Thermocouples

Thermocouples are used in different temperature-sensing applications in turbine engines. However, despite their abundant use in aircraft systems, we are often not aware of how these seemingly simple devices actually work. This article will shed a little light on the basic operational principles of thermocouples.

Theory

Thermostats and thermocouples use dissimilar metals in their operation. Thermostats relate to expansion and contraction of metals. They are based on the difference in expansion rates of dissimilar metals that are joined together. The difference in expansion rates causes the combined metals to physically deflect one way or the other as temperature changes are noted.
Thermocouples also use dissimilar metals. Unlike thermostats, thermocouples don't rely on the physical expansion and contraction of metals. Instead, they 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 that is generated is proportional, and is related to the temperature difference between the two points, not just the temperature of the junction that is being measured. The EMF generated is then measured by a millivoltmeter or potentiometer incorporated into the circuit in order to determine temperature.

Electrostatic potential in metals

In a normal metals, if there is a temperature difference between any two points in the metal, there will be 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.

Thermocouples

If you consider electrostatic potential, if we attach a voltmeter to the thermocouple, the 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 different temperature measurement points.

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 page 16 shows color codes for different type 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.