What is a thermocouple?
A thermocouple is an electrical circuit that can be used to measure temperature. It is called a thermo-“couple” because two wires of different metallurgy are connected in two places to make two electrical junctions. One of the two junctions is generally held at a reference, or cold temperature and one at a hotter or measurement temperature. The current generated in the electrical circuit between the two junctions is proportional to the difference in temperature between the two junctions. In real life, the second, or cold-junction, of a thermocouple circuit is generally included as part of the measuring device and the sensor that is referred to as a thermocouple is just a probe with wires made up of different materials. This leads to some confusion because people tend to think of a thermocouple as a single measurement point sensor. The relationship between the thermocouple output and the temperature is quite non linear. Different metallurgies produce different outputs and the do have a
• Thermocouple Types, Letter Designations, Construction The ISA in the USA started the standards on Thermocouple nomenclature that includes the letter conventions used to describe certain alloy wire pairs. This is now carried on around the World and ASTM Committee E20 on Temperature provides the standard (ASTM E 230) that is now the American National Standard for the letter designation, the calibration tables, the color coding and recommended use limits of the most common types of thermocouple wire pairs • Calibration Tables-Downloadable Courtesy of Pyromation in Fort Wayne, Indiana USA • Calibration Tables-The NIST Manual for ITS-90 Calibration Tables From the NIST website. • Recommended Use Limits and Tolerances Maximum temperatures by thermocopule type and construction and wire size, part of the information from the ASTM E230 Standard • Thermocouple Wire Color Codes: USA and Others Courtesy of ISE, Instrument Service & Equipment, Cleveland Ohio, USA.
A thermocouple is a temperature probe that uses two dissimilar metals join at two places. The voltage created when the metals are subjected to a temperature gradient is used to calculate the temperature. There are many different types of thermocouple, each designed for a specific temperature and sensitivity range.
Its definition depends on the application. For example, a thermocouple is also sort of a switch or control device which “couples” or turns on the switch so to speak when combustion has reached a certain temperature. The heat at which the circuit will close (turn the switch on) is controlled by the type of metals used and the density of the material. One common use is as a safety device in a modern gas appliance. A thermocouple is used as a safety device to make sure that there is a pilot flame or ignition source present before gas is allowed to pass into the burner. Once it heats up, circuit closes, energizing the contacts that allow the gas valve to open, and burner ignites.
