A resistance thermometer, also known as a resistance thermal device (RTD), is a thermometer specifically for metals that pushes electricity through the metal and judges its temperature based on the resistance. Its chemical inertness and other factors means platinum is the most common metal used in a resistance thermometer. There are different RTD types, judged mostly on how many wires are included in the device; more wires means better accuracy. While it is a dependable device, an RTD is not used at temperatures above 1,112° Fahrenheit (600° Celsius) or below -518°F (-270°C) because of complications. Industrial workers judge the temperature of metal based on its resistance by using a resistance thermometer. When a metal heats up, its resistance increases; if the RTD encounters a higher resistance, then the metal is very hot. By correlating these two factors, an RTD can arrive at an accurate temperature. Pushing an electrical current through the metal and simultaneously reading the current in the metal does this. While several metals can be used as a core in a resistance thermometer, the most common is platinum. One reason for this is that platinum is chemically inert, so it will not react with nearby chemicals. Another, more important reason, is that platinum is resistant to high heat and will remain stable under harsh conditions. This makes a platinum core especially suited for this purpose. The internal core of the resistance thermometer may not change from unit to unit, but the wiring configuration is often different. RTDs come in two-, three- and four-wire designs, each being more accurate than the last. The wires are made of copper and have their own resistance. Fewer wires means the RTD is not as capable of handling the resistance without throwing off the accuracy of the temperature, while more wires gives the RTD better resistance, so it can better judge a metal’s temperature. Two-wire versions are best for approximate temperatures and are much cheaper; three- and four-wire versions are best for exact temperatures but are more expensive. The resistance thermometer is considered dependable and versatile, but it does have limitations. At temperatures exceeding 1,112°F (600°C), it becomes difficult to keep the metal sheath of the RTD from contaminating the platinum core. Contamination means the measurements will become inaccurate, making the RTD useless. At very low temperatures, such as -518°F (-270°C), it becomes impossible to judge temperature based on resistance, because the resistance is coming from impurities, not the metal itself.
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