Thermocouples

The mutual type is not very satisfactory. The alternating current to be measured divides, part passing through the galvanometer and part through the load. Since the impedance of these two parallel paths varies with frequency, the current division also varies, and a shunting error is introduced. Furthermore, a reversal error exists, causing a different thermocouple output for each direction of current flow.

In the contact type of thermocouple the shunting error is negligible, and the reversal error is very small and may usually be neglected. The calibration may readily be made with direct current and is independent of frequency except at very high frequencies where skin effect alters the effective resistance and the stray capacitance shunts current out of the heater.

The separate-heater type has no reversal error and is especially suitable for high-frequency measurements where the capacitance of the galvanometer to ground would be objectionable. Maximum sensitivity is obtained by binding the thermocouple junction and the heater together with a bead of heat-conducting but electrical insulating material.

At low frequencies, such as over the audio range, about the only effect of inserting a thermocouple in a circuit is to add resistance to the circuit. At radio frequencies, however, the equivalent circuit¹⁹ for a separate-heater type thermocouple is as shown in Fig. 25. Because of the stray inductances and capacitances, a calibration made with direct current, or with low frequencies, does not hold at extremely high frequencies.

Figure 24. Illustrating three types of thermocouples and how they would be connected to measure the current taken by a load. The dissimilar metals forming the thermocouple are A and B. The heater is marked H.

For measuring an alternating current, the thermocouple is inserted in the circuit as in Fig. 24. For measuring an alternating voltage, the thermocouple and a series resistance are connected as in Fig. 26. The voltage between the line wires is E = I(R+r), where R is the protective or current-limiting resistor, r is the resistance of the thermocouple heater (this should be accurately measured) and I is the current obtained from the calibration curve. These arrangements take power from the source and accordingly alter circuit conditions. There are instances when thermocouples cannot be used and when vacuum-tube measuring devices must be employed.

Figure 25. The high-frequency equivalent circuit of a thermocouple of the separate-heater type. A and B are the thermocouple wires, and H is the heater. L1 and L2 are the inductances of the connecting leads and the heater. C1 is the capacitance between connecting leads. C2 is the capacitance between heater and thermocouple. C3 is the capacitance of the insulating bead between the thermo-junction and heater. Mutual inductance (not shown) exists between the heater and thermocouple. (Adapted from Reference 19.)