Thermocouples: basic theory of these temperature-sensing devices

Basic theory of these temperature-sensing devices
By Joe Escobar

Thermocouples are used in many different temperature-sensing applications. However, despite their abundant use in aircraft systems, many mechanics are not aware of how these simple looking devices operate. This article will shed some light on the basic operational principles of thermocouples.

Theory
Thermocouples are based on the principle that when two dissimilar metals are joined, a predictable voltage will be generated that relates to the difference in temperature between the measuring junction and reference junction. A thermoelectric electro magnetic field (EMF) is generated within the thermocouple when the ends are maintained at different temperatures. The magnitude of the EMF is proportional, and is related to the temperature difference between the two points, not just the temperature of the measuring junction. The EMF generated can then be measured by a millivoltmeter or potentiometer incorporated into the circuit in order to determine temperature.

Electrostatic potential in metals
In a normal metal, a temperature difference between any two points in the metal will result in an electrostatic potential difference, as long as no electrical current is allowed to flow between the two points. This difference is proportional to the temperature difference between the two points (T1 and T2) as shown in Figure 1.

Thermocouples
In the previous example, an attached voltmeter is actually measuring the potential drop across all the metal between its terminals - including the strip of metal and the wires used to connect it to the voltmeter terminals. In order to counterbalance the added potential drop caused by the connecting wires, a thermocouple uses a strip of metal dissimilar to the first that is attached in series to the first so that the ends are also between the temperatures T1 and T2 (see Figure 2). The thermocouple consists of two dissimilar metal wires or semiconducting rods welded together at their ends. One of the two junctions, called the hot or measuring junction, is exposed to the temperature to be measured. The other junction, referred to as the cold or reference junction, is maintained at a known reference temperature.
The table on this page shows properties and construction of various thermocouples. In standard practice, the negative lead is color coded red. In addition, the negative lead is usually shorter than the positive lead, and the large pin on a thermocouple connector is the negative conductor.
Thermocouples are chosen for an application based on their temperature range, chemical resistance of the thermocouple or sheath material, abrasion and vibration resistance, and installation requirements. The two types common to aircraft are Type J and Type K.

Type J
Type J thermocouples are composed of a positive leg of iron and a negative leg of Constantan (45 percent nickel and 55 percent copper) wire. The iron lead is the positive (magnetic) lead and is color coded black. The Constantan lead is negative (non-magnetic) and is color coated red.
Type J thermocouples are usable from zero to 870 degrees Celsius for the largest wire sizes, although smaller wire sizes should operate in correspondingly lower temperatures. These are the recommended type thermocouples for use in reducing atmospheres.