Thermocouple Reference Guide & FAQ

A thermocouple is a temperature sensor. It is comprised of two dissimilar metals, joined together at one end and connected at the other end to a read output device known as a thermocouple thermometer.

A thermocouple works based on the theory of the thermoelectric effect. Electricity and changes in temperature (heat or cold) are very closely related. Anyone with a notebook computer sitting on their lap will attest that electricity can produce heat, but most people are unaware of the inverse, that heat can produce electricity! It is upon this effect that a thermocouple operates. In 1821, Thomas Johann Seebeck discovered that when any metal conductor is subjected to a change in temperature, it will generate a voltage. Also discovered was the fact that each type of metal conductor produces a different voltage for the same change in temperature.

Measuring the voltage given off by a change in temperature of a metal conductor involves connecting another conductor to complete an electrical circuit. This new conductor will also experience the change in temperature and develop its own voltage, opposing the original. When two dissimilar metals are used each will generate a differing voltage and the difference between the two can be measured.

A thermocouple-based temperature measurement system consists of a thermocouple or probe and a thermocouple thermometer (read output device). Extension wire is often used if the thermocouple thermometer can not be placed close enough to the actual thermocouple being read.

A probe is a thermocouple covered with a sheath on the measuring end (hot junction) and usually contains an industry standard connector for connecting to a thermocouple thermometer on the other end (cold junction). Probes have slower response times but are more resistant to the environment and last longer.

There is no easy answer to this question. The decision must be based on factors in the application. In general,thermocouples are very inexpensive. The main limitation is accuracy. Accuracy of less than 1°C can be difficult to achieve.

Again there is no easy answer to this question. Application factors are considered in the final decision. Major factors are: Temperature Range, Temperature Accuracy, Response Time, Environment, and Cost. When using electrical devices additional factors may contribute to the decision, like Electrical Noise.

Although almost any two types of metal can be used to make a thermocouple, standards have been made in the industry for using the best suited combinations. Thermocouples are built in a variety of industry-standard materials. These materials combined are called thermocouple types. Each type has a different range of measurable temperature. These industry standards are listed in the table below: