+ Temperature measurements are amongst the most common and the most important measurements made in controlling industrial processes.
+ Changes that are commonly used to monitor temperature are the expansion or contraction of solids, liquids or gases, the change in electrical resistance of conductors, semiconductors and thermoelectric e.m.f.s. The control system which are used to measure the temperature is as follows
+ The most common electrical method of temperature measurement uses the thermocouples.
+ The basic principle of this is, if two different metals are joined together, a potentiometer difference occurs across the junction.
+ The potential difference depends on the metals used and the temperature of the junction.
+ When both junctions are at the same temperature, there is no net e.ni.f. But if there is a difference in temperature between the junction the e.m.f. will be produced.
+ This e.m.f. will depend upon the two metals and the temperature between the junctions. One junction is held at 0° C and the equation which is used to find out the e.m.f. is
Fig. 2.39. Thermocouple
+ There are three e.m.fs present in a thermoelectric circuit. In this the Seebeck e.m.f. is caused by the junction of dissimilar metals and the Pettier e.m.f. is caused by a current flow in the circuit, and the
Thomson e.m.f. which results from a temperature gradient in the materials.
+ It is observed that all thermocouple circuits must involve at least two junctions. In that one of the junctions senses the desired or unknown temperature.
+ This junction is called the hot or measuring junction. The other junction is usually maintained at a known fixed temperature and this junction is called the cold or reference junction.
+ If the temperature of the reference or cold junction is known, the temperature of the hot or the measuring junction can be calculated by using the thermoelectric properties of the materials.
+ If thermocouple circuit can have other metals in the circuit and they will have no effect on the thermoelectric e.m.f.
+ A thermocouple can be used with the reference junction at a temperature other than 0° C.
+ For that we assume a 0° C junction and the correction has to be applied using the law of intermediate temperatures.
The equation used in this is
Fig. 2.41. Law of intermediate temperature

+ Here to maintain the 0° C at one junction a compensation circuit is used to provide an e.m.f. which varies with the temperature of the cold junction.
+ When it is added to the thermocouple e.m.f. it will generate a combined e.m.f. This is shown in
Fig.2.42.
Fig. 2.42. Compensation thermocouple
+ In the above Fig.2.42, the wires from the measuring junction are screwed directly to an isothermal block terminal strip.
+ The temperature of the block is ambient temperature.
+ This reference temperature is measured by semiconductor sensor and compensation circuitry develops a voltage Ecomp which is combined with measuring junction and the net voltage across the voltmeter = T (Temperature being measured).
+ The isothermal block can accept many thermocouple pairs in multichannel instruments with microprocessor computing power since the T (reference junction sensor now sends its temperature data to the computer which computes the needed voltage correction for each thermocouple.
+ The thermocouples like E, J, K and T are relatively cheap and it has accuracies of about ± 3%.
+ The noble metal thermocouples are very high cost compared with this and it has accuracies of about
±1% better than the base metal thermocouples.
+ Thermocouples are used in applications ranging from measurement of room air temperature to that of a liquid metal bath. The problems which may be encountered are
1. Faulty reference junction,
2. Installation faults,
3. Junctions formed by users may involve excessive temperatures or faulty soldering techniques,
4. Gross errors can result due to wrong installation of thermocouple.
2. Resistance Temperature Detectors (RTDs)
+ Resistance temperature detectors (RTDs) or resistance thermometers are basic instruments for measurement of resistance.

+ The materials used for RTDs are Nickel, Iron, Platinum, Copper, Lead,
Tungsten, Mercury, Manganin, Silver, etc.
+ The resistance of most metals increases over a limited temperature range and the relationship between
Resistance and Temperature is shown below.
Fig. 2.43. Resistance temperature detector
+ The Resistance temperature detectors are simple, and resistive elements in the form of coils of wire and it is shown in the above Fig.2.44.
+ The equation which is used to find the linear relationship in RTD is
Fig. 2.44. RTD element
Constructional Details ofRTDs:
+ The platinum, nickel and copper in the form wire are the most commonly used materials in the RTDs.
+ Thin film platinum elements are often made by depositing the metal on a suitable substrate wire- wound elements involving a platinum wire held by a high temperature glass adhesive inside a ceramic tube.
+ This is shown in Fig.2.45.
Fig. 2.45. Construction Detail of RTD

Salient Features ofRTDs:
1. High degree of accuracy.
2. Resistance thermometer is interchangeable in a process without compensation or recalibration.
3. It is normally designed for fast response as well as accuracy to provide close control of processes.
+ Thermistor is a semiconductor device that has a negative temperature coefficient of resistance in contrast to positive coefficient displayed by most metals.
+ Thermistors are small pieces of material made from mixtures of metal oxides, such as Iron, cobalt, chromium, Nickel, and Manganese.
+ The shape of the materials is in terms of discs, beads and rods.
+ The thermistor is an extremely sensitive device because its resistance changes rapidly with temperature.
+ The resistance of conventional metal-oxide thermistors decreases in a very non-linear manner with an increase in temperature is shown in the Fig.2.46 below.
+ The change in resistance per degree change in temperature is considerably larger than that which occurs with metals.
Fig. 2.46. Thermistors
+ The simple series circuit for measurement of temperature using a thermistor and the variation of resistance with temperature for a typical thermistor is shown in the below Fig.2.47.
Fig. 2.47. Thermistor
+ The thermistor is an extremely sensitive device because its resistance changes rapidly with temperature.
+ Thermistors have many advantages when compared with other temperature sensors.
+ The main disadvantage is highly non-linear behaviour.

(a) Thermodiodes:
+ Thermodiode is widely used method for measuring temperature. When the temperature of doped semiconductors changes, the mobility of their charge carriers changes and this affects the rate at which electrons and holes can diffuse across ap-n junction.
1. Measurement of temperature,
2. Control of temperature,
3. Temperature compensation,
4. Measurement of thermal conductivity,
5. Measurement of power at high frequencies,
6. Measurement of composition of gases,
7. Providing time delay,
8. Vacuum measurements.
+ The difference in voltage and current through the junction is a function of the temperature. The equation which is used to find the I is
+ From the above equation the voltage ‘ V’ is proportional to the temperature on Kelvin scale and the potential difference measurement across a diode at constant current is used to measure the temperature.
(b) Transistor:
+ In Thermo transistor the voltage across the junction between the base and the emitter depends on the temperature.
+ A common method is use of two transistors with different collector current and finding the difference in the base-emitter voltages between them, and this difference is the measure of temperature.
Fig. 2.48. Transistor
+ The thermo transistors can be combined with circuit components on a single chip to give a temperature sensor.
+ This is shown in the above Fig.2.48.

Fig. 2.49. Bimetallic thermostat
+ A Bimetallic thermostat consists of two different metal strips bounded together and they cannot move relative to each other.
+ These metals have different coefficients of expansion and when the temperature changes the composite strips bends into a curved strip, with the higher coefficient metal on the outside of the curve.
+ The basic principle in this is all metals try to change their physical dimensions at different rates when subjected to same change in temperature.
+ This deformation may be used as a temperature- controlled switch, as in the simple thermostat.
+ The Fig.2.49 shows the Bimetallic thermostat which was commonly used with domestic heating systems.