A thermocouple consists of a pair of different metal or alloys joined together at both ends. One end, measuring junction, is placed where temperature is to be measured. The two conductors extend out of the measurement area to the reference junction. An electromotive force (emp or mV) is produced which is function of temperature difference between the two junctions and material of the wires.
E = k (T1 – T2)
|Thermocouple working principle|
Since the mV generated by the thermocouple is the function of temperature difference between the two junctions, control or compensation of reference junction temperature is required.
Seven types of thermocouples (TC) have been given letter designation by ISA (Instrument Society of America). These are listed in table along with normal working temperature measurement range. A brief description of important thermocouples, which are extensively used in steel industries, is produced in the following paragraphs.
TYPE COMPOSITION RANGE EMF WIRE B Pt-6% Rh vs. 0 to 0 to 24 R Pt vs . -50 to -0.226 to 24 S Pt vs. -50 to -0.236 to 24 J Iron vs. -210 to -8.096 to 8 K Chromel vs. -270 to -6.458 to 8 T Copper vs. -270 to -6.258 to 14 E Chromel -270 to -9.835 to 8