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.

When engineers pick a thermocouple for a particular measuring application, they choose a thermocouple based on its temperature range, chemical resistance of the thermocouple or sheath material, abrasion and vibration resistance, and installation requirements. The table on page 14 illustrates the thermocouple types, the material the negative and positive leads are made of and useful application temperatures.

The two types of thermocouples commonly used in aviation applications are Type J and Type K.

Thermocouple types

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 coded red.

Type J thermocouples are usable from zero to 870 C 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.

Type K. Type K thermocouples are composed of a positive leg of Chromel® (90 percent nickel and 10 percent chromium) color coded yellow, and a negative leg of Alumel® (95 percent nickel, 2 percent aluminum, 2 percent manganese, and 1 percent silicon), color coded red. These are usable from -36 to 1,260 C.

There are differences that can change the amount of current produced by thermocouples in each type group. These are wire thickness and thermocouple length.

Wire thickness

Wire thickness affects the thermocouple temperature range and sensitivity. Thicker wire thermocouples have a longer life at higher operating temperatures, while thinner wires have a higher degree of sensitivity.

Thermocouple length

The length of the thermocouple is also taken into consideration. It needs to be long enough so that the effects of heat conduction from the measuring junction of the thermocouple do not affect the reference junction.

Troubleshooting thermocouples

When looking at thermocouples, they can be checked with an ohmmeter. Thermocouples should show low resistance. As a word of caution, indicators can be damaged by the voltage produced by a multi-meter. Ensure that the meter and the probe have been unhooked prior to checking for resistance.

Whenever you replace a thermocouple, it is extremely important to replace it with a thermocouple of the same style and composition as specified in the maintenance manual. Even a seemingly minor act of substituting a thermocouple with one of a different length or wire thickness can significantly affect the performance of the system.