Thermocouple Meters

Author: E.E. Kimberly

A sensitive D'Arsonval instrument connected to a thermocouple measures the current caused by heating of the couple junction. If the couple be heated by a small resistor, the meter may be calibrated in terms of current in the resistor. That is the principle of the thermocouple ammeter extensively used for measurement of high-frequency currents such as those used in radio communication. The voltage of a thermocouple is proportional to the difference between the temperature of its heated junction and the temperature of the other ends of the couple metals.

If no temperature compensation were provided,the accuracy of the instrument would decrease as the ambient temperature in which it is used changed from that at which it was calibrated. To avoid such errors, the ends of the heater wires are commonly connected to rather massive metal blocks to which are also connected the instrument leads. This is called "cold junction compensation" and helps to minimize temperature errors. A circuit for a thermocouple-type instrument is shown in Fig. 16-13.

Fig. 16-13. Circuit for Heater Thermocouple-Type Instrument With Temperature Compensation