Resistance temperature detector
A resistance temperature detector (RTD) is a temperature sensor that senses temperature by means of changes in the magnitude of current through, or voltage across, an element whose electrical resistance varies with temperature. These types of sensors provide a change in resistance proportional to a change in temperature. Resistance temperature detectors have been used for making accurate temperature measurements. They utilize a resistance element whose resistance changes with the ambient temperature in a precise and known manner. The resistance temperature detector may be connected in a bridge circuit which drives a display calibrated to show the temperature of the resistance element. Most metals become more resistant to the passage of an electrical current as the metal increases in temperature. The increase in resistance is generally proportional to the rise in temperature. Thus, a constant current passed through a metal of varying resistance produces a variation in voltage that is proportional to the temperature change. Platinum is the most commonly used metal for RTDs due to its stability and nearly linear temperature versus resistance relationship. Platinum also has the advantages of chemical inertness, a temperature coefficient of resistance that is suitably large in order to provide readily measurable resistance changes with temperature, and a resistance which does not drastically change with strain. Other types of RTDs include copper, nickel and nickel alloys. Resistance temperature detectors are usually fabricated by winding a fine diameter wire on a bobbin or mandrel which is made of a ceramic material such as aluminum oxide. Then, the wire-wound bobbin is coated with an cement-like insulating material and is installed in a protective tube.
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