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Friday, 26 July 2019

Bimetal Relay and Thermocouples Working Principle

Bimetal Relay Working Principle :

A thermostatic bimetal element is based on the theory that metals expand on heating and contract on cooling. Two strips of different metals with different co-efficients of expansion are welded together lengthwise and when heated it bends due to difference in co-efficient of linear expansion of the two metals and regains back it shape when the temperature is removed. Commonly used materials for bimetallic strips are :

(a) High co-efficient of expansion materials Nickel, Iron, Constantan.
(b) Low co-efficient of expansion materials Alloy of iron and nickel.

When two metals are joined together, it is called bi-metals. When these two metals have different co-efficient of linear expansions, it gives an advantage of using it in the field as a thermostat. On application of heat to those bi-metals, each of the metal in the bimetal expands depending upon its co-efficient of linear expansion and since they are fused together, the net result will be bending. The bimetal will bend towards the side of least co-efficient of linear expansion. This phenomenon is used in thermostat to cut off the supply when excessive current flows through bimetals. Figure shows a thermostat using bimetals.

Thermocouples Working Principle :

Thermocouples are used for the measurement of temperature. Depending on the range of temperature to be measured, proper materials are chosen for a thermocouple. When two wires of different metals are jointed at their ends and the junction ends are maintained at different temperatures then an e.m.f. exists across the junctions. The e.m.f. causes a current to flow from hot junction to cold junction upto a certain maximum temperature. If the temperature of hot junction is increased beyond the maximum value, then the current decreases and becomes zero at a particular temperature. If the temperature of the hot junction is still increased, then the current reverses and increases. Current produced in this way by heating the junctions of different metals are known as thermo electric current. The e.m.f. are called thermo e.m.f.

Thermo e.m.f. depends upon the types of metals or alloys and on the difference of temperature between the junctions. The e.m.f. produced by a thermo couple is very small and is measured by a sensitive moving coil multi voltmeter. The temperature of the hot junction, at which maximum current flows is a constant for a given couple, and is known as the neutral temperature for the couple.

The materials used for thermocouple should have the following properties

(a) The e.m.f produced should not change rapidly with time.
(b) Should have high resistance to corrosion and should not oxidise.
(c) The e.m.f. generated per degree should be large enough.
(d) The couple should be used through a broad range of temperatures.
(e) Cost should be less.

Table gives the list of thermocouples with the temperature range and e.m.f. at 500°C.

List of Thermocouples

Temperature range (°C)
e.m.f. at 500 ° C (mV)
Copper / Constantan
200 to 400
Iron / Constantan
0 to 900
Nickel Chromium / Nickel
0 to 1100
Platinum / Platinum rhodium
500 to 1400

The thermo electrical e.m.f. has an important application in certain measuring instruments. In Figure the cold junction A is maintained at constant temperature and the temperature of junction B is slowly increased, high resistance milli voltmeter indicates a definite potential difference for any given temperature of junction B.
Circuit for Thermocouple EMF

The instrument can be calibrated to read the junction B temperature directly and employed to measure the temperature of a hot body with which the junction B is placed in contact. This forms a very convenient method of measuring temperatures at remote points or in positions where ordinary thermometers cannot be introduced. The instrument may be joined upto the junctions by quite long conductors, provided they are of low resistance.

The same principle can be applied to the measurement of electric currents. The current to be measured is passed through a short length of resistance of wire or strip. A thermocouple is fixed permanently in contact with or in close proximity to it, and is connected to a high resistance multi voltmeter. The instrument gives a deflection depending on the temperature of resistance or strip, which in turn depends upon the current flowing and is calibrated to read directly in amperes. Such an instrument is termed as thermo ammeter. The thermo ammeters are of importance in measuring alternating currents of high frequency, for which the ordinary types of measuring instruments are quite ineffective.

When a number of thermocouples are connected in series or in parallel, it is called a thermopile. The series of circuit provide high sensitivity than one thermocouple. Some of the circuit diagram is given in Figure.
Thermocouple Circuit
The connection leads are commonly used to contact the measuring junction with the reference junction when the connections are greater than 6 metres. The extension lead should have better mechanical characteristics, lowest resistance and lower cost than that of the thermocouple wire. Material combination of thermocouples can be used upto a temperature of 1150°C.

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