A voltage that thermocouples develop can be used in measuring temperature.
Thermocouples are devices that develop a voltage when heated. That voltage can be used in temperature measuring and sensors. The device consists of two dissimilar metals joined together. When the junction is heated, a voltage is produced, and the temperature is related to the voltage. The resistance temperature detector (RTD) is similar to a thermocouple but uses the change in resistance as a temperature sensor.
Low Temperature Thermocouple (0K to 450 C)
Generally low temperature thermocouples are fabricated in three different configurations. One is a copper conductor joined with a conductor made of copper-nickel alloy. A second type uses a nickel-chromium alloy conductor joined with a nickel-aluminum conductor. But the configuration perhaps best known for low temperature applications uses a nickel-chromium alloy conductor joined with one of a copper-nickel alloy. The ultimate use of the thermocouple determines the gauge of the conductors.
Thermocouple for Temperatures Greater Than 450 C
Higher temperature thermocouples normally use platinum conductors joined to conductors made of a platinum-rhodium combination. Since it has a higher melting temperature, the rhodium content is increased for higher temperature applications. Iridium-rhodium thermocouples are sometimes used for temperatures of 2150 C to 2250 C, and tungsten-rhenium alloys are used for very high temperatures, but thermocouples of these materials are quite expensive. As with the lower temperature thermocouples, the gauge of the conductor is dictated by the specific application.
Thin-Film Ceramic Thermocouples
NASA is developing thin-film ceramic thermocouples for surface measurement in propulsion system research. Thermocouples made of chromium silicide and tantalum carbide are being tested in temperatures in excess of 1500 C. Additionally, two thin-film indium-tin-oxide-based thermocouples have been tested to 1500 C. Initial results indicate that the silicide and tantalum carbide compositions have a voltage output approximately 15 times that of the standard platinum-rhodium thermocouples.
Resistance Temperature Detector (RTD)
Most RTDs use a coil of fine wire wrapped around a ceramic core. Resistance increases as the temperature increases; therefore, a resistance measurement will directly correlate with temperature. Platinum is the most commonly used material in an RTD, but nickel and copper wire are also used. RTDs are accurate and stable, and are immune to electrical interference --- making them useful in manufacturing environments.
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