A resistance temperature detector (RTD) is a temperature sensor which operates on the principle of measurement that the electrical resistance of material changes with temperature.
The interaction between an RTD’s resistance and the surrounding temperature is highly predictable, allowing precise and accurate temperature measurement. The resistance of the RTD can be calculated and the temperature can be determined by supplying an RTD with a constant current and measuring the resulting voltage drop across the resistor.
How RTD works and what are the common resistance materials for RTDs?
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Thermometer resistance uses metals which alter their electrical resistance when heated. The different materials used in the construction of RTDs provide a different relationship between resistance and temperature.
Resistance temperature detectors (RTDs) is made of temperature-sensitive materials such as platinum nickel and copper whereas the most widely used materials for industrial RTD are platinum.
In the working process of RTD, the bridged circuit can be used in such cases when the change in resistance value is very small in term of temperature.
The constant electric current comes into the bridged circuit and voltage drops across the resistor are measured. This can also determine the temperature by using a calibration expression to convert the RTD resistance value.
Main components of RTD
RTD platinum resistance element
This is the real RTD temperature control component. The range in length of the elements is from 1/8″ to 3″. There is also a range of types. The standard temperature coefficient is an alpha of .00385 and the standard resistance is 100 Ω at 0C.
RTD outside diameter
The most common outside diameter is 1/4 in the US or 6mm (.236″) for non-US applications. However, outside diameters range from .063″ to .500″
RTD tubing material
For assemblies up to 500 F, 316 stainless steel sheet is commonly used above 500 F it is advisable to use Inconel 600.
RTD process Connection
All regular fittings used with thermocouples such as compression, welded or spring-loaded are included in process attachment fittings.
RTD wire configuration
RTDs are available in 2, 3 and 4 wire configurations. For industrial applications, 3 wire configurations are the most common. The standard wire insulation materials are Teflon and Fiberglass. The Teflon is resistant to moisture and can be used up to 400F whereas Up to 1000 F may be used with Fiberglass.
RTD cold end termination
RTDs can be ended on a cold end with plugs, bare wires, terminal heads and any thermocouple reference joints.