Thermocouples are used to sense the temperature variations. In tea industry most of the tea driers are using Iron-constantan thermocouples Fe-K (J) type.
When two dissimilar metals are joined and the jointed end is subjected to heat, it will develop mill volts and it is measured to identify the temperature associated to it. ( It is known as SEE BECK effect.)
Ferrous means iron (Fe), constantan (K) is an alloy of Copper and nickel .Generally Fe-K Thermocouples are used which can measure up to 650 °.Purity of the sensing element and the perfectness of the fusion joint (welding) are important to get proper accuracy in temperature sensing.To measure the temperature up to 950 °C Chromel –Alumel (K) type thermocouples are used. To connect thermocouple to the temperature meter compensating cable are to be used.It is important to use the same type of compensating cable as that type of thermocouple is very essential to get proper accuracy in temperature readings.
As per British standard POSITIVE (+) element should have a colour YELLOW and NEGETIVE (-) element should have BLUE and overall BLACK sheath.
( As per American and DIN standards POSITIVE element WHITE and NEGETIVE element RED with overall BLACK and in DIN standard POSITIVE (+) RED and NEGETIVE (-) BLUE and Overall BULE colour sheath)
RTD (PT-100) Resistance Temperature Detectors are used to measure low temperatures.
But due to three wire connection and sensing elements are fragile in nature tend to create frequent problems.