Thermocouples: basic theory of these temperature-sensing devices

Feb. 1, 2002
Thermocouples

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.
figure 1
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.
figure 2

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.

Thermocouple Types
TypePositive lead material Negative lead material Useful application temp.
J
Iron
Constantan
200-1400 ºF
95-760 ºC
K
Chromel
Alumel
200-2300 ºF
95-1260 ºC
T
Copper
Constantan
-330-660 ºF
-200-350 ºC
B
Platinum 30%, Rhodium
Platinum 6% Rhodium
2500-3100 ºF
1370-1700 ºC
R
Platinum 13%, Rhodium
Platinum
1600-2640 ºF
870-1450 ºC
S
Platinum 10%, Rhodium
Platinum
1800-2640 ºF
980-1450 ºC

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.

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 degrees Celsius.
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
When looking at thermocouples, they can be checked with an ohmmeter. They 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 a thermocouple is to be replaced, 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 affect the performance of the system.

Thermocouple color codes
Type
Sheath Color
Negative Lead
Positive Lead
J
Black
Red
White
K
Yellow
Red
Yellow
T
Blue
Red
Blue
B
Grey
Red
Grey
R/S
Green
Red
Black