What is Thermocouples? Standard Thermocouples

Thermocouple

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.

Standard Thermocouples

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.

                                                           TABLE

TYPE              COMPOSITION        RANGE          EMF                WIRE

                                                            (Deg C)           (mV)                GAUGE

B                     Pt-6% Rh vs.               0 to                  0 to                  24

                        Pt-30% Rh                  1820                13.81

R                     Pt vs    .                       -50 to               -0.226 to          24

                        Pt-13% Rh                  1768                21.10

S                      Pt vs.                           -50 to               -0.236 to          24

                        Pt-10% Rh                  1768                18.69

J                       Iron vs.                        -210 to             -8.096 to          8

                        constantan                   760                  42.92

K                     Chromel vs.                 -270 to             -6.458 to          8

                        Alumel                                    1372                54.87

T                      Copper vs.                   -270 to             -6.258 to          14

                        constantan                   400                  20.869

E                      Chromel                      -270 to             -9.835 to          8

                        constantan                   1000                76.358