Thermocouples

Converting heat to electrical energy, thermocouples are an inexpensive way to measure a wide range of temperatures. Selecting the optimum metal composition of a thermocouple is based on application temperature, atmospheric pressure, necessary length, accuracy, precision and cost.

When replacing a thermocouple, it is absolutely essential that the replacement type exactly matches that of the measuring instrument. Thermocouples differentiate in the way they compute the voltage output curves.

It is of utter importance that the same thermocouple, and extension wire, be used from the sensing element to the measuring element.

Major errors can develop if this practice is not followed:

Wire size of the thermocouple – The wire size selected in the thermocouple sensor depends upon the application. Generally, a larger gauge wire will last longer in high temperature conditions. If sensitivity is the prime concern, then a smaller gauge wire should be used.

Length of thermocouple probe – The effect of heat radiation from the hot end of the thermocouple must be minimized; therefore the thermocouple probe must have sufficient length to counter. Unless there is sufficient immersion, the readings will be low. Thus, it is suggested that the thermocouple be immersed a minimum distance, equivalent to four times the outside diameter of a protection tube, or well.

Location of thermocouple – Thermocouples must be positioned in a way that they have a definite temperature relationship with the work load. Usually, the thermocouple is located between the work load and the heat source: and should be located approximately 1/3 the distance from the work load to the heat source.