A resistance temperature detector measures the relationship between electrical resistance and temperature. As they are almost invariably made of platinum they are often called PRTs (platinum resistance thermometers). Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ... General Name, Symbol, Number platinum, Pt, 78 Chemical series transition metals Group, Period, Block 10, 6, d Appearance grayish white Atomic mass 195. ...
There are two broad categories, "film" and "wire-wound" types.
Film thermometers have a layer of platinum on a substrate; the layer may be extremely thin, perhaps 1 micrometre. Advantages of this type are relatively low cost and fast response. Such devices have improved in performance although the different expansion rates of the substrate and platinum give "strain gauge" effects and stability problems.
Wire-wound thermometers can have lesser accuracy, especially for wide temperature ranges. The coil diameter provides a compromise between mechanical stability and allowing expansion of the wire to minimise strain and consequential drift.
The current international standard which specifies tolerance and the temperature to electrical resistance relationship for platinum resistance thermometers is IEC 751:1983. The sensitivity is a nominal 0.385 ohm/°C. By far the most common devices used in industry have a nominal resistance of 100 ohms at 0 °C, and are called Pt-100 sensors ('Pt' is the symbol for platinum). The word substrate can mean the following: In biochemistry, a substrate is a molecule which is acted upon by an enzyme. ... A micrometre (American spelling: micrometer), symbol µm, is an SI unit of length. ... Ohm may refer to: The scientist Georg Ohm. ...
Temperature to resistance equation
For the range -200 to 0 °C:
(the Callendar-Van Dusen equation), where
RT = resistance at T°C
R0 = resistance at 0°C
T = temperature in °C
For the range 0 to 850 °C:
where
A = 3.9083 × 10-3
B = -5.775 × 10-7
C = -4.183 × 10-12
Advantages and limitations
Advantages of platinum resistance thermometers:
High accuracy
Low drift
Wide operating range
Suitability for precision applications
Limitations:
RTDs in industrial applications are rarely used above 660 °C. At temperatures above 450 °C it becomes increasingly difficult to prevent the platinum from becoming contaminated by impurities from the metal sheath of the thermometer. This is why laboratory standard thermometers replace the metal sheath with a glass construction.
Resistancetemperaturedetectors operate on the principle of changes in electrical resistance of pure metals and are characterized by an almost linear positive change in resistance with temperature.
RTDs are able to sense temperatures with extreme repeatability and low drift error from –200°C to +850°C. The temperature coefficient of the RTD element is positive.
RTD (T) is the resistance of the RTD element at temperature T ºC, RTD0 is the specified resistance of the RTD element at 0ºC and, T is the temperature of the RTD on the environment to be measured in degrees Celsius.
A resistancetemperaturedetector measures the relationship between electrical resistance and temperature.
The sensitivity is a nominal 0.385 ohm/°C. By far the most common devices used in industry have a nominal resistance of 100 ohms at 0 °C, and are called Pt-100 sensors ('Pt' is the symbol for platinum).
RTDs in industrial applications are rarely used above 660 °C. At temperatures above 450 °C it becomes increasingly difficult to prevent the platinum from becoming contaminated by impurities from the metal sheath of the thermometer.
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