A resistance thermometer measures temperature by passing electricity through metal and judging its temperature based on resistance. Platinum is the most common metal used due to its chemical inertness and resistance to high heat. The number of wires in the device determines accuracy, with more wires being better. However, it has limitations and cannot be used at temperatures above 1.112°C or below -518°F.
A resistance thermometer, also known as a resistance thermal device (RTD), is a metal-specific thermometer that drives electricity through metal and judges its temperature based on resistance. Its chemical inertness and other factors indicate that platinum is the most common metal used in a resistance thermometer. There are several types of RTDs, judged primarily by how many wires are included in the device; more threads means better accuracy. While a reliable device, an RTD is not used in temperatures above 1.112° Fahrenheit (600° Celsius) or below -518°F (-270°C) due to complications.
Industrial workers judge the temperature of metal based on its resistance using a resistance thermometer. As a metal heats up, its resistance increases; if the RTD meets a higher resistance, the metal is very hot. By relating these two factors, an RTD can achieve a precise temperature. Pushing an electric current through the metal while simultaneously reading the current in the metal does this.
While several metals can be used as the core in a resistance thermometer, the most common is platinum. One reason is that platinum is chemically inert, so it won’t 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 particularly suitable for this purpose.
The inner core of the resistance thermometer may not change from one unit to another, but the wiring configuration is often different. RTDs come in two-, three-, and four-wire designs, each more accurate than the last. The wires are copper and have their own resistance. Fewer wires mean the RTD cannot handle resistance without compromising temperature accuracy, while more wires give the RTD better resistance, so it can better gauge the temperature of a metal. Two-wire versions are best for approximate temperatures and are much cheaper; the three- and four-wire versions are best for exact temperatures but are more expensive.
The resistance thermometer is considered reliable and versatile, but it has limitations. At temperatures above 1.112°C, it becomes difficult to prevent the metal sheath of the RTD from contaminating the platinum core. Contamination means that measurements will become inaccurate, rendering the RTD useless. At very low temperatures, like -600°F (-518°C), it becomes impossible to judge temperature by resistance, because the resistance comes from the impurities, not the metal itself.
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