Data Acquisition Web

Thermocouples are temperature sensors suitable for use with any make of instrument designed or programmed for use with the same type of thermocouple. 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 the reference junction (connection to the measuring device).

The selection of the optimum thermocouple type (metals used in their construction) is based on application temperature, atmosphere, required length of service, accuracy and cost. When a replacement thermocouple is required, it is of the utmost importance that the type of thermocouple type used in the replacement matches that of the measuring instrument. Different thermocouple types have very different voltage output curves. It is also required that thermocouple or thermocouple extension wire, of the proper type, be used all the way from the sensing element to the measuring element. Large errors can develop if this practice is not followed.

Most thermocouples (unless otherwise specified) are constructed using a metallic sheath and high purity mineral oxide insulation. This provides a rugged and bendable design that exceeds the temperature rating of non noble metal calibrations. Mineral oxide coated type thermocouples have a high insulation resistance and are suited for most process applications.

If you use thermocouples, you need to have some way to interpret their small and non-linear output voltages. There's lots of ways to do it, the simplest being a measurement on the output voltage and looking up the value in a table of millivolts dc versus temperature AND correcting for the cooler junction not being at 0 °C. The high end is to hook the thermocouples up to a modern readout display or a DAQ module plugged into a PC and read away!

However, it is not a fool proof business and there are many subtle things about thermocouples and their uses that have made well-intentioned engineers, who thought they understood them, look like fools. The hooking up and reading out are the easy parts of a measurement. The selection, installation details and the conditions of use play a big role in obtaining a measurement that is accurate and reliable. The devil is in the details and "simple" thermocouples have a lot of details!