How thermocouple wire is used to measure temperature

Put simply, a thermocouple is used as a device to measure temperature in a given application. Thermocouples are used in a wide range of industries, from the food industry to aeronautical and metals processing as a means of ensuring both workplace safety as well as control production.

A thermocouple is the sensor which consists of two dissimilar metals which have been joined together at the sensing end. There are a number different thermocouple types (J, K, R, N etc.). All have different combinations of metals/alloys providing different temperature ranges and responses to temperature changes

At the sensing end of the thermocouple, a change in temperature is detected and produces a millivolt value, which is relevant to the difference in temperature according to the different alloys used in the thermocouple. This gives the value of the change in temperature from the reference end to the sensing end.

Thermocouple wire is graded according to the purity of the alloys used in making the sensor. They can be broken down into 3 categories:

Thermocouple grade: This is the wire that is used in the thermocouple as well as in cables that can be used at elevated temperatures. Its ability to be used is only limited by the sheath temperature rating or cable insulation rating. This type of wire is highly accurate and more costly than extension wire.

Extension grade: Slightly lower grade.  This means the alloys are not as pure as the thermocouple grade wire. It is used as an ‘extension’ cable which starts from the sensor and joins to the control or measuring room/ device such as an RKC temperature controller. It is typically used where the cable runs though ambient temperature conditions that are less than 200^oC, or less than the cable insulation rating such as PVC. It is cost-effective due to the length that may be needed for various applications. Be cautious that this type of wire should never be used in place of thermocouple grade wire.

Compensating cable: This uses alloys that are different to the thermocouple alloys, but produce a millivolt signal that is equivalent to the actual thermocouple alloys over a limited range. You will find compensating cables installed where the alloys or metals in the thermocouple are expensive, making the compensating cables much more cost effective to use. You will often see compensating cables used for Type K installations, and will always be used for Noble or Precious Metal installations due to the high cost of the alloys used in these type of applications.

Wires are insulated and colour coded for easy identification. The most common colour code system in use in Australia is the USA ANSI M96.1. This is what Pyrosales uses for easy identification. To see the colour chart, click here.

The types of wire insulation that are available include: PVC, Nylon, Siicon Rubber, PTFE, FEP, Fibreglass and other high temperature Synthetic materials. The insulation is determined according to the application and the protection needed. It is important for wires to have insulation, to eliminate corrosion and to avoid the thermocouple wires shorting out.