Thermocouple System

Thermocouple temperature indicators show the air temperatures in the heater duct of anti-icing systems and in the exhaust systems of jet engines.

A thermocouple is a junction or connection of two unlike metals; such a circuit has two junctions. When one of the junctions becomes hotter than the other, an electromotive force is produced in the circuit. By including a galvanometer in the circuit, this electromotive force can be measured. The hotter the high temperature junction (hot junction) becomes, the greater the electromotive force. By calibrating the galvanometer’s dial, in degrees of temperature, the galvanometer becomes a thermometer. The galvanometer contains the cold junction.

The thermocouple thermometer systems used in naval aircraft consist of a galvanometers indicator, a thermocouple or thermocouples, and thermocouple leads. Some thermocouples consist of a strip of copper and a strip of constantan pressed tightly together. Constantan is an alloy of copper and nickel. Other thermocouples consist of a strip of iron and a strip of constantan. Others may consist of a strip of Chromel and a strip of Alumel.

The hot junction of the thermocouple varies in shape, depending on its application. Two common types, gasket and rivet, are shown in Figure 6-39. In the gasket thermocouple, the rings of two dissimilar metals are pressed together, forming a spark plug gasket. Each lead that connects back to the galvanometers must be of the same metal as the thermocouple part to which it connects. For example, a copper wire connects to the copper ring, and a constantan wire connects to the constantan ring. Thermocouple leads are critical in makeup and length because the galvanometers are calibrated for a specific set of leads in the circuits.

Thermocouples: (A) gasket type; (B) rivet type.