Materials used for Thermocouples
For the formation of the thermocouple at least two metals should be joined together to form two junctions. The thermocouple cannot be formed if the two junctions are not formed. Both the junctions are maintained at different temperatures. One is at the known temperature and the other at the unknown temperature. There are several combinations of the metals that can be used to form the thermocouple. However, there certain metals and their combinations that work better than the other combinations and they have become standard for certain ranges of temperature and for specific applications.
Some of the elements used commonly for thermocouple are:
1) Copper as independent element
2) Iron as independent element
3) Platinum as independent element
4) Rhodium as an independent element
5) Iridium as an independent element
6) Constantan: combination of 60% copper and 40% nickel
7) Chromel: combination of 10% chromium, 90% nickel
8) Alumel: combination of 2% aluminum, 90% nickel and remainder silicon and manganese
Properties of Some Combinations of Materials Used for Thermocouples
Here some of the properties of commonly used combinations of the elements for thermocouples.
1) Copper – constantan: used for -300 to 650 F. Inexpensive, high emf output
2) Chromel – constantan: used for 0 to 1000 F. Highest emf output, good stability
3) Iron – constantan: used for 0 to 1500 F. Inexpensive, high emf output, iron oxidizes after 1500F
4) Chromel – alumel: used for 600 to 2000 F. Resistant to the temperature within the specified range
5) Platinum – 10% rhodium: used for 1300 to 2850 F. Expensive and gives low output, resistant to oxidation, stable, used only for high temp.
How to Choose the Thermocouple Elements
Here are some of the factors that should be considered while selecting the material combinations for thermocouples.
1) The combination of the materials should produce sufficiently high electromotive force (emf) when they are connected. This ensures that the emf can be measured easily and the value of the temperature can be obtained correctly.
2) The combination of materials should have high resolution or fast response time: This means for every degree change in the temperature of the junctions there is sufficient emf generated within the circuit so that it can be sensed and measured easily.
3) Higher stability: The properties of the combination should remains stable for the specified temperature range.
4) Resistant to oxidation: The materials used for the thermocouple should be resistant to the oxidation, this is especially very important for the materials that are used at high temperatures since they tend to oxidize at those temperatures.
For all the material combinations the size of the wire is very important. If higher temperatures are to be measured, the diameter of the wire should be more, but in that case time response to the temperature change increases. Hence some compromise will have to be made between the response of the thermocouple and its life.
Shapes of Thermocouples
Shapes of Thermocouples
Thermocouples are available in various forms and shapes. They are made of various diameters, length, sheath material, combinations of the materials mentioned above, lead wire lengths etc. The most commonly used shapes are beads and probes. The beads shaped thermocouples are highly inexpensive and have very fast response time. Probes are available in the market for measuring temperature in different applications like industrial, medical, scientific, food etc. The connectors used with the probes come with round pins, called standard connectors, or the flat pins, called as miniature connectors.
When selecting the thermocouple for any application, one should consider the range of the temperature to be measured, the response time required, accuracy and the surrounding environment. As per the existing conditions one can select proper materials combinations and proper shape of the thermocouple.
1) Book: Mechanical Measurements by Thomas G. Beckwith and N. Lewis Buck