Thermocouple Circuit Considerations


#1

A circuit of thermocouples is like any other circuit of electricity. There is one or more EMF sources that can be batteries, a generator, or the hot and cold junctions in this case. There is a load, the indicator, and to connect the circuit together there are electrical conductors that have resistance. As always, the current in this circuit is governed by the law of Ohm:

I = E/R

Galvanometer Instruments

Thermocouple galvanometer-type indicators with mechanical cold junction compensation are either designed to be used with a stated resistance external circuit (this resistance value is usually marked on the dial) or have an internal adjustable resistor.

In the latter case, the external circuit resistance shall not exceed a specified maximum value and the adjustable resistor shall be adjusted to give the specified total circuit value. Where
no internal resistance adjustment is provided, the instrument must be used together with an external ballast resistor see figure A.

When installing one of these instruments, the usual practice is to wind the ballast resistor on a small bobbin with constant wire. The constantan wire length is chosen to make up the total resistance required. The bobbin is integrated with one of the indicator terminals on some instruments. shown in figure B.

Potentiometric Instruments:

The thermocouple EMF is opposed to an equal and opposite potential from the potentiometer in a potentiometric device ; there is no current in the circuit and therefore the value of the circuit resistance is irrelevant.

Potentiometric indicators of thermocouples used to be quite common but are not found so frequently now. However, if, as is often the case, the thermocouple indicator is a strip chart recorder, a potentiometric instrument is almost certain

Electronic Instruments

The input circuit “seen” by the thermocouple is a high-impedance amplifier in modern electronic tools for thermocouple indication, whether analog or digital devices. Again, the thermocouple circuit has negligible current, and since the thermocouple circuit resistance is in the order of 100 ohms whereas the amplifier input is likely to be a megohm or more, the external circuit resistance effect is negligible.

Electronic instruments make it much more versatile for their designer to compensate for cold junctions.

The cold junction correction can be arranged after the input amplifier. This has the advantage that the voltage levels used can be in the order of several volts of amplitude instead of a few millivolts, making it easier to achieve a higher degree of compensation accuracy. This arrangement is shown in above figure.

Thermocouple input circuits are available as encapsulated electronic modules. These modules contain input amplifier and cold junction compensation