TheItalian scientist Stanislao Cannizzarro defined the chemicalformula of the water molecule, H2O, in 1860.

Water is a powerful substance that acts as a medium formany reactions, which is why it is often referred to as theuniversal solvent. Although pure water is a poor conduc-tor of electricity, natural impurities found in water cantransform it into a relatively good conductor. Water hasunusually high boiling (100C/212F) and freezing(0C/32F) points. It also shows unusual volume changeswith temperature. As water-cools, it contracts to a maxi-mum density of 1 gram per cubic centimeter at 4C (39F).Further cooling actually causes it to expand, especiallywhen it reaches the freezing point. The fact that water isdenser as a liquid than as a solid explains why an ice cubefloats in a beverage, or why a body of water freezes from thetop down. While the density property of water is relativelyunimportant for beverage cooling, it has a tremendousimpact on the survival of the aquatic life inhabiting a bodyof water.

The physical and chemical properties of water results inpart, from the strong attraction that hydrogen atoms havefor each other in water molecules. Because of this phenom-enon, the energy required to melt ice and boil liquid wateris greater than might otherwise be predicted.

Salts and contaminates in water disassociate into compo-nents called ions. In many cases, ions in water are calledimpurities (especially when referring to pure water), while inother aqueous solutions such as hydrochloric acid or sodiumhydroxide, the ions define the actual chemical composition.

Ion Product Constant of WaterWater molecules are in continuous motion, even at low tem-peratures. Occasionally, when two water molecules collide, ahydrogen ion is transferred from one molecule to the other(Figure 1). The water molecule that loses the hydrogen ionbecomes a negatively charged hydroxide ion. The watermolecule that gains the hydrogen ion becomes a positivelycharged hydronium ion. This process is commonly referredto as the self-iionization oof wwater.

Figure 1. Self Ionization of Water

The self-ionization of water is limited and can be written asa simple dissociation (Figure 2). In fact, at 25C in purewater, each concentration of hydrogen ions and hydroxideions is only 1 x 10-7 M. In theoretically pure water, theresult of this disassociation allows a minute electrical cur-rent to flow whose value is 0.056 microSiemens/cm (S/cm)or 18.21 megohms cm) at 25C. It is important to note thatthe amounts of hydrogen and hydroxide ions produced fromthis reaction are equal. This is why pure water is oftendescribed as a neutral ssolution.

Figure 2. Dissociation of Water

In all other aqueous solutions, the relative concentrations ofeach of these ions are unequal. In fact, when more of oneion is added to the solution, the concentration of the otherdecreases. The following equation describes this relationship:

[H+] [OH-] = 1 x 10-14 (mol/L)2 = KW

The product of the hydrogen and hydroxide ions is alwaysequal to 1 x 10-14 (mol/L)2. Therefore, if the concentrationof one ion increases by a factor of 10, then the concentra-tion of the other ion must decrease by a factor of 10. Sincethis relationship is constant, it is given the symbol KW,which is called the ion-pproduct cconstant ffor wwater.

3

Chapter 1 INTRODUCTION

What is Conductivity?Electrolytic conductivity is the measure of the ability of asolution to conduct an electric current and is sometimesreferred to as specific conductance. Electrolytic conduc-tivity is defined as the inverse or reciprocal of electricalresistance (ohms) and uses measurement units called mhoswhere one millionth of a mho equals one micromho.Resistivity as the inverse of conductivity is defined as themeasure of the ability of a solution to resist an electric cur-rent flow.

Water is a polar solvent. That is, the water molecule has anuneven distribution of electrons, causing one portion of themolecule to be somewhat positive, and another somewhatnegative. As a result, water molecules cannot carry a netcharge. Therefore, water is not an efficient conductor ofelectrical current unless impurities or dissolved substancesare present.

In an electric field, water molecules will gravitate towardboth electrodes (Figure 3). However, dissolving specific sub-stances to introduce impurities in water can greatly increaseits conductivity. These dissolved substances are calledelectrolytes. Electrolytes in water dissociate into positivelyand negatively charged ions that are free to move about thesolution. In a beaker containing a pair of electrodes con-nected to a DC voltage source, positively charged particleswill migrate toward the negative electrode (anode) and neg-atively charged particles will migrate toward the positiveelectrode (cathode). The migration of particles is the electriccurrent flow.

The conductivity measurement is directly affected by thenumber of dissolved ions in the solution and will increase asthe quantity and mobility of ions increases. The higher theconductivity reading, the better ability the solution has toconduct electricity. Conversely, the lower the conductivityreading, the poorer ability the solution has to conductelectricity.

Why is Conductivity of Interest?Conductivity is an indication of the quantity of ions con-tained in a solution. In ultrapure water, for example, minuteamounts of ions affecting the conductivity measurement byas little as 0.05 S/cm can produce unwanted deposits onplated parts, cause significant problems in manufacturingsemiconductors, and damage turbine components used inthe power industry. In the manufacture of sulfuric acid(approximately 100,000 S/cm for a 99% concentration of

H2SO4 at 25C), the large number of H2SO4 ions dissociatedin solution defines the concentration of the acid. The con-ductivity of the solutions used in these various applicationsmust be measured and kept within acceptable limits. Just astemperature is controlled for maximum process efficiencies,so too must conductivity be controlled.

Electrical Conductivity in LiquidsThe conduction of electricity through all forms of matter isnormally associated with the flow of electrons. However, themechanism of electrical conduction through a liquid is sig-nificantly different than electrical conduction through asolid (Figure 3). When a potential is applied to a solid con-ductor, the flow of current is virtually instantaneous, and isproportional to the applied potential. Additionally, differenttypes of materials conduct electrical charges with differentefficiencies. In metals, there are free electrons that are avail-able for conduction, even at fairly low temperatures. Onemajor characteristic of metals is that resistances increase asthe metal is heated because of the decrease in electronmobility. Conversely, the resistance of semiconductors andinsulators decreases with increasing temperature becausethe number of charge carriers increases. In semiconductors(and insulators), more energy is required to excite electronsto be available to conduct a charge.

In ionic compounds, entire ions may diffuse to conduct elec-tricity, though these have very low mobilities. Applying apotential to a liquid conductor causes current to be carriedthrough the solution by dissolved particles (ions) that haveelectrical charges. Dissolved ions move slower than elec-trons, depending on their geometry, the applied potential,and the temperature of the solution.

Figure 3. Method of Conduction in Liquid andSolid Conductors

4

Chapter 2 BASIC THEORY OF CONDUCTIVITY

Smaller ions generally move through a solution more rap-idly than larger ones. The hydrogen ion [H+] and thehydroxyl ion [OH-] are extremely mobile due to their geom-etry, as shown in Figure 4.

Figure 4. Size of Ions Relative to Each Other

Polarization When a DC voltage is applied to the electrodes of a conduc-tivity sensor, positive (H+) ions and negative (OH-) ionsmove to the positive and negative electrodes of the sensor(Figure 5). The ions gain or lose electrons, and are convertedto hydrogen gas at the negative electrode and oxygen gas atthe positive electrode. These gases effectively form an insu-lation barrier at the electrodes, increasing the apparentresistance that in turn decreases the apparent conductivityof the solution. Using an AC voltage, at 60 Hertz or higher,and increasing the cross sectional area of the electrodes, vir-tually eliminates the effects associated with polarization.However, there are practical limits to increasing the elec-trode surface area. Electrode size cannot become too largedue to the physical constraints of sensor installation.

Mhos versus SiemensThe electrical measurement of conductivity is the inverse(reciprocal) of ohms (1/ohms) or mhos. In the 1960s, theinternational committee for electrical measurement statedthat all measurements (electrical) will be named after theperson(s) who pioneered that measurement. In the 1800s,Dr. Siemens performed electrical experiments on the resis-tivity of water. Thus, the unit of measurement for conduc-tivity was changed from mhos to Siemens, and is exactly thesame measurement:

1/ohm = 1 mho = 1 Siemen

1000 micromhos/cm = 1000 microSiemens/cm (S/cm)

The Conductivity SpectrumIf all salts and free ions are removed from water, whatremains is referred to as theoretically ppure wwater. Theore-tically pure water has, at 25C, a specific conductance of0.056 S/cm and a resistivity of 18.21 megohms cm. Highquality condensed steam and distilled or demineralizedwater have specific conductances of 1.0 S/cm or less. Thisvalue is caused by about 0.5 ppm of dissolved salts. Table 1and Figure 6 (on the next page) show typical conductivityranges of various substances.

Table 1. Conductivity Ranges of CommonSolutions

Pure Water_____________________0.05 S/cmDemineralized Water _____________0.1 to 1.0 S/cmDistilled Water__________________1 to 10 S/cmTap Water _____________________100 to 1000 S/cmPolluted Water__________________1000 to 10,000 S/cmSea Water _____________________30,000 to 50,000 S/cm5% Sodium Chloride Solution ______70,000 S/cm10% Sulphuric Acid Solution _______140,000S/cm

5

Figure 5. Demonstration of Polarization

TDS Relationship to Conductivityand ResistivityTDS (Total Dissolved Solids) is the amount of solids dis-solved in a water sample. SS (Suspended Solids) is theamount of solids that have not dissolved into a water sam-ple, or solids that are insoluble in water. TS (Total Solids) isthe sum of TDS and SS. In laboratory analysis, measure-ments of these parameters are made by filtering and weigh-ing to determine SS, then drying and weighing to determineTDS.

In a process stream, TDS is commonly measured with a con-ductivity analyzer. However, this measurement is only anapproximation because it is based on a multiplication factor

of 0.4 to 0.75 times the raw conductivity value. The varia-tion is due to the type of dissolved solid(s) that are in thesample. Table 2 can be used to convert between conductiv-ity, resistivity, and TDS.

Conductivity/ConcentrationRelationshipsDifferent solutions with different conductivities, do notalways have directly proportional concentrations of salts orsolids in solution. In dilute solutions, an increase in conduc-tivity is linear with an increase in concentration of the solu-tion provided an electrolyte is dissolved in the solution andthere are no interactions between the solution and the dis-solved electrolyte. When these two conditions are met, the

6

Figure 6. Specific Conductances of Various Solutions

Table 2. Conductivity/Resistivity/TDS Conversions

Conductivity Resistivity Dissolved SSolids(S/cm) (ohms ccm) Parts/millionat 225C aat 225C (ppm)

Ultrapure Water0.056 18,000,000 0.0277 0.059 17,000,000 0.0294 0.063 16,000,000 0.0313 0.067 15,000,000 0.0333 0.072 14,000,000 0.0357 0.077 13,000,000 0.0384 0.084 12,000,000 0.0417 0.091 11,000,000 0.0455 0.100 10,000,000 0.0500

High Purity Water0.111 9,000,000 0.0556 0.125 8,000,000 0.0625 0.143 7,000,000 0.0714 0.167 6,000,000 0.0833 0.200 5,000,000 0.100 0.250 4,000,000 0.125 0.333 3,000,000 0.167 0.500 2,000,000 0.250 1.000 1,000,000 0.500

Conductivity Resistivity Dissolved SSolids(S/cm) (ohms ccm) Parts/millionat 225C aat 225C (ppm)

Pure Water1.110 900,000 0.556 1.250 800,000 0.625 1.430 700,000 0.714 1.670 600,000 0.833 2.000 500,000 1.000 2.500 400,000 1.250 3.330 300,000 1.670 5.000 200,000 2.500 10.00 100,000 5.000

dissolved electrolyte is said to be completely dissociated. Anexample is sodium chloride (NaCl). In solution, NaCl formsNa+ and Cl- ions which exist as separate entities and areindividually surrounded by water molecules. The water mol-ecules form a barrier around the ions. Being separate, theNa+ and Cl- ions are in a state capable of carrying current.

This not the case with all electrolytes. When Ammonia,(NH3), is dissolved in water, it reacts with the water to formAmmonium Ions, (NH4

+) and OH- Ions. (NH3+H2O > NH4+

+ OH-)

The attraction between the NH4+ and OH- ions is so great

that the barrier created by the surrounding water moleculescannot keep them all apart. The NH4

+ and OH- ions con-stantly break through the water molecule barrier to formammonium hydroxide (NH4OH). Consequently, the numberof ions available to carry electric current is quite small andthe resulting conductivity is lower than that of the com-pletely dissociated solution.

Plotting concentration versus conductivity curves graphi-cally portrays another important physical property of solu-tions. Sulfuric acid (H2SO4) is an example of this. It iscompletely dissociated in low concentrations and its con-ductivity is directly proportional to concentration. TheH2SO4 in the solution dissociates to form H

+ and HSO4-

ions. At a specific point of concentration, the attractionbetween the ions breaks down the weakened water moleculebarrier and the total number of ions in solution is at themaximum. At this point, the conductivity of the solutionreaches a maximum value. Any further increase in the con-centration of the solution results in more and more ionscoming together, or associating, producing fewer ions avail-able to conduct current. The curve of concentration versusconductivity (Figure 7) for sulfuric acid begins to dropwhich, in effect, results in two (or more) concentration val-ues for the same conductivity.

Figure 7. Concentration versus Conductivity forSulfuric Acid

High Purity Water Measurements High purity water is defined as water specially processed toreduce or eliminate suspended solids, dissolved solids,organic, ionic and microbiological contaminants; this termis commonly used in the power, microelectronics and phar-maceutical industry.

In a solution of sodium chloride (NaCl), the sodium andchloride ions disassociate, providing the vast majority ofions capable of carrying current. In this solution, only thecurrent carrying capability of the Na+ and Cl- ions aremeasured while the concentration of H+ and OH- ions in thewater are ignored because there concentration is only about1/10,000,000th equivalent weight per liter of solution. Forexample, in a 0.1% solution of NaCl in water, this wouldrepresent approximately 1/10,000,000th gram of H+, and20/10,000,000th grams of OH-, as compared to four gramsof Na+ and six grams of Cl-. As the Na+ and Cl- ions areremoved from the solution, a point is reached where the H+

and OH- ions constitute an appreciable percentage of theions present in the solution. At this point, the contributionof the H+ and OH- ions to the conductivity of the solutioncan no longer be disregarded. When the concentration of theH+ and OH- ions, and the Na+ and Cl- ions are equal, theconcentration of the conductivity solution value of H+ andOH- ions is four times greater than Na+ and Cl- ions.

pH versus ConductivityMeasurementsA frequently asked question when making water qualitymeasurements is whether conductivity or pH should bemeasured.

pH is specific to hydrogen ions it will not respond to otherions that may be present in solution. For example, if ironsulfate were dissolved in the sulfuric acid solution discussedabove, the measurement of pH, the acid concentration,would be unaffected. However, because conductivityresponds to all ions present, the measurement of conductiv-ity will change with changes in iron sulfate content, whilethe acid content remains constant. When determiningwhether to measure pH or conductivity, always considersolution concentrations and ion activity.

Example: Should pH or conductivity be used to measure theconcentration of sulfuric acid? This depends on the param-eters of the process. If the concentration of the acid is lessthan 0.5%, pH or conductivity measurement could be used.If the acid concentration is greater, conductivity measure-ment should be used. The limitation for using pH measure-ment is that the hydrogen sensitive measuring electrodebecomes saturated at high concentrations of hydrogen ions.Conductivity sensors are not hampered by high concentra-tions of hydrogen ions.

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Effects of Temperature onSolution ConductivityIn a solid, electrons move almost instantly upon the appli-cation of a voltage potential. In a solution, the electroncharge is carried by ions. Therefore, the applied potential,species and concentration of the ion, and temperature of thesolution determine the speed at which the charge is trans-ferred.

The resistance of a solid conductor has a positive tempera-ture coefficient. Resistance increases with increasing tem-perature. Solid conductors vary chemically and possessdifferent positive temperature coefficients.

In solutions, ion mobility increases with increasing temper-ature. This increase in ion mobility means that ions willcarry more electrons per unit of time, resulting in a decreasein solution resistance and a corresponding increase of con-ductivity which is a negative temperature coefficient.

The mobility of the dissociated ions depends on the temper-ature of the solution, and is not always linear because of thecontaminants in solution.

Temperature CompensationElectrical conduction in aqueous solution is related to thetemperature, solution concentration, and the specific con-ductivity of the ions in the solution (Figure 8). Temperaturecompensation corrects the conductivity measured at thesample temperature to what that value would be if the sam-ple was at a reference temperature (normally 25C).

Solution conductivity varies with ion concentration as wellas temperature and must be related to conductivity meas-urements at a reference temperature. The temperature coef-ficient (TC or temperature compensation factor) ofconductivity solutions in water is typically positive, linear,and ranges between 2% and 7% per C, depending on thenature and concentration of the conducting ions. Watersolutions typically have a compensation factor of 2% per C.An analyzer or meter that is properly temperature compen-sated will read conductivity corrected to the reference tem-perature and the temperature coefficient curve, regardless ofsample temperature.

For example, water with a TC of 2% per C (0.02) and500 S/cm at 25C, would actually read 510 S/cm at 26C,an increase of 10 S/cm, if the measurement was not prop-erly temperature compensated.

Information on specific temperature coefficients for mostchemicals can be found in published reference books andperiodicals.

The overall accuracy of a conductivity system is governedto a large extent by the accuracy of its temperature com-pensator rather than by any other single factor and, manytimes, by all other factors combined.

Figure 8. Temperature vs. Conductivity

Non-linear TemperatureCompensationIn high purity water (1-18.3 megohms cm), temperaturecompensation is non-linear. The conductivity versus tem-perature is not as easily defined. Special resistivity instru-ments are used to monitor high purity water and havepre-programmed electronics for handling the conductivityversus temperature equation.

Resistance Measurement ofConductivityMeasuring the resistance of the solution and calculating itsreciprocal determine conductance. The Whetstone Bridge(Figure 9), due to its inherent accuracy, is normally used tomake this resistance measurement. The conductivity sensorforms one section of the bridge (R1 and R2), and from thebalancing arm (R3 and Rs), the resistance of the solutionbeing measured can be calculated.

A practical means of providing temperature compensation isto introduce into the bridge circuit a resistive element (Rt)that will change with temperature at the same rate as thesolution being measured. This temperature compensator armof the Whetstone Bridge can be:

8

Manuala potentiometer calibrated in temperature.

Automatican RTD or thermistor in good thermal con-tact with the sample being measured.

Regardless of the type of device used to accomplish the tem-perature measurement, accurate compensation for tempera-ture changes requires the temperature coefficientcompensator to match that of the solution being measured,and that the temperature compensator be in direct contactwith the solution.

Figure 9. Whetstone Bridge

NOTE: Conductivity measurements are an AC resistancemeasurement.

9

Contacting Style SensorsContacting style conductivity sensors have their electrodes indirect contact with the solution being measured (Figure 10).

To understand contacting conductivity sensors and their con-struction, the concept of a cell cconstant (K) and a standard ccellrequires discussion.

A contacting style conductivity sensor with a cell constantof 1.0 has two electrodes, each one square centimeter inarea, spaced one centimeter apart. The volume betweenthese electrodes is one cubic centimeter (Figure 11).

Since the volume of measured solution is the area of theelectrode times the distance between the electrodes, themathematical relationship does not change if one dimensionincreases and the other decreases proportionally.

This configuration results in a one-centimeter cube of liquidbetween the electrodes, and is referred to as the StandardCell, where its two plates are the electrodes. By definition, astandard cell has a cell constant (K) of 1.0.

Generally, a standard cell is only used in laboratory condi-tions where flow and pressure are controlled to prevent thedimensions and geometry of the electrodes from varying.

Industrial contacting style conductivity sensors are manu-factured to be robust, and dimensionally stable, and areoffered in various mounting configurations to address manydifferent installation requirements.

Figure 11. Configuration of a standard cell

The cell constant (K) is determined by the formula:

K = L (distance between plates) / a (area of plates)

K = 1 cm / 1 cm2 = 1 cm-1

10

Chapter 3 CONDUCTIVITY SENSORS

Figure 10. Contacting Style Conductivity Sensors

Industrial contacting style conductivity sensors built in thesame production run under identical circumstances will notbe exactly dimensionally alike. Therefore, a method must beprovided to calibrate the cell constant of each sensor. Cellconstant calibration is accomplished by first measuring theresistance of a solution (Rs) with a reference ccell with aknown cell constant. Next, the resistance of the same solu-tion is measured with the sensor to be calibrated (Rc). Thesemeasurements must be made at the same temperature.

Consider a solution that measures 50 ohms with the refer-ence cell and 500 ohms with the uncalibrated sensor. Thecell constant (K) can then be calculated:

K = Rc/Rs

K = 500/50 = 10

If the solution measured 5 ohms in the uncalibrated sensor,its cell constant would be:

K = 5/50 = 0.1

The ratio Rc / Rs is defined as the cell constant (K) for themeasuring sensor. It can be used to convert readings madewith the previously uncalibrated sensor to specific conduc-tance using the following equations:

Specific Resistance = Measured Resistance/Cell Constant

Specific Conductance = (Cell Constant) x(Measured Conductance) = mho/cm or S/cm

The cell constant (K) also serves the very important purposeof determining the appropriate conductivity cell for thedesired measuring range. Electrolytic conductivity measuringinstruments have been designed for a specified resistancerange, for example, infinity to 100 ohms. The correspondingrange in S/cm would be 0 to 10,000 microSiemens.Remember:

1 S/cm corresponds to 1,000,000 ohms

1,000,000 S/cm corresponds to 1 ohm

The quotient obtained by dividing 1,000,000 by the resist-ance value yields a S/cm value. Example: 100 ohms =1,000,000/100 or 10,000 S/cm. Use the same procedure toconvert S/cm to ohms. Example: 200 S/cm =1,000,000/200 or 5,000 ohms.

Electrode MaterialsElectrode materials affect the conductive properties of con-tacting style conductivity sensors. In the past, platinum andgold, which exhibited high performance characteristics,were used as electrode materials. When aggressive solutions(concentrated acids and caustics) were measured, noble met-als that resisted oxidation were used to eliminate dissolvingof the electrode. Today, most electrodes are made of tita-nium, graphite, or stainless steel. These materials have goodperformance characteristics in addition to exhibiting goodcorrosive resistance in most applications.

Non-contacting (Electrodeless)Style SensorsConductivity measurements are often made in solutionswhich tend to coat, foul, or damage the surface of conven-tional contacting style conductivity sensors. When measur-ing solutions over 10,000 S/cm with electrode type sensors,large cell constants must be used. These sensors have smallelectrode surface areas and, consequently, are very suscep-tible to fouling and polarization which introduce measure-ment errors.

Non-contacting or electrodeless conductivity sensors donot have electrodes in direct contact with the solution beingmeasured (Figure 12). They are immune to the normal prob-lems associated with contacting style conductivity sensors.Electrodeless style conductivity sensors were specificallydeveloped to solve these problems.

Figure 12. Electrodeless Conductivity Sensors

Operation

The electrodeless style conductivity sensor operates byinducing an alternating current in a closed loop of solutionand measuring the magnitude of this current to determinethe conductivity of the solution (Figure 13 on the next page).

The electrodeless conductivity sensor contains a pair ofToroidal coils arranged so that the solution passes throughthe center of both coils, effectively forming a loop. TheseToroidal coils operate similarly to a transformer. Think ofeach Toroidal coil as an independent winding of a trans-former, and the solution being measured as the dielectricresistance which is converted to specific conductance. Thedielectric resistance varies with the amount of ions presentin the solution being measured because they serve toincrease the magnetic coupling between the coils.

The Toroidal coils and internal electronics are contained ina waterproof housing to isolate them from direct contactwith the solution being measured. the term electrodelessconductivity is derived from the fact that the solution neveractually contacts the torroids.

11

When to use Contacting versusElectrodeless SensorsThe measuring ranges of contacting and electrodeless styleconductivity sensors are almost identical. Contacting stylesensors typically measure from theoretically pure water(0.056 S/cm) to concentrated salt solutions in theSiemens/cm range. Electrodeless style sensors are somewhatlimited at the low end of the measuring range. Their usefulrange is from about 25 S/cm up to concentrated solutionsin the Siemens/cm range.

By knowing these inherent limitations, the only other fac-tors in deciding whether to use a contacting or electrodelessstyle sensor is its mechanical design, maintenance require-ments, and cost. In general, contacting style sensors shouldonly be used in clear fluids to preclude maintenance prob-lems associated with electrode coating, fouling, and physi-cal damage. Electrodeless style sensors perform morereliably in applications where coating, fouling and slurrieswould render a contacting style sensor inoperable.

Sensor InstallationBest performance comes from a conductivity system thathas its sensor mounted properly. Conductivity sensors areusually installed in-line (flow-thru) or immersed (submer-sion) in an open vessel.

In flow through applications the following should beadhered too. For contacting conductivity sensors, flowshould contact the tip end of the sensor and completelysurround the sensor before exiting. Electrodeless conductiv-ity sensors must have a flow path that flows through thecenter of the sensor and completely surrounds the sensor

before exiting. Flow rate in either case should be within themanufacturers specifications and held constant. In immer-sion applications, such as in a tank, care must be given thatthe sensor is mounts at least four inches from the tank wallto avoid any capacitance effect of the tank wall that willaffect the measurement. High quality cable should be usedfor wiring and have as few junctions as possible. The bestwiring method is direct connection of the sensor to the ana-lyzer, avoiding an interconnect cable and junction box. Theuse of splices is highly discouraged. The analyzer should bemounted and properly grounded according to the manufac-turers specifications.

Some common installations are shown in Figures 14through 18.

Figure 14. Typical Non-Contacting Sensor UnionMount Hardware

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Figure 13. Operation of Electrodeless Conductivity Sensors

Figure 15. Typical Non-Contacting ImmersionMounting Into Vessel

Figure 16. Typical Non-Contacting Gate ValveMounting

Figure 17. Typical Non-Contacting SanitaryMount Hardware

13

Figure 18. Typical Contacting Conductivity Sensor Installations

1 Insertion mounting 5 End of pipe immersion2 Insertion mounting 6 Non-metallic sensor, end of pipe immersion3 Non-metallic sensor, insertion mounting 7 Sanitary (CIP) flange mounting4 Boiler wall insertion mouting 8 Ball valve insertion for compression-style sensor

with extended sensor body

12

4

75

6

3

8

In general, the following guidelines apply to all conductiv-ity sensor installations:

1. Mount the sensor so that it is totally surrounded bythe solution being measured.

2. Mount the sensor so that air or sediment cannotcontact the sensor.

3. Keep the flow past the sensor constant and withinspecified levels.

4. Keep wiring clean and dry.

AnalyzersUntil the advent of the microprocessor-based conductivityanalyzer, many of the calculations (algorithms) necessaryfor accurate conductivity/resistivity measurement could notbe effectively implemented. The flexibility of the micro-processor enables the inclusion of multiple concentrationranges, linear and non-linear ranges, and for measuring sys-tems using two sensors, dedicated math functions for TDS

(Total Dissolved Solids), % passage or % rejection, and dif-ference, all within the same analyzer.

Also, separate alarms can be dedicated to a data point, setto a math function, or configured for diagnostic and or tem-perature alarms. Typically, resolution can be read to 3-1/2digits after the decimal point. Other microprocessor-basedanalyzer attributes include system diagnostics for the sensorand analyzer.

It is interesting to note that analog analyzers still have meritand can be advantageous. For specific applications onlyrequiring moderate accuracy, the generally lower-pricedanalog instruments may be the best choice.

Analyzer InstallationThe analyzer should be installed according to the manufac-turers specifications. Pay particular attention to grounding.Ground loops commonly cause havoc with conductivityreadings.

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Interconnect (Extension) CableSome installations may require the sensor to be located fur-ther from the analyzer than the standard sensor cablelength, typically 20 ft. (6 m). In these cases, use an inter-connect or extension cable recommended by the sensormanufacturer and a suitable junction box to protect thesplicing between the sensor and interconnect cables. Use ofother cables may introduce errors due to cable capacitanceeffect, noise intrusion, or line loss. In general, low conduc-tivity ranges necessitate shorter cable runs, while higherranges enable longer runs. Be sure to follow good wiringpractices and use as few termination points as possible. Ifcabling must be spliced, a soldered splice is better than acrimped splice. Make sure to protect any splices from mois-ture. Sealing the splice with a non-conductive type RTV orsilicone sealant is highly recommended.

Calibration of ConductivitySystemsThe old adage of an analyzer is only as good as its calibra-tion holds true for conductivity analyzers, and is especiallycritical when measuring high purity water (HPW). Wet cal-ibration to known standards is the accepted method for cal-ibrating most conductivity measurement systems. However,it is extremely difficult, if not impossible, to wet calibrateconductivity measurement systems used for HPW measure-ment. The difficulties in preparing qualified samples of HPWstandards without error are monumental.

Qualified HPW samples are unstable and costly to make.They are, however, available in small quantities as 5 S/cm,and 15 S/cm standards from NIST (National Institute ofStandards and Testing). Cost as of this writing is $360 for a500 ml sample. These standards are only good for one timeuse, and only under laboratory conditions. Therefore thismay not be the best choice for calibrating a system used tomeasure HPW conductivity.

A better method that some analyzers offer is a dry calibra-tion method in which samples are not required. This cali-bration is also known as the true cell constant (K)method. To perform this method, the true cell K not justthe nominal cell K must be known. The analyzer then usesthe entered true cell K in its measuring algorithm to cal-culate the conductivity value:

Conductivity = 106 x Kcell

Rcell [1 + S (C - 25)]

Where:Conductivity is in S/cm at 25CKcell = true cell constantRcell = resistance in ohmsS = the slope of the solution with temperature.C = temperature in degrees C.

CalibrationConductivity sensors and their associated temperature com-pensators require only a one-point calibration. It may beinconvenient or impossible for the operator to calibrate theconductivity sensor to the analyzer using known solutionsor temperature baths. This is especially true when operatingat conductivity or temperature points that are at theextremes of their ranges, or that would require unstable orhazardous solutions.

For these reasons, a dry calibration makes calibrating eas-ier and more accurate. The conductivity sensor is preciselymeasured at the factory for both conductivity slope andtemperature offset. The sensor is then labeled with the truecell K and temperature T factor for entry into the ana-lyzer. The assumption with dry calibration is that the char-acteristics of the sensor have not changed; the electrodes areclean and not deformed or eroded away. If the physicalcharacteristics of the sensor have not changed from its man-ufactured state, dry calibration is valid. This makes the drycalibration method viable for any conductivity range.

Dry Method CalibrationThis method is the most accurate calibration method andonly requires entry of the true cell K and true temperatureT factor of the sensor into the analyzer to complete sys-tem calibration. The analyzer then applies these valuesdirectly to the measuring algorithm.

Wet Method CalibrationThis method enables the analyzer to interpret the cell con-stant (K). The sensor is immersed into a sample of knownvalue. After the sensor and sample temperatures have equal-ized, the known sample value is entered into the analyzer.The analyzer then calculates the true cell K and uses thisvalue in the measuring algorithm.

This method is most useful when using electrodeless sensors,because their cell K is usually only a nominal cell K.Electrodeless sensors are typically used in applications withconductivity ranges in which stable calibration samples canbe readily made and will remain stable during the calibra-tion process.

Maintenance Schedule/CleaningConductivity sensors, like any other sensor installed in aprocess, will eventually succumb to contamination from theprocess. Regular cleaning of the conductivity sensor willassure long, reliable, and accurate service.

Conductivity sensors are typically made of durable, corro-sion-resistant materials. Other than cleaning to remove con-taminants, no other maintenance should be required.Careful examination of the electrodes should be performedto make sure they have not been chemically attacked,

15

eroded, or physically altered. A slow degradation of the sen-sor can be hard to spot until its too late.

Cleaning should be done with a combination of soaks andrinses in a solution of water and detergent. This usuallyremoves most contaminants. Persistent contaminants mayrequire soaking in weak acid or caustic solutions for briefperiods of time, followed by thorough rinsing with cleanwater to remove and neutralize any residual acid or caustic.

NOTE: Always follow all safety procedures when usingchemicals to clean conductivity sensors.

Do not use a brush or other abrasives to clean conductivitysensors. These can bend or abrade the cell parts (metal,graphite) of the sensor, altering its cell constant. Alwaysfollow the manufacturers recommended procedure andschedule.

For optimal performance of conductivity systems that aredry calibrated, it is recommended to have the manufac-turer periodically re-certify the true cell K and tempera-ture T factor of the sensor. For optimal operation of aconductivity measuring system an annual re-certification isall that is required.

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A discussion of applications would not be complete withoutfirst considering advantages and disadvantages of measur-ing conductivity.

Conductivity offers a fast reliable means of measuring theionic content of a aqueous sample stream. The measurementcan be made in-line on a continuous basis in processes fromultrapure water to thick slurries. Accuracy, repeatability, andreliability of better than 1% of the measured range is easilyachieved. Todays robust sensors readily enable insertion inprocess temperatures above 250C at line pressures of300 psi.

The non-fouling ability of electrodeless conductivity sensorsare ideal for use in slurries such as those found in miningoperations, food processing, waste streams and chemicalproduction.

Conductivity measurements do have some disadvantages.Conductivity is a non-specific ion measurement. Conduc-tivity systems measure all ions in a sample in the aggregate(total). Also, usable measurements cannot be made in manyorganic compounds (alcohols, sugars, etc.), petroleum prod-ucts, or other non-ionic solutions.

Table 3 shows a list of industries and applications that arerepresentative of the many processes where conductivity isused.

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Chapter 4 COMMON APPLICATIONS

Table 3 Conductivity Applications

Industry Applications

Chemical Production Concentrations* Purity Leak Detection* (Heat Exchange) Cleaning*Scrubbers*Waste Streams Cooling Towers

Power Generation Steam Production* Condensate Return*Boiler Blow-down*Cooling Towers* Leak Detection* (Heat Exchangers) Demineralizers*Reverse Osmosis Cleaning*Concentration*Waste Streams Desalination*

Textiles Dye Vats* Bleaching*Washing* Rinsing*Waste Streams Cooling Towers* Concentration*

Iron & Steel Cooling Towers* Cleaning*Waste Streams Pickling Baths*

Metals Finishing Plating Rinsing* ConcentrationPickling BathCleaning*Waste Streams

Brewing & Beverage Cleaning* Rinsing*Concentration*

Industry Applications

Mining Leaching Concentration*Waste Streams Leak Detection* (Heat Exchanges)

Semiconductors Reverse Osmosis Deionization*Electro DeionizationConcentration*Waste Streams Rinsing*

Food Processing Tomato Paste PicklingWaste Streams Chemical Peeling*

Papermaking Concentration Black Liquor*BleachingBrown Stock* Waste Streams Cooling Towers*

Printing Fountain Control*

Water Production Reverse Osmosis Demineralization*Deionization*Electro DeionizationDesalination*Nano FiltrationDistilled Water*

Petroleum Drilling Mud Control Concentration*Waste Streams

Pharmaceuticals Reverse Osmosis Deionized Water* Concentration*Waste Streams

*These application are briefly described in the following pages.

Demineralizers/Deionizers

Two-bed Demineralizers

In a two-bed demineralizer, synthetic resins adsorb cationsand anions in the water as they pass through the cation andanion beds. A conductivity sensor located in the dischargeof the cation bed will signal resin depletion by a drop inconductivity. A conductivity sensor located in the dischargeof the anion bed will signal the depletion of resin in eitherbed by a rise in conductivity. In large demineralizers, addi-tional conductivity measuring systems are commonly usedto monitor the strength of the regenerate acid and caustic,and check the rinse before the demineralizer is put back inservice. A properly operating deionizer train will removebetween 98-99.9% of all ionic impurities. The final productwater will have a conductivity of 0.056 S/cm which is con-sidered theoretically pure.

Unlike aqueous solutions of higher conductivities (morethan 25 S/cm), high purity water (HPW) requires specialconsiderations for accurate conductivity measurements.Sensors must be constructed of materials that do not read-ily dissolve in HPW, which would add ions. The sensor mustnot have voids where air can become trapped, causingunstable readings. It must have a very fast acting and accu-rate ( 0.1C) temperature compensation device with itstrue cell K and T factor must be known. The analyzermust be microprocessor-based and include algorithms spe-cific to HPW measurement.

Mixed-bed Demineralizers

In a mixed-bed demineralizer, the anion and cation resinsare mixed. This has the effect of a number of two-bed dem-ineralizers in series and produces higher quality productwater. A conductivity sensor is used to monitor the dis-charge. A rise in conductivity signals depletion of eitherresin, similar to the two-bed system. Additional conductiv-ity measuring systems can be used to monitor the acid andcaustic regenerate solutions as well as the rinse-down.

Heat Exchanger LeakageProduction of many liquid products are heated by steam ineither plate or coil type heat exchangers. The steam con-densation from the exchanger is collected through a steamtrap and returned to a condensation tank. A break in thesteam coils can let potentially corrosive materials return tothe boiler. Many industrial chemicals applications flowingthrough heat exchangers contain conductive impurities.Conductivity measuring systems are used to provide analarm or actuate an automatic dump valve if leaks occurcausing high conductivity values. The same measurementsapply when heat exchangers are used for cooling hotsolutions.

Even if the product being manufactured does not containions, hydrocarbon products for example, the same conduc-tivity measurements are necessary. The resulting readings

will be quite different however. In this case, a leak wouldcause the conductivity value to drop because of the dilutionof the heating or cooling medium.

A much more effective approach to heat exchanger moni-toring is to use a two-sensor input analyzer, with sensorsinstalled on the inlet and outlet of the heat exchanger.Instead of reading the absolute conductivity of each sensor,the difference between them is monitored. If there is no leak,there will be no difference. If there is a leak, it will show upimmediately as a difference.

Distilled WaterThe purity of distilled water is determined by conductivity.Conductivity is measured at the discharge line of the still. Ifthere is a storage vessel for this water, this too can bemonitored.

Chemical PeelingIn canning processes, many fruits and vegetables such aspotatoes, onions, peaches, etc., are placed on a conveyorand run through a hot caustic soda bath to loosen theirskins. The strength of the caustic solution is monitored byplacing a conductivity sensor between the discharge of thebath and inlet of the circulating pump used to keep the con-centration equal throughout the bath. The bath eventuallybecomes contaminated with dirt, peel, and other foreignmaterial, and its viscosity increases. A change in conductiv-ity signals when the bath must be cleaned.

Boiler Operation

Boiler Blowdown

Dissolved solids concentrate in boiler water where steamingoccurs. Depending on the boiler design, proper boiler oper-ation requires that the solids concentration be maintainedbelow some limit. This is accomplished by blowing down(draining) a portion of the boiler water and maintaining thewater level by adding feedwater with a low solids content.Blowdown is normally measured as TDS (Total DissolvedSolids).

Condensate

Monitoring returned condensate is one of the largest uses ofconductivity measuring systems. Where steam is manufac-tured, the return condensate may be contaminated withacid, caustic, plating solution, brine, etc. These contaminatesare easily detectable by conductivity analyzers. In otherprocesses, the condensate may contain materials such assugar syrup, contaminated oils, etc. which are non-ionic butwhich contain impurities that are ionic. Conductivity meas-urement is a practical way to detect these impurities.

When electricity is generated by steam, a water-cooled con-denser is usually used after the turbine that reduces pressureand temperature to insure a liquid sample for making a con-

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ductivity measurement. Leakage of small amounts of thiscooling water into the high purity condensate, subsequentlyused as feedwater is a serious matter.

Several schemes have been devised to improve the sensitiv-ity for the detection of such a leak. The most successful ofthese has been the Larson-Lane Condensate Analyzer (U.S.Patent 2,832,673). A major limitation to the sensitivity ofany measurement is the presence of amines, usually addedto the boiler to maintain an alkaline atmosphere to inhibitcorrosion. The amines are soluble and conductive in waterand mask the conductivity changes when leakage occurs.The Larson-Lane Condensate Analyzer removes the interfer-ence of amines by passing the sample through a smallcation exchange bed. Not only are the amines removed inthis process, but also additionally, the salts in the leakageare converted to corresponding mineral acids. Because theacids are about three times more conductive than the salts,the resultant conductivity indication is far more sensitivethan the standard measurement. In many utility steam sta-tions, the Larson-Lane Analyzer is used not only on thecondensate discharge, but also on heater drains and deaer-ators.

Steam

Steam can be checked for carryover of boiler water solids byusing proper sample conditioning equipment and conduc-tivity measuring systems. Where steam is employed for itsheat content or where it is used to drive a reciprocatingsteam engine, sensitivities below 1 ppm TDS are not gener-ally required. In this case, a proper sampling system consistsof a suitably sized cooling coil and a throttling valve to dropthe temperature and pressure. The conductivity of the sam-ple is then monitored.

Where steam is used to operate a turbine, much more strin-gent requirements are placed on the steam generator. Smallamounts of solids will foul a turbine and reduce its effi-ciency. The use of a sample cooler to reduce steam temper-ature and pressure and provide a condensed sample is acommon practice at solids levels above 1 ppm TDS in thesteam. However, if turbine operation requires steam puritysomewhat better than this for prolonged service, more elab-orate sample conditioning is necessary.

The Larson-Lane Steam Analyzer (U.S. Patent 2,832,673) isa combination thermal and chemical analyzer which sub-stantially increases the sensitivity of the conductivity meas-urement. Not only dissolved solids, but also volatileconstituents such as carbon dioxide, ammonia and amines,contribute to the conductivity of the condensed steam.Degassing of the sample is a requisite for high sensitivityand accuracy in measuring conductivity as a means ofmeasuring solids in steam. Mechanical degassing is effectivein stripping carbon dioxide. The more soluble ammonia andamines are not appreciably removed. However, by precedinga reboiling stage with ion exchange through a cation resinbed, ammonia and amines are completely removed, and the

boiler water salts are converted to their respective acidswhich are readily detectable by a conductivity measure-ment. Bicarbonates are converted to carbonic acid which isdecomposed and driven off in the reboiling stage. Mineralacids formed from the salts have about three times moreconductivity than the salts. Therefore, the overall sensitivityto mineral carryover is enhanced, and is on the order of 100ppb TDS in the steam.

Cooling TowersWith growing emphasis on the conservation of water, cool-ing towers are prominent as a means of dissipating largeheat loads from chemical production, air conditioning sys-tems, and power generation. Because evaporative losses inthe cooling tower concentrate the solids in the circulatingwater, conductivity measurement is an effective way to con-trol Blowdown and make-up water.

RinsingRinsing of ionic solutions from a variety of materialsaccounts for a large use of conductivity instrumentation.Products like milk of magnesia, salt cake, titanium dioxide,paint pigments, and latex curd fall into this category.Measurement and control of the conductivity of the effluentin the washing of these materials helps to insure consistenthigh quality.

Adequate rinsing of metal and plastic in the electroplatingindustry has special significance. Not only is adequate rins-ing of corrosive cleaning and plating solutions from partsimportant in maintaining their finish quality, but alsoequally important is the reduction in the volume of waterbeing used. Continuous blow-down in this process effec-tively promotes adequate rinsing and consistent platingquality while minimizing water volume and waste volumethat must be treated.

SemiconductorsUltrapure water is an excellent solvent, leaving no residue.It is used throughout the semiconductor manufacturingprocess for rinsing wafers and other substrates. Monitoringthe quality of the deionized water used in these processes isa critical part of quality control programs for this industry.

MetalsCleaning with mixtures of caustic, polyphosphates, and othercompounds is used throughout the metal fabricating indus-try, starting with the mill and continuing through to the fin-ishing shop. Changes in the cleaning bath concentration arecaused by drag-in and drag-out as well as by dilution.Conductivity effectively monitors the concentration anddirectly controls additions of make-up water or chemicals.

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Acid ManufactureConductivity measuring systems are used in the manufac-ture of many primary industrial chemicals including sulfu-ric, nitric, hydrochloric, phosphoric, and hydrofluoric acids.Many of these measurements are made at high acid concen-trations, where conductivity will actually decrease with anincreasing acid concentration.

Scrubbing TowersThe composition and gradual depletion of the chemical con-tents of many industrial scrubbing towers are effectivelymonitored by the measurement of conductivity. For exam-ple, a 10% caustic soda solution is commonly used to scrubacid vapors from process gases and exhaust air. As hydroxylions in the caustic soda are replaced by chloride, cyanide,bicarbonate, etc., the conductivity of the solution decreases.This decrease is proportional to the degree of depletion ofthe original charge of caustic soda.

Pulp and PapermakingPulp and paper mills make varied use of conductivity instru-ments. In addition to the common uses such as detection ofcontamination of steam condensate, control of dilution ofliquid caustic, etc., two special uses are noteworthy.

Conductivity measurement of black liquor during the diges-tion cycle provides an indication of the cook, and the pre-diction of the kappa number. Following the cookingoperation, the digested fiber undergoes several stages ofblack liquor extraction, washing, and a final rinse in thebrown stock washer. Measuring the effluent conductivity ofthe brown stock washer is an effective means of determin-ing completeness of washing cycle. Because most pulp andpaper process have high solids and have species that aresticky electrodeless conductivity sensors are commonlyused to measure conductivity.

DesalinizationConversion of brackish and saline waters to potable water isbecoming increasingly important. Regardless of the conver-sion process (distillation, freezing, reverse osmosis, or ionexchange), conductivity measurements provide inexpensiveand reliable monitoring of the product quality.

High Purity WaterConductivity measurement is the prime measurement forimpurities in high purity water (HPW). By its very nature,HPW is difficult to produce and measure. It is unstable andvery corrosive. HPW is the universal solvent. One methodfor producing HPW involves using a reverse osmosis unitfollowed by mixed-bed deionization. This type of a systemwill typically produce theoretically pure water with a con-ductivity value of 0.056 S/cm.

Reverse OsmosisDuring reverse osmosis (RO), water is drawn first through afilter unit to remove suspended solids, and then sent on tothe RO unit. In the RO unit, pressure is applied to water onone side of a semi-permeable membrane, forcing the waterto diffuse through the membrane. Minerals, salts and col-loids are rejected, while purified water passes through. Totaldissolved solids (TDS) removal can be 90% or higher withconductivity readings below 5 S/cm.

The RO process functions well with waters that have highdissolved solids. In fact, RO units are often used to removethe majority of dissolved solids from water destined for anion exchange system. The product water from the reverseosmosis unit is then passed through a two-bed or mixed-beddeionization unit to remove the remaining ions that arepresent. This step is sometimes referred to as polishing thewater.

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BIBLIOGRAPHY

1. Beckman, 1958, Principles of Industrial Solution Conductivity Measurements, Beckman Instruments, Inc.

2. Leeds and Northrup, 1988, On Line Conductivity and Resistivity Measurements, Leeds and Northrup

3. Osmonics, 1999, Pure Water Handbook, Osmonics

4. Power Engineering, 1987, Guide to Water Treatment, Diversey Water Technologies

5. Thornton and Associates, 1998, Resistivity Measurement Techniques, Thornton and Associates

6. John C. Russ, 1996, Materials Science and Engineering, North Carolina State University, Raleigh NC

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Anion: A negatively charged ion in an aqueous solution.

Blowdown: In boiler operation, the purge of a small portionof concentrated boiler water from the system to maintain adesired level of dissolved and suspended solids in the sys-tem.

Cation: A positively charged ion in an aqueous solution.

Concentrate: In crossflow filtration, the portion of a feedstream which does not permeate the media, but retains theions, organics, and suspended particles which are rejectedby the media.

Concentration: The amount of material contained in a unitvolume of fluid; the process of increasing the dissolvedmaterial per unit volume.

Condensate: Water obtained through evaporation and subse-quent condensation. Normally the water resulting from con-densing plant steam originally generated in a boiler. (Watercondensed in a water still operation is usually called distil-late.)

Contaminate: A source of contamination, an impurity. Anysubstance in water which is not H2O.

Deionization ((DI): The process of removing ionized salts(theoretically up to 100%) from water through speciallymanufactured ion exchange resins. Deionization typicallydoes not remove organics, viruses or bacteria, exceptthrough accidental trapping in the resin, or when usingspecially made strong base anion resins which remove gramnegative bacteria.

Demineralization: The process of removing minerals fromwater, usually through deionization, reverse osmosis, or dis-tillation.

Dissolved SSolids: The residual material remaining after fil-tering the suspended material from water and evaporatingthe solution to a dry state.

Distillate: The product water from distillation formed bycondensing vapors.

Distillation: The process of condensing steam from boilingwater on a cool surface. Most contaminants do not vaporizeand, therefore, do not pass to the distillate. Distillationremoves nearly 100% of all impurities.

Effluent: The final output of liquids exiting a process.

Electrodialysis: Dialysis that is performed with the aid of anelectromotive force applied to electrodes adjacent to bothsides of the membrane.

High PPurity WWater ((HPW): Water specially processed toreduce or eliminate suspended solids, dissolved solids,organic, ionic and microbiological contaminants; this term

is commonly used in the power, microelectronics and phar-maceutical industry.

Ion: An atom or molecule that has lost or gained one ormore electrons, thereby acquiring a net electric charge.

Ion EExchange: A process in which ions are preferentiallyadsorbed from a solution for equivalently charged ionsattached to small solid structures (resin).

Membrane: A highly engineered polymer film containingcontrolled distributions of pores. Membranes serve as a bar-rier permitting the passage of materials only up to a certainsize, shape, or character. Membranes are used as the separa-tion mechanism in reverse osmosis, electrodialysis, ultrafil-tration, nanofiltration, and microfiltration, and as discfilters in laboratories, and as pleated final filter cartridges,particularly in pharmaceutical and electronic applications.

Mixed-bbed TTank: An ion exchange tank consisting of bothcation and anion resins mixed together. Provides the mostcomplete deionization of water, up to 18.3 megohms cmresistivity. Commonly used to polish water already treatedby two-bed ion exchange tanks or reverse osmosis.

Molecule: The smallest physical unit of a substance, com-posed of one or more atoms, that retains the properties ofthat substance.

Osmosis: The spontaneous flow of water from a less con-centrated solution to a more concentrated solution througha semi-permeable membrane until energy equilibrium isachieved.

Osmotic PPressure: The measurement of the potential energydifference between solutions on either side of a semi-permeable membrane. A factor in designing reverse osmosisequipment. The applied pressure must first overcome theosmotic pressure inherent in the chemical solution to attaingood purification and flux.

pH: An expression of hydrogen ion concentration, specifi-cally the negative logarithm of the hydrogen ion concentra-tion. The pH measurement range is 0-14, with 7 as neutral,0-7 as acid, and 7-14 as alkaline (base).

ppb: An acronym for parts per billion, commonly consid-ered equivalent to micrograms per liter (g/L).

ppm: An acronym for parts per million, commonly consid-ered equivalent to milligrams per liter (mg/L).

ppt: An acronym for parts per trillion, commonly consideredequivalent to nanograms per liter (ng/L).

Precipitate: An insoluble product of a chemical reaction ofsoluble compounds in water.

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GLOSSARY

Precipitation: The process of producing a precipitate froman aqueous chemical reaction, usually a crystalline com-pound that grows in size to be settleable.

Reagent-ggrade WWater ((ASTM): Water that meets the stan-dards for reagent use promulgated by American Society ofTesting and Materials. Four grades, RI through RIV, havebeen established for specifically intended uses.

Regeneration: In ion exchange systems, the process of usingeither an acid, alkali, or salt solution to remove the accu-mulated cations or anions. The cation exchange resins takeon hydrogen ions and the anion exchange resins take onhydroxide ions to restore themselves to the original hydro-gen or hydroxide form when using strong acid and strongalkali solutions for the process.

Rejection: In crossflow membrane systems, the process ofretaining contaminants at the membrane that are largerthan the membrane pore sizes. In a membrane system,expressed as a percent of the total presence of those con-taminants.

Resin: Specially manufactured polymer beads used in ionexchange processes to remove dissolved salts from water.

Resistivity: In aqueous solutions, the property of a substanceto resist the flow of electricity; the measurement of thatresistance. the inverse of conductivity. Measured by a resis-tivity monitor, and described in ohms cm.

Reverse OOsmosis ((RO): The separation of one component ofa solution from another component by flowing the feedstream under pressure across a semi-permeable membrane.RO removes ionized salts, colloids, and organics down to150 molecular weight. May also be called hyperfiltration.

Saturation: The point at which a solution contains enoughof a dissolved solid, liquid, or gas so that no more will dis-solve into the solution at a given temperature and pressure.

Scaling: The build-up of precipitated salts on a surface, suchas pipes, tanks, or boiler condensate tubes.

Semi-ppermeable MMembrane: A membrane which allows asolvent such as water to pass through, while rejecting cer-tain dissolved or colloidal substances.

Sepralator: In crossflow membrane systems, a modular andreplaceable spiral-wound membrane cartridge or element.

Solutes: Matter dissolved in a solvent.

Suspended SSolids ((SS): Solid organic and inorganic particlesin a solution that are held in suspension.

TDS: An acronym for Total Dissolved Solids.

Two-bbed TTank: A pairing of cation and anion exchangetanks, typically operating in series. Best used for the deion-ization of relatively high volumes of water. Capable ofproduct water resistivity of up to 1 megohmcm.

Ultrapure WWater: Highly treated water of high resistivitywith no organics; usually used in the semiconductor andpharmaceutical industries.

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