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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.
Pirani was aware of the gas thermal conductivity investigations of Kundt and Warburg [4] (1875) published thirty years earlier and the work of Marian Smoluchowski [5] (1898). In 1906 he described his "directly indicating vacuum gauge" that used a heated wire to measure vacuum by monitoring the heat transfer from the wire by the vacuum environment.
The two top thermocouple junctions are at temperature T 1 while the two bottom thermocouple junctions are at temperature T 2. The output voltage from the thermopile, ΔV , is directly proportional to the temperature differential, ΔT or T 1 - T 2 , across the thermal resistance layer and number of thermocouple junction pairs.
A temperature sensor, such as a thermometer, thermocouple, or resistance temperature detector, is inserted in the open end of the tube, which is usually in the open air outside the piping or vessel and any thermal insulation. Thermodynamically, the process fluid transfers heat to the thermowell wall, which in turn transfers heat to the sensor ...
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.
For instance, setting those up so that the temperature in the morning at 7 a.m. should be 21 °C (69.8 °F), makes sure that at that time the temperature will be 21 °C (69.8 °F), where a conventional thermostat would just start working at that time.
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.