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RTD stands for Resistance Temperature Detector. RTDs are sometimes referred to generally as resistance thermometers. An RTD is a temperature sensor that measures temperature using the principle of the resistance of a metal changes with temperature.In practice a small electric current transmitted through a piece of a metal which are platinum ,copper or nickel(the rtd element or resisitor ) located in the area where temperature is to be measured.The resistance value of the RTD elements then measured by an instrument. This resistance value is then correlated to temperature.They are used in many industrial applications below 600 °C, due to higher accuracy and repeatability. Platinum is the most stable resistance-temperature relationship over the largest temperature range and best metal for RTDs because it follows a very linear resistance-temperature relationship and it follows the R vs T relationship in a highly repeatable manner over a wide temperature range and the .The unique properties of platinum make it the material of choice for temperature standards over the range of -272.5 °C to 961.78 °C, and is used in the sensors that define the International Temperature Standard. Platinum is chosen also because of its chemical inertness
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