What is thermocouple?
Thermocouples consist of two metallic wires of different materials, joined at one end. This junction constitutes the measurement point (hot junction). The other end is called a cold junction.
The heating of the measuring junction causes an electrical voltage, approximately proportional to the temperature. (Thermoelectric effect, Seebeck effect). This voltage (EMF electromotive force) is due to two factors: the different electrode density of the two materials and the difference in temperature between the hot spot and the cold spot.
This means that a thermocouple does not measure the absolute temperature but the differential temperature between:
T1 hot junction
T2 cold junction
Since electrical voltage measurement is usually performed at room temperature, the voltage value would indicate too low a temperature value since room temperature is subtracted. To maintain the value for the absolute temperature, “cold junction compensation” is applied.
In the past (in some calibration laboratories still today), this compensation was done by immersing the cold tip in an ice bath.
In modern instruments with thermocouple input (eg transmitters, portable meters or panel-mounted instruments etc.) an electronic cold junction compensation is incorporated.
Each metal has a specific electronegativity. (Electronegativity = tendency of atoms to attract or emit electrons)
To achieve maximum thermal stress, specific combinations of materials are applied to create thermocouples with vastly different electronegativities. These material combinations have certain limitations – due for example to the maximum application temperature.
The following standards define thermocouples:
IEC 60584-1: Thermocouples - Basic values of thermal voltages
IEC 60584-2: Thermocouples - Limit deviations of thermal stresses
IEC 60584-3: Thermocouples: Thermocouple cables and compensation cables
What types of thermocouples are there?
Thermocouples are available in different metal combinations or calibrations to suit different applications. The three most common are Type J, K, and T calibrations, of which the Type K thermocouple is the most popular due to its wide temperature range and low cost. The Type K thermocouple has a nickel-chromium positive lead and a nickel-aluminum negative lead.
There are type R, S, B, G, C and D calibrations for high temperature that offer performance up to 2320°C. These are made of precious metals (platinum/rhodium and tungsten/rhenium) and are therefore relatively expensive.
Each calibration has a different temperature range and working environment. Although thermocouple calibration dictates the temperature range, the maximum range is also limited by the diameter of the thermocouple wire. That is, a very thin thermocouple may not be able to reach the desired temperature range.
Our Maximum Temperature Guide for Thermocouples shows the maximum temperatures for each type of thermocouple and wire diameter. This guide also provides upper temperature limits for mineral-insulated thermocouple probes in common sheath diameters.
How to choose a type of thermocouple?
Because thermocouples measure over very wide temperature ranges and are relatively robust, thermocouples are widely used in industry. The following criteria are used in the selection of a thermocouple:
- Temperature range
- The chemical resistance of the thermocouple or sheath material
- Abrasion and vibration resistance
- Installation requirements (may need to be compatible with existing equipment; existing holes may determine probe diameter)
The thermocouple produces a voltage output that can be correlated to the temperature that the thermocouple is measuring. Documents in the box below provide the tolerances and temperature for a given type of thermocouple. Most documents also provide the thermocouple temperature range, error limits, and environmental conditions.