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An RTD is a temperature sensor which measures temperature using the principle that the resistance of a metal changes with temperature.
An electrical current is transmitted through a piece of metal (the RTD element or resistor) located in proximity to the area where temperature is to be measured. The resistance value of the RTD element is then measured by an instrument. This resistance value is then correlated to temperature based upon the known resistance characteristics of the RTD element.

How do RTDs Work?
RTDs work on a basic correlation between metals and temperature. As the temperature of a metal increases, the metal's resistance to the flow of electricity increases. Similarly, as the temperature of the RTD resistance element increases, the electrical resistance, measured in ohms (Ω), increases. RTD elements are commonly specified according to their resistance in ohms at zero degrees Celsius (0 C). The most common RTD specification is 100 Ω, which means that at 0 C the RTD element should demonstrate 100 Ω of resistance.

Platinum is the most commonly used metal for RTD elements due to a number of factors, including its (1) chemical inertness, (2) nearly linear temperature versus resistance relationship, (3) temperature coefficient of resistance that is large enough to give readily measurable resistance changes with temperature and (4) stability (in that its temperature resistance does not drastically change with time).

Other metals that are less frequently used as the resistor elements in an RTD include nickel, copper and Balco.

RTD elements are typically in one of three configurations:
(1) a platinum or metal glass slurry film deposited or screened onto a small flat ceramic substrate known as "thin film" RTD elements, and
(2) platinum or metal wire wound on a glass or ceramic bobbin and sealed with a coating of molten glass known as "wire wound" RTD elements.
(3) A partially supported wound element which is a small coil of wire inserted into a hole in a ceramic insulator and attached along one side of that hole. Of the three RTD elements, the thin film is most rugged and has become increasingly more accurate over time.