A Resistance Thermometer Detector measures the relationship between electrical resistance and temperature. As they are almost invariably made of platinum they are often called PRTs (Platinum Resistance Thermometer).

General description

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 concerns.
• Wire wound Thermometers can have greater accuracy, especially for wide temperature bof the coil diameter providing 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. The sensitivity is a nominal 0.38 ohm/°C. By far the most common devices used in industry have a nominal resistance of 100 ohms at 0 °C.

Temperature to resistance equation

For the range -200 to 0 °C

   Rt = Ro [ 1 + At + B(t*t) + C(t-100 °C)-(t*t*t) ]

For the range 0 to 850 °C

   Rt = Ro [ 1 + At + B(t*t) ]
     A =  3.9083 10-3
     B = -5.775 10-7
     C = -4.183 10-12

Advantages/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 the reason why laboratory standard thermometers replace the metal sheath with glass construction.