Thermocouples contain two dissimilar metals bonded to form a bimetallic junction.
Heat forces the junction to generate an electromotive force or voltage that corresponds to the temperature of the junction. Higher temperatures produce a greater EMF though the change is small. Voltage output of a typical thermocouple shifts about 50 µV/°C.
The types of metals used in making thermocouples determine their qualities and are identified by a letter code. The four most common types are J, K, T, and E.
One advantage thermo-couples have over other temperature sensors is that they need no electrical power to operate — they generate their own. A thermocouple connected directly to a sensitive voltmeter forms a simple high-temperature thermometer called a pyrometer.
The simplicity of a thermocouple is offset by two problems. First, the voltage output is not linear with temperature. In the simple pyrometer, the meter uses a nonlinear scale drawn to match the calibration curve of the thermocouple. Other applications require linearization of the thermocouple output before use. Most electronic temperature displays have built-in linearization curves set to match the type of thermocouple connected. The technician simply programs the display for the type.
The second problem concerns parasitic voltages created by other "thermo-couple junctions" within the circuit. Remember, two different metals in contact form a thermocouple that generates an EMF. For example, a technician connects copper wire extensions to the thermocouple leads to make them longer. Each copper/thermocouple joint generates another voltage that interferes with the output from the thermocouple. Thermocouple manufacturers make special extension cables to reduce this error. The extensions are made from the same material as the thermocouple, thus minimizing parasitic voltages.
Another technique used to minimize the effects of parasitic voltages is the cold junction. Another thermocouple is formed to revert the connecting lead so both are the same type of wire. This second junction is maintained at a known temperature, typically 0°C. As the output of the thermocouple at 0°C is known, that value is added to the measured output to determine the true output and thus the actual temperature. As the same wire now connects both sides of the thermocouple to the equipment, the parasitic voltages created by the connection are equal but opposite, so they cancel each other. Today's modern thermocouple systems perform the actions of a cold junction electronically.