Thermocouple Operation:

Thermocouples will cause an electric current to flow in the attached circuit while subjected to changes in temperature. An amount of current which will be generated is dependent on the temperature difference among the measurement and reference junction; the features of the two metals used; and the features of   the   attached   circuit. Figure described a simple thermocouple circuit.

Figure: Internal Construction of a Typical Thermocouple

Figure: Simple Thermocouple Circuit

Heating the measuring j u n c t i o n o f   t h e thermocouple generates a voltage that is greater than the voltage across the reference junction. The difference among the two voltages is proportional to the difference in temperature and could be measured on the voltmeter (in millivolts).  For ease of operator use, a few voltmeters are set up to read out directly within temperature by use of electronic circuity.

Other application provides only the millivolt readout.  In sequence to convert the millivolt reading to its corresponding temperature, you have to refer to tables like the one display in Figure.  These tables could be obtained from the thermocouple manufacturer, and they list the specific temperature corresponding to a series of millivolt readings.

FigureTemperature-vs-Voltage Reference Table