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A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction.A thermocouple produces a temperature-dependent voltage as a result of the Seebeck effect, and this voltage can be interpreted to measure temperature.
The ring under the spark plug is used to transfer the heat from the plug to the thermocouple. The gauge and cold junction are usually calibrated at room temperature, 72 °F (22 °C). Because the thermocouple is calibrated for room temperature, the gauge readings will only be 100% accurate at that engine compartment temperature.
A special form of the Pirani gauge is the pulsed Pirani vacuum gauge where the sensor wire is not operated at a constant temperature, but is cyclically heated up to a certain temperature threshold by an increasing voltage ramp. When the threshold is reached, the heating voltage is switched off and the sensor cools down again.
Thermowells are typically installed in piping systems and subject to both hydrostatic and aerodynamic forces. Vortex shedding is the dominant concern for thermowells in cross-flow applications and is capable of forcing the thermowell into resonance with the possibility of fatigue failure not only of the thermowell but also of the temperature sensor.
Thermoelectric sorting functions similarly to a thermocouple but involves an unknown material instead of an unknown temperature: a metallic probe of known composition is kept at a constant known temperature and held in contact with the unknown sample that is locally heated to the probe temperature, thereby providing an approximate measurement ...
A difficulty during this measurement is that the human skin is not particularly suitable for the mounting of heat flux sensors. Also, the sensor has to be thin: the skin essentially is a constant temperature heat sink, so added thermal resistance has to be avoided. Another problem is that test persons might be moving.
One thermocouple monitors heat transferred by convection or radiation, while the other responds to ambient temperature. The detector responds when the first sensing element's temperature increases relative to the other. Rate of rise detectors may not respond to low energy release rates of slowly developing fires.
These elements work with temperatures to 300 °C (572 °F) without further packaging, but can operate up to 600 °C (1,112 °F) when suitably encapsulated in glass or ceramic. Special high-temperature RTD elements can be used up to 900 °C (1,652 °F) with the right encapsulation.