Resistance Temperature Detectors, also known as RTD's, are wire wound and thin film devices which incorporate pure metals or certain alloys that increase in resistance as temperature increases and, conversely, decrease in resistance as temperature decreases. RTDs are similar to thermocouples in that they convert changes in temperature to voltage signals by the measurement of resistance. As an RTD gets hotter the resistance measured on it's element becomes larger, as it cools the resistance becomes smaller.
The most commonly used type of RTD element is Platinum, often referred to as PRTs or PRT100s. Platinum elements are popular because they can be used over a wide range of temperatures and feature a quick response time. Platinum's coefficient of resistance is nearly linear, as such, by using a platinum element resolutions of ±0.1 °C or better are possible.
Other elements can be used in RTD's which are less costly. The most commonly used alternatives are Copper and Nickel. The advantage to this is that the RTD will cost less money to manufacture and have a lower replacement value. The drawback is that these elements have more restriction on their ranges due to non-linearities in the resistance vs. voltage curve. Also, there are problems with wire oxidation which are known to occur when copper is used as the element.
RTDs have a stable output for long period of time, are easy to recalibrate, and give accurate readings over relatively narrow temperature spans. When compared to thermocouples their disadvantages are their higher sensitivity to vibration, smaller overall temperature range, and greater initial cost.
To get the maximum life from your RTD it is recommended that you use a protective well and terminal head. The well serves to protect the RTD from being damaged by the gas or liquid being measured. Protecting wells are available in stainless steel, carbon steel, Inconel, or cast iron, and they are used for temperatures up to 1100°C.