All other thermocouple instruments are integral components to systems that use or generate heat, usually by gas.
There are many different types of thermocouples, including type K, which is by far the most common for general use, as well as type E, J and N, which are also universal, platinum types B, R and S that use platinum or platinum-rhodium alloy wires, as well as T, C and M, all of which use different metal alloys as temperature sensors and have varying temperature ranges. In general, thermocouple devices operate in temperatures ranging from -420º F all the way up to 3300º F, although a single thermocouple alone isn’t able to vary in temperature this much.
Thermocouple instruments are safety devices that prevent gas leaks, explosions, over heating and system damage in the petrochemical, oil and gas, water heating, pharmaceutical, food, brewing, power generation and metal fabrication industries. Furnaces, gas fireplaces, gas water heating systems and diesel engines also use thermocouples as heat sensors for temperature measurement of liquids or gases.
All thermocouple instruments are simple devices composed of only a few parts-two dissimilar metal alloy wires that are joined together at one end, insulating and protective tubing around the wires, and a thermostat, which attaches to the other end of the wires. Portable thermocouple instruments are battery powered and most likely have a digital readout. All thermocouples use the conjoined end wires as a sensor that is placed directly in the area to be measured or monitored. The heat generates electrical voltage that conducts its way up the two wires.
The voltage changes in proportion to the temperature, therefore the voltage indicates a temperature difference between the area at the sensor end of the wires and the room temperature thermostat attached to the opposite end of the conducting wires. That temperature difference is deduced from a measurement of the difference in thermoelectric potentials developed along the wires. If the thermocouple measures a temperature that is too high or low, it will signal an alert or system shut down. The most accurate thermocouple instruments are platinum or platinum-rhodium types R and S, but the most widely used are K because of its wide temperature range. Type K uses aluminum and nickel alloy for one wire and a chromium nickel alloy for the other, both of which are inexpensive metals.