Professor Charlton S. InaoProfessor Mechatronics System

DesignDefence Engineering College

Bishoftu, Ethiopia

PE-4030Chapter 2/b Part two

Instructional Objectives To understand the working principle and applications of the following

sensors: 1. Liquid Flow Sensor

1.1 Orifice1.2 Turbine Meter

2. Level Sensor2.1 Floats 2.2 Differential Pressure

3. Temperature Sensor3.1 Liquid in Glass3.2 Bimetallic Strip3.3 Thermistors

3.4 Electrical Résistance Thermometers3.5 Thermocouples

4. Light Sensor • To practice how to select sensor based on industrial requirements.

Flow Sensors

1.1 OrificeThe orifice plate is simply a disc ,

with a central hole, which is placed in

the tube through which the fluid is

flowing

The pressure difference is measured

between a point equal to the diameter

of the tube upstream and a point

equal to half of the diameter

downstream. It does not work well

with the slurries. The accuracy is

typically about + 1.5% of full range

and is non-linear.

• The turbine flowmeter consists of a multi-bladed motor that is supported centrally in the pipe along which the flow occurs.The fluid rotates the motor , the angualr velocity being approximately proportional to the flow rate. The rate of the revolution of the rotor can be determined using a magnetic pick up which produces an induced emf pulse every time the rotor blade passes it as th e blades are made from magnetic material or have small magnets mounted at their tips.

1.2 Turbine Meter

The pulses are counted and so the number of revolutions of the rotor can be determined. The meter is expensive with a n accuracy of typically about + 0.3%

Level Sensors

A direct method of monitoring the level of liquid in a vessel is by monitoring the movement of the float. . The displacement of the float causes a lever arm to rotate and so move a slider across the potentiometer. The result is an output of voltage related to the height of the liquid.

2.1 Floats

2.0 Indirect Method 1. Monitoring of the weight of the

vessel by load cell Weight= AhρgNote: hρg = P

2. Measurement of pressure at some point in the liquid P= hρg

Float Swtich

2.2Differential PressureThe differential pressure cell determines the pressure difference between the liquid at the base of the vessel and atmospheric pressure, the vessel being open to atmospheric pressure.

The differential pressure cell monitors the difference in pressure between the vase of the vessel and the air or gas above the surface of the liquid.

Temperature Scales• Celsius(º C)- common SI unit of relative temp• K=C +273• Kelvin(K)-Standard SI unit of absolute

thermodynamic temperature• Fahrenheit-(º F)English unit of relative

temperature. T= 9/5C +32• Rankine(ºR) English system unit of absolute

thermodynamic temperature. R=F +460

Temperature Measurements

Temperature Sensors

• 3.1 Liquid in Glass-A simple non electrical temperature measuring

device which typically uses alcohol or mercury as the working fluid, which expands and contracts relative to the glass container. When making measurements in a liquid, the depth of immersion is important

Temperature Sensors• 3.2 Bi–Metallic StripAnother nonelectrical temperature

measuring device. I tis composed of two or more metal layers having different coefficient of thermal expansion. Since these layers are permanently bonded together, the structure will deform when temperature changes, due t to the difference in the thermal expansions of the two metal layers. The deflection can be related to the temperature of the strip.

The mechanical motion of the strip makes or breaks an electrical contact to turn a heating or cooling system On or OFF.

Temperature Sensors

• 3.3Resistance Temperature Detector(RTDs)RTD is constructed of metal wire wound around

a ceramic or glass core and hermetically sealed. The resistance of the metallic wire increases with temperature. The resistance Temperature relationship is approximated by the following linear expression:

R=Ro[1 +α(T-To)]

Where To=reference temperature Ro= resistance at the reference temperature α=calibration constant

The reference temperature is usually the ice point of the water(0º C).

The most commonly used metal in RTD is platinum, because of its high melting point, resistance to oxidation, predictable tem characteristics, and stable calibration values.

The operating range of typical platinum RTD is –220 deg centigrade to 750 deg centigrade.

3.4 Thermistor-is a semiconductor device whose resistance changes exponentially with temperature. Thermistors have much narrower operating ranges than RTDs.

Its resistance –temperature relationship is usually expressed in the form

R= Roe [β(1/T-1/To)]

Where To= reference temperature β =a calibration constant called the characteristic temperature of the material

Temperature Sensors

• 3.5 ThermocouplesTwo dissimilar metals in contact

form a thermoelectric junction occur in pairs, resulting in what is called thermocouple.This is known as Seebeck effect.The thermocouple voltage is directly proportional to the junction temperature difference

V= α(T1-T2)

Where α is called the Seebeck coefficient; T1 and T2 is the junction temperature of metals A and B.

Thermocouple Circuit

The Five Basic Law of Thermocouple

1. Law of leadwire temperatures. The thermoelectric voltage due to two junctions in a circuit consisting of two different conducting metals depends only on the junction temperature s T1 and T2. The temperature environment of the leads away from the junctions(T3,T4,T5) does not influence the measured voltage.

2. Law of intermediate leadwire metals. A third metal C introduced in the circuit constituting thermcocouple has no influence on the resulting voltage as long as the temperature of the two new junctions(A-C and C-A) are the same(T3=T4). A voltage measurement device that creates two new junctions can be inserted into the thermocouple circuit without altering the resulting voltage.

3.Law of Intermediate Junction Metals. If a third metal is introduced within the junction creating two new junctions(A-C and C-B) , the measured voltage will not be affected as long as the two new junctions are at the same temperature(T1=T3).If T1=/T3, the effective temperature at C is the average of the temperatures(T1 +T3)/2.

4.Law of Intermediate Temperatures. Junction pairs at T1 and T3 produce the same voltage as two sets of junction pairs spanning the same temperature range(T1 to T2 and T2 to T3), therefore V1/3=V1/2+ V2/3

The voltage resulting from measuring temperature T1 relative to T3, is the same as the sum of the voltages resulting from T1 relative to T2 and T2 relative to T3.

5. Law of the intermediate metals. The voltage produced by two metals A and B is the same as the sum of the voltages produced by each metal A and B relative to the third metal C.

VA/B=VA/C +VB/C

The results supports the use of standard reference metal(e.g . Platinum) be used a s a basis to calibrate all other metals.

Thermocouple Configuration

Thermocouple Data

Thermocouple

Thermocouple Type, Materials, Range, Sensitivity

Thermocouple Junction Temperature and Output voltage

Junction Temperature (C) Output Voltage (mV)

0 0

10 0.507

20 1.019

30 1.536

40 2.058

50 2.585

60 3.115

70 3.649

80 4.186

90 4.725

100 5.268

Light Sensors

Since distance is velocity multiplied by time, wavelength can be expressed as the velocity of electromagnetic waves multiplied by the time of one cycle of frequency f. Since the accepted speed of light is 186,000 miles per second or 300,000,000 meters per second, this is: ë(in meters) = 300,000,000 meters/sec × 1/f(in seconds) or, ë(in meters) = 300/f(in MHz) If visible light (white light) is passed through a prism, , the visible light separates into its color components.

The electromagnetic spectrum is divided into radio waves and light waves by frequency. Light waves are further divided by into infrared, visible, ultraviolet and X-rays. The spectrum is either expressed in frequency or wavelength. Wavelength is the distance that an electromagnetic wave travels through space in one cycle of its frequency.

The Electromagnetic Spectrum

The frequency of visible light is from 400 million megahertz to 750 million megahertz. The wavelength is from 750 nanometers (10−9) to 400 nanometers. Light sensors extend into the infrared frequency range below visible light and into the ultraviolet light frequency range above visible light. Cadmium sulfide sensors are most sensitive in the green light region of visible light, while solar cells and phototransistor sensors are most sensitive in the infrared region.

Light Sensors

Light sensor diodes make the resistance of the circuit decreases and the current increases as the light/illuminance increases, at constant voltage.

•Used in control of street lamp•Used in the automatic /digital camera•Used in the automotive and military industry

Selection of Sensors1. Identify the nature of the measurement

required• Variable to be measured• Nominal value• Range of Value• Accuracy required• The required speed of measurement• Reliability required• Environmental conditions

2. Identify the nature of the output required from the sensor, this determining the signal conditioning requirements in order to give suitable output signals from the measurement.

3. Identify the possible sensors, taking into account such factors as range, accuracy, linearity, speed of response, reliability, maintainability, life, power supply requirements, ruggedness, availability and cost.

4.Identify the signal conditioning requirements. Eg. Measurement of level of a corrosive acid in a vessel.

Using a load cell, which gives an electrical output, calibrated to the level, ie. When empty and when full.

The End