Thermocouples and Controls

A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference. Thermocouples are a widely used type of temperature sensor for measurement and control. They are in general inexpensive and interchangeable, can be supplied fitted with standard connectors and adaptors and can measure a wide range of temperatures. The main limitation is accuracy: system errors of less than one degree Celsius can be difficult to achieve although different types are available to compensate for this.

Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges. Properties such as resistance to corrosion may also be important when choosing a type of thermocouple. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by using compensating cables which are less costly than the materials used to make the sensor. Thermocouples are usually standardised against a reference temperature of 0 degrees Celsius. Temperature control instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements often referred to as PID control or fuzzy logic.

Thermocouples are widely used in science and industry; applications include temperature measurement for injection molding and extrusion machines, kilns and other industrial processes where temperature control and monitoring is essential. Common thermocouple types are J, K and T type.