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TEMPERATURE MEASUREMENT About This Chapter We are learning this chapter 6 hours for 16 marks. Temperature is the most important parameter required to be measured and control in many industrial applications. We are learning measurement of very small and very high temperature. The method used may be electrical or non-electrical or it may be contacting or contactless.

4.1 TEMPERATURE MEASUREMENT f^CTl Temperature is thermal state of body. It is the property, which tells us the degree of hotness or coldness measured along some temperature scale. Temperature is fundamental characteristics of thermal state of body. Thermal state of body determines whether desired chemical reaction will take place or not and whether physical condition of manufactured product is correct or not. Thus, measurement and control of temperature is essential in process plants and industries.

When body is heated or cooled, various primary effects result. One of these results may be used for measurement of temperature. The various effects may be : 1. Change in phase or physical state (e.g. Solid, liquid and vapour).

2. Change in physical dimensions : Metal expands on heating and contracts on cooling.

3. Variation in electrical properties : With increase in temperature resistance of wire increases.

4. Seebeck effect : When two dissimilar metals are joined together to form two junctions, and if two junctions are maintained at different temperatures, emf is generated.

5. Bimetallic strip when heated, it bends in direction of metal having low coefficient of expansion.

6. Change in intensity of total radiation emitted : If a body is heated, It starts emitting radiant energy. The amount of radiant energy emitted is proportional to its temperature.

7. Change in temperature also causes change in volume and pressure of gas.

4.1.1 Temprature S c a l es Temperature is degree of hotness or coldness. This hotness or coldness I indication of heat and is compared by giving some reference of unit i.e. temperal scale. Temperature scales makes it possible to measure, identify and compare heat level of various substances. This instrument used to measure temperatur the thermometer. The thermometer is provided with suitable temperature scale) measure the value of temperature. Commonly used temperature scales are : 1. Centigrade i.e. Celcius Scale (°C) : In centigrade scale freezing point and boiling point of water are used as points. The centigrade scale, abbreviated °C, assigns 0°C to the ice point 100°C to the steam point. The interval between steam point and ice poir divided into 100 equal parts. Each part is of 1°C. Boiling point of water °C °F °K °R 100 212. 373 672

Freezing point of water 0 32 273 492 Freezing point of -17.7 0 255.3 460 concentrated salt water solution Asolute zero -273 -459.7 0 0 Fig. 4.1 : Temperature scales Fahrenheit scale (°F) : In Fahrenheit scale ice point is taken as 32°F and steam point is taken as 21 The interval between steam point and ice point is divided into 180 equal Each part is of 1°F. Temperature as read on Fahrenheit ther%nometer (reconverted to centigrade values by 100 180 (°F - 32) Kelvin scale (°K) : Theoretically, there is a condition of no molecular motion, hence no The temperature is at this point is absolute 0 temperature i.e. lowest pc temperature. The Kelvin scale has 0 point at absolute zero temperature level, ice point is taken as 273°K and steam point as 373°K. Temperature as read can be converted to °C values by °K = °C + 273.

Measurement and Control 4 -3 Temperature Measurement 4. Rankine scale (°R) : In this scale ice point is taken as 492 °R and steam point as taken as 672 °R. The Rankine scale is sum times referred as absolute Fahrenheit scale.

4.2 CLASSIFICATION OF T E M P E R A T U R E MEASURING INSTRUMENTS Temperature measuring instruments may be classified as : 1. Glass thermometer : It may be mercury, alcohol or any other liquid filled.

2. Pressure thermometer : (a) Liquid filled : It is filled with liquid mercury, ethyl alcohol. (b) Vapour filled : Commonly filled vapours are methyl chloride, sulphur dioxide having non-uniform scale. (c) Gas filled : Commonly filled gas is nitrogen having fast response and linear scale.

3. Bimetallic thermometer : In this, differential expansion of two dissimilar metals is used.

4. Electrical resistance thermometer (RTD) : In this, temperature is measured by resistance of electrical wire.

5. Optical Pyrometer : It measures temperature by matching luminosity of the hot body with calibrated source.

6. Radiation Pyrometer : It measures temperature by absorbing radiation.

7. Therrnister : It measures temperature by relating change in resistance with change in temperature.

Table 4.1 : Devices suitable for different temperature range Name Temperature range

1. Glass thermometer : (a) Mercury filled (b) Mercury + Nitrogen filled (c) Alcohol filled - 38 to 760 °F - 38 to 1000 °F - 95 to 150 °F 2. Pressure thermometer : (a) Liquid filled (b) Gas filled (c) Vapour filled - 300 to 1000 °F - 450 to 1000 °F - 300 to 600 °F 3. Bimetallic thermometers - 100 to 1000 °F 4. Thermocouples - 300 to 2000 °F 5. Resistance thermometers - 400 to 1800 °F 6. Thermisters - 150 to 500 °F 7. Pyrometers : (a) Optical (b) Radiation More than 1400 °F More than 1000 °F 4 -4 Temperature Me; 4.3 NON-ELECTRICAL METHODS Non-electrical methods of temperature measurement are : 1. Expansion thermometers. (a) Expansion of solid. ' • Bimetallic thermometer. (b) Expansion of liquid.

• Liquid in glass thermometer. • • Liquid in metal thermometer. (c) Expansion of gas.

2. Filled system thermometers.

• Gas filled thermometer.

• Liquid filled thermometer.

• Mercury filled thermometer.

• Vapour pressure thermometer 3. Pyrometers.

4.4 BIMETALLIC THERMOMETER -Bimetallic strip in helix form -Bulb Fig. 4.2 : Bimetallic strip in helix form

Measurement and Control 4 -5 Temperature Measurement Two different materials having different coefficient of thermal expansion rigidly joint together, one on other to form a bimetallic strip. When bimetaflic strip is fixed at one end, and heated from free end, it bends in the direction of material having low thermal coefficient of expansion. The bending moment of free end is connected to the pointer, which moves over calibrated scale. Usually bimetallic strip is wound in the form of helix or in spiral form. Its one end is fastened permanently to outer casing to forrh stopper and other end is connected to pointer. A pointer moves over a circular dial as helix coils and uncoils with temperature variation.

A typical bimetallic, thermometer used between - 40°C to 320°C consist of Invar (36% Ni, 64% Fe) has low coefficient of expansion when joined to Ni-Mo alloy.

A thermal well is provided at its outer tubing made of brass, steel or stainless steel for protection against corrosion and breakage. , Bimetallic strip in spiral form Fig. 4.3 : Bimetallic strip in spiral form Advantages of Bimetallic Thermometer: 1. Rugged construction.

2. Simple and convenient design.

3. No maintenance is required.

4. It gives ± 1% accuracy.

5. Overloading can be tolerated.

6. Low cost.

Disadvantages of Bimetallic Thermometer : 1. Material is subjected to creep at high temperature. . 2. Speed of response is low.

Advantages of Bimetal Helix Thermometer : 1. Low cost 2. Tough, not easily broken.

3. Easily installed and maintained.

4. Wide temperature range.

5. Deflection is more, hence sensitivity is more.

[S-09] [S-09] Measurement and Control 4-6 Temperature 4.5 LIQUID IN GLASS THERMOMETER It consists of glass envelope filled in with liquid and calibrated indicati marked on it. The envelope comprises a thick walled glass tube with a capill and a spherical or cylindrical bulb filled with liquid. The two parts are fused and top of c a p i l l a ry tube is sealed. The size of capillary tube depends on : 1. Size of sensing bulb.

2. Liquid filled in it.

3. Temperature range of thermometer.

—Safety reservoir

-Stem -Scale Thermometric liquid Bulb Fig. 4.4 : Liquid in glass thermometer Advantages of Mercury : The change in temperature causes fluid toi and rise up t he stem. The volume enclosed in s t em above liquid may contain 1 or it may be filled with inert gas. The liquid filled in thermometer a thermometric fluid.

Table 4.2 : T h e different 1 thermometric fluids used Thermometric fluid Range 1. Mercury 2. Alcohol 3. Toluene 4. Methyl chloride -35°C to 510°C - 80°C to 70°C -80°C to 100°C 0°C to 50°C Generally liquid used is mercury.

Advantages of Mercury : 1. It has linear coefficient of expansion.

2. It does not s t i ck to glass surface.

3. It is clearly visible in transparent glass.

Measurement and Control 4 -7 Temperature Measurement Advantages of Liquid Filled Thermometer : 1. Simple to use and relatively cheaper.

2. No auxiliary power is required.

3. Wide range of measurement.

4. Good accuracy.

Disadvantages of Liquid Filled Thermometer : 1. They are fragile in nature.

2. Can not be adopted for automatic recording 3. Time lag in measurement.

PRESSURE GAUGE THERMOMETER Operation of pressure gauge thermometer is based upon the expansion of liquid, gas or vapour in the bulb of the thermometer. It consists of bulb containing a liquid, gas or vapour, which is immersed in environment. The bulb is connected by capillary tube to some pressure measuring device, this pressure measured is calibrated to indicate temperature.

Pressure Bulb or Sensor:

The sensing element is pressure bulb, which contains a thermometer fluid and is placed where temperature is measured. The bulb is made of cylindrical piece of metal tubing closed at one end and connected to capillary tubing at the other end.

The size of bulb used depends upon the type of fluid used, temperature span of system and the length of capillary tube used with it.

A capillary tube is used to connect the bulb with pressure measuring device. The length of capillary tube is made as short as possible in order to improve dynamic performance. The capillary tube is made of copper or steel for systems using fluids other than mercury. When mercury is used as the transmitting fluid, stainless steel capillary tube is used.

Pressure Measuring Device: Pressure measuring device may be bourdon tube, diaphragm or bellow. The displacement of this is amplified to operate a pointer over a stationary scale. Scale may be calibrated to give temperature.

Advantages of Pressure Gauge Thermometer: 1. They are most economical and versatile for measurement of temperature.

2. They are rugged in construction and little chance of damage. ' 3. Remote indication up to (50 m is possible.

4. Accuracy is ± 1%.

4.7 ELECTRICAL METHODS There are three types of electrical method based temperature measuring instruments, which is listed as, PLATINUM RESISTANCE THERMOMETER (I Resistance Temperature Detector (RTD) or Resistance thermometer measures temperature using platinum, nickel or copper wire whose resistance changes with change in temperature.

Resistance thermometer works on the principle of positive temperature coefficient of resistance i.e. as temperature increases, resistance offered by thermometer also increases. The resistance of wire at t°C is given by, Where, Rt = Resistance at t°C. R0 = Resistance at 0°C. a0 = Resistance temperature coefficient t t = Change in temperature.

Fig. shows the modern platinum resistance thermometer. It consists of pure, well-annealed platinum wire wound on thin strip of insulating mica or ceramic, and placed in porcelain sheath.

Free ends of platinum wire are attached to long lead of low resistance copper wires.

To measure the change in resistance, bridge network is used. The resistance thermometer is connected to one of the arm of Wheatstone bridge circuit. When resistance thermometer is subjected to temperature variation, the Wheatstone bridge gets unbalanced. The galvanometer deflection can be directly calibrated to get temperature.

The unknown temperature T is given as

Where, RT = Resistance of wire at temperature t.

R0 = Resistance of wire at 0°C. R1oo = Resistance of wire at 100°C. Platinum is especially suited for RTD as it withstand high temperature while maintaining excellent stability. Tungsten has a relatively high sensitivity, but it is used for high temperature applications as it is extremely brittle and difficult to work.

Copper is occasionally as economical alternative up to temperature 120°C. The material used for RTD should have following characteristics: The change in resistance of material per unit change in temperature should be as large as possible.

Material should have high value of resistivity so that minimum volume of material can be used.

Resistance of material should have continuous and stable relationship with temperature.

Thus, commonly used materials for resistance thermometer are: Advantages of Resistance Thermometer: 1. It can be used for wide range of temperature from -200°C to 1200°C. 2. It has linear characteristic.

3. It has ± 0.15% accuracy.

4. It has high sensitivity.

5. Installation is easy.

Disadvantages of Resistance Thermometer: 1. Response is slow.

2. Possibility of current leakage between resistance element and ground.

3. Balancing of the bridge takes time.

THERMISTER Thermistor is thermally sensitive variable resistor available in two types.

1. Negative Temperature Coefficient Thermistor: In this type of thermister, as temperature increases its resistance decreases. It is made of semiconducting materials. They are manufactured by sintering powder mixtures of metallic oxides such as manganese, nickel, cobalt, copper, iron or uranium. The size and configuration can be controlled so that rods, beads, discs or washer shape can be produced. The whole assembly of thermister may be coated with glass to give strength and protection.

Resistance variation with temperature is non-linear. The resistance variation is from few ohms to mega-ohms. The non-linearity of thermister can be corrected by connecting ordinary low resistance in parallel with it.

The size of bead type thermister is 0.25 to 0.50mm in diameter and disc type thermister is 1 inch in diameter Advantages of Thermister: 1. It is small and compact, having high sensitivity.

2. It has low specific heat; hence it does not take very much heat 3. It is physically strong and rugged.

4. Relatively high temperature coefficient of resistance.

5. It can be used for extremely low temperature measurement with great accuracy.

Disadvantages of Thermister: 1. It has non-linear characteristic.

2. It requires external energy source.

4.10 THERMOELECTRIC METHODS Thermoelectric method of temperature measurement works on Pielter effect and Thomson effect in which emf is generated across the two junctions of dissimilar metal wires and the magnitude of emf measured is directly proportional to change in temperature, e.g.

Thermocouple.

THERMOCOUPLES Working Principle: Thermocouple works on Seebeck effect. It states that 'when two dissimilar metal wires are joined together so as to form two junctions, if one junction is kept reference temperature and another junction is heated, it gives emf between two terminals of metal wires and the magnitude of this emf measured is directly proportional to change in temperature'.

Thermoelectric effect takes place in two ways: Peltier Effect: A potential difference always exists between two dissimilar metals in contact with each other.

Thomson Effect: A potential gradient exists even in a single conductor having temperature gradient.

In commercial instruments the thermocouple materials are so chosen that the 'Peltier' and Thomson' emf act in such a manner that the combined value is maximum and varies directly with temperature.

Law of Intermediate Temperature': The thermal emf produced when a circuit of two homogeneous metals exists between a first and second temperature, and thermal emf produced when the same circuit exists between the second and a third temperature, are algebraically equal to the thermal emf produced when the circuit exists between first and third temperature, and is the law of intermediate temperature.

Law of Intermediate Metals: In a circuit consisting of two dissimilar homogeneous metals having the junctions at different temperatures, the emf developed will not be affected when a third homogeneous metal is made a part of the circuit, provided the temperatures of its two junctions are the same.

Construction and Working: If junctions 'A' and 'B' are maintained at same temperature, no current will flow around the circuit since the emf in the circuit will be equal and opposite. If however 'A' is heated to a higher temperature than XB' then current will flow since the emf at one junction will be greater than the opposing emf at the other junction.

Fig. shows a thermocouple consisting of two wires, one iron, one constantan (i.e. copper nickel alloy), with milli voltmeter coupled to copper wire.

Fig. shows AB and AC as thermocouple wires. If junctions B and C are maintained at same temperature, the introduction of third wire BC will not affect the emf generated. Here A will be hot junction and B with C will form cold junction. Thermocouple wires AB and AC shown as iron

and constantan respectively can be made of various metals and alloys depending on operating temperatures. The wire BC is generally long compared to thermocouple wire and is made of copper. In practical thermocouple system a cold junction B and C may be at relatively high temperature due to environment.

Thermocouple Characteristic: When the temperatures at reference and hot junction are same then the reading shown by milli-voltmeter is zero. When the temperature of sensing junction increases, due to temperature difference it circulates current through dissimilar metals creating potential drop across it. This is measured by milli-voltmeter. Therefore, the reading shown by voltmeter is directly proportional to change in temperature.

The relationship between emf induced and change in temperature is given by

Where a, b and c are material constant, e is emf induced, t is change in temperature.

From above equation, we can say that for smaller values of temperature bt2 and ct2 are very small which are to be neglected. Therefore e = at which possess linear characteristics and for larger values of temperature it had non-linear characteristics.

Desirable Characteristics of Thermocouple Material: 1. The emf produced per degree of temperature change must be sufficient to facilitate detection and measurement.

2. Temperature-emf relationship should be linear.

3. It should maintain calibration without drift for longer period of time.

4. It should be highly resistant to oxidation, corrosion and contamination.

5. It should withstand high and fluctuating temperature.

Advantages of Thermocouple: 1. It does not require external excitation source.

2. Design is very simple.

3. More sensitive, it follows temperature change with small lag time and therefore suitable for rapid changing temperatures.

4. Cheaper than resistance thermometers.

Disadvantages of Thermocouple: 1. It has a non-linear characteristic at higher temperature.

2. Lower accuracy, hence cannot be used for precision work.

Thermo-emf Measurement: The output emf of thermocouple, as a result of difference between temperatures at measuring junction and reference junction can be measured by:

1. Measuring the output voltage directly with a permanent moving coil (PMMC) instrument. A milli-voltmeter is used since the output is the order mV. Measuring the output voltage with the help of a D.C. potentiometer Measuring the output voltage after amplifying it The output emf of thermocouple is in range of millivolt and it can be digital milivoltmeter.

Voltmeter is basically a current sensitive device and therefore the meter reading will be dependent on emf generated by thermocouple, including the resistance of the connecting wires. The complete system, consisting of thermocouple leads and the milivoltmeter may be calibrated directly to give a reasonable correct determination of temperature.

QUARTZ THERMOMETER Quartz crystal is used for measurement of temperature. The method of temperature measurement is based on the sensitivity of the resonant frequency of a quartz crystal to temperature change.

We get linear relationship when proper cut is given to quartz crystal. Hammond discovered a new orientation called the "LC" or linear cut which provides a temperature frequency relationship of 1000 Hz/째C over a temperature range of 40째C to 230째C. The relationship between the frequency f and temperature T is given by, Where A, B and C are constants and f0 is the resonance frequency at reference temperature T. In LC cut crystal, the coefficients B and C are negligible and resonance frequency and temperature are linear.

The commercial models for the instrument use electronic counters and readout for the frequency. Normal resonator frequency is 28 MHz and the sensor output is compared with a reference frequency of 28.208 MHz supplied by a reference oscillator. The frequency difference is detected and converted to pulses and passed on to an electronic counter, which provides digital display of the magnitude of the temperature.

Advantages of Quartz Thermometer: 1. Various types of probes are available.

2. Resolution upto 0.001째C can be obtained.3. Remote sensing upto 3000 m is possible.

4. Fast response.

PYROMETERS Pyrometry is the technique for measuring temperature without physical contact.

It depends upon the relationship between the temperature of a hot body and the electromagnetic radiation emitted by the body. When body is heated, it emits thermal energy known as ^radiation'. Pyrometry is the technique for determining body's temperature by measuring its electromagnetic radiation. Two types of pyrometer used are: 1. Radiation pyrometer.

2. Optical pyrometer.

Pyrometer is contactless method of temperature measurement. Operation of pyrometer is based on the measurement of radiant energy emitted by hot body. It consists of lens to focus

radiant energy at some point. If distance of object from the lens is more, radiant energy will be lost in atmosphere instead of concentrating at fixed point and accuracy of measurement will be decrease, therefore it is desirable to have short distance of object from lens.

Radiation Pyrometer Radiation Pyrometer Operation of Radiation Pyrometer is based on the measurement of energy emitted by hot body as index of its temperature. It operates according to 'Stefan Boltzmann law'.

The pyrometer head consists of metal housing containing a fused silica lens and a thermopile.

The lens end is inserted into a small opening in the furnace. Radiations from hot body enter the lens. The radiant energy is focused by moving lens on thermopile. Due to hot incident radiation on thermopile, it develops an emf. The developed emf is measured by milli-voltmeter, which can be calibrated to give temperature of hot body. The fused silica lens is used as it withstands the high temperature. To avoid the absorption and reflection of certain wavelength by lens, concave mirror is used as shown in Fig. It is used to measure temperature above 400째C. Advantages of Radiation Pyrometer: 1. It has fast speed of response.

2. It can measure the temperature higher than the thermocouple.

3. It can measure the temperature of moving objects like molten metal.

4. It is less affected by corrosive atmosphere.

Disadvantages of Radiation Pyrometer: 1. It has non-linear scale. 1 2. Emissivity of material affects the measurement.

Applications of Radiation Pyrometer: 1. It measures the temperature that is above the limit of thermocouple.

2. It measures the temperature in environment, which contaminates the thermocouple.

3. It measures temperature of moving object.

4. It is also used to measure average temperature of large surface Optical Pyrometer It is based on monochromatic radiation from hot body whose temperature is to be measured.

When radiation from heated body at high temperature falls within the visible region of electromagnetic spectrum, it has fixed colour. An image of radiating source is produced by lens and made coincide with filament of lamp. In optical pyrometer- wavelength of radiation accepted is restricted by colour filter and its brightness is compared with standard lamp.

The current through the lamp filament is made variable so that lamp intensity can be adjusted.

The filament is viewed through the eyepiece and filter. The current through the filament is so adjusted that filament and image are of equal brightness.

When brightness of source and image produced is same, we can say both temperatures are same.

However if the temperature of filament is higher than that required for equal brightness, filament become too bright as shown in Fig and if temperature of filament is lower, it becomes too-dark as shown in Fig. The intensity of light of any wavelength depends on temperature of radiating body and temperature of filament depends upon the current flow through the lamp, the optical pyrometer can be directly calibrated in terms of filament current. But filament current depends on resistance of the filament, therefore modern type of pyrometers are calibrated in terms of resistance of filament.

The optical pyrometer can measure temperature upto 1400째C but with the use of absorption of screen, range can be increased up to 3000째C. Optical pyrometer is used for measurement of temperature of furnace, molten metal and other heated material.

Advantages of Optical Pyrometer: 1. Optical pyrometer is portable.

2. It is light in weight and can be carried to any place.

3 It can be used to detect the temperature of moving object.

4. It is also used to detect the temperature of distant places, as it is contactless measuring device.

5. It measures the high temperature with accuracy.

Disadvantages of Optical Pyrometer: 1. It measures the temperature of only hot surfaces, which radiates energy.

2. More chances of human error while adjusting the image.

3. Device is very costly.