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A thermocouple consists of two conductors of different materials (usually metal alloys) that produce a voltage in the vicinity of the point where the two conductors are in contact. The voltage produced is dependent on, but not necessarily proportional to, the difference of temperature of the junction to other parts of those conductors. Thermocouples are a widely used type of temperature sensor for measurement and control and can also be used to convert a temperature gradient into electricity.
Total harmonic distortion (THD) determines the difference of the real signal form from perfect sine wave.
Type B thermocouples use a platinum–rhodium alloy for each conductor. One conductor contains 30% rhodium while the other conductor contains 6% rhodium. These thermocouples are suited for use at up to 1800°C / 3272°F. Type B thermocouples produce the same output at 0°C and 42°C / 32°F and 107.6°F, limiting their use below about 50°C / 122°F.
Type C (tungsten 5% rhenium – tungsten 26% rhenium) thermocouples are suited for measurements in the 0°C to 2320°C / 32°F to 4208°F range. This thermocouple is well-suited for vacuum furnaces at extremely high temperatures. It must never be used in the presence of oxygen at temperatures above 260°C / 500°F.
Type E (chromel–constantan) has a high output (68 µV/°C) which makes it well suited to cryogenic use. Additionally, it is non-magnetic. Wide range is -50 to 740°C / -58 to 1364°F and Narrow range is -110 to 140°C / -166 to 284°F.
Type J (iron–constantan) has a more restricted range than type K (-40 to +750°C / -40 to 1382°F), but higher sensitivity of about 55 µV/°C. The Curie point of the iron (770°C / 1418°F) causes an abrupt change in the characteristic, which determines the upper temperature limit.
Type K is the most common general purpose thermocouple with a sensitivity of approximately 41 µV/°C, chromel positive relative to alumel. It is inexpensive, and a wide variety of probes are available in its −200°C to +1250°C / -328°F to + 2282°F range.
Type M thermocouples use a nickel alloy for each wire. The positive wire (20 Alloy) contains 18% molybdenum while the negative wire (19 Alloy) contains 0.8% cobalt. These thermocouples are used in vacuum furnaces for the same reasons as with type C. Upper temperature is limited to 1400°C / 2552°F. It is less commonly used than other types.
Type N (Nicrosil–Nisil) (nickel-chromium-silicon/nickel-silicon) thermocouples are suitable for use between -270°C and 1300°C / -454 and 2372°F owing to its stability and oxidation resistance. Sensitivity is about 39 µV/°C at 900°C / 1652°F, slightly lower compared to type K.
Type R thermocouples use a platinum–rhodium alloy containing 13% rhodium for one conductor and pure platinum for the other conductor. Type R thermocouples are used up to 1600°C / 2912°F.
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