Question

a) Describe the principal of operation of each of the following detecting elements

i) Resistance thermometer

ii) Thermo - couples

iii) Thermisters

b) State one application for each of the detecting elements referred to above

ANS

a)

i)The resistance thermometer works on the principal that electriacl resistance of a metal varies with  temperature . The sketch  shows resistance thermometer, utilising the wheatstone bridge principal.

ii)Thermo- couples work on the principal where two different metals are joined to form a closed circuit and exposed to heat at their different junctions, this will cause a current to flow which can be calibrated to measure voltage. See Sketch.

iii)Thermistors work on the principal that they use the change in resistance to measure temperature. The thermistor is a semi - conductor material, which can be formed into many shapes under pressure, depending on it's application.

b)One application for each of the described is;

Resistance thermometers can be used for furnace temperature detecting, aternator air temperature windings

Thermocuople can be used to measure exhaust gas temperature

Thermistors can be used to measure motor winding temperatures.