When you want to measure temperature, use a thermocouple, a device consisting of two different metal wires joined together to form a junction. As this video mentions, when exposed to heat or cold, the thermocouple generates a tiny yet measurable voltage. A thermocouple feedthrough provides a hermetic seal for each wire as it passes through a sealed environment. Only where the two wires form a junction do the wires not have a feedthrough covering.
The scientist Thomas Seebeck first identified the effect of heat or cold on the differing wires, thus, we refer to the process as the Seebeck effect. Although nearly any two metals could work together, scientists have identified the metals that, when joined, produce the most accurate and linear response, as well as the best temperature response and corrosion resistance. The top combinations include:
– nickel and chrome to form Chromel
– copper and nickel to form constantan
– nickel, manganese, and aluminum to form Alumel.
Using metals from this short list results in thermocouples used in many common devices, such as home thermostats, oven controls, diesel engines, and kilns. Some of these applications use thermocouple vacuum feedthroughs, and others use thermocouple pressure feedthroughs. A separate instrument, such as a voltage meter or thermometer measures and displays the heat or cold generated by the thermocouple.