A platinum resistance thermometer (PRT) is a device which determines the temperature by measuring the electrical resistance of a piece of pure platinum wire. The piece of platinum wire is referred to as a temperature sensor. When manufactured carefully these devices offer an excellent combination of sensitivity, range and reproducibility.
The electrical resistance of many metals (e.g. copper, silver, aluminium, platinum) increases approximately linearly with absolute temperature and this feature makes them useful as temperature sensors. The resistance of a wire of the material is measured by passing a current (AC or DC) through it and measuring the voltage with a suitable bridge or voltmeter, and the reading is converted to temperature using a calibration equation.
The most reproducible type of sensor is made from platinum because it is a stable unreactive metal which can be drawn down to fine wires but is not too soft. Using very pure wires, thermometers can be made with closely similar resistance characteristics and achieve good reproducibility in use.
The length and diameter of the platinum wire used in a thermometer are often chosen so that the resistance of the device at around 0 ºC is 100 ohms. Such a sensor is a called a PT100 sensor, and its resistance changes by approximately 0.4 ohms per degree Celsius. Using a typical 1 mA measuring current, at around 0 ºC a PT100 sensor would have a voltage drop of around 100 mV across its terminals and this would change by approximately 0.4 mV per degree Celsius, which thus makes sensitive thermometry available to anyone with a high resolution voltmeter or resistance bridge. In many instruments the measurement is converted so that the reading is directly in temperature.
Platinum sensors care manufactured to comply with a standard curve within a specified tolerance. The international standard, IEC 751, is published in the UK as BS EN 60751. The Class A tolerance for a PT100 sensor is ± 0.25 ºC over the range 0 ºC to ±200 ºC. However, better uncertainties can be achieved if the sensor is suitably protected in a steel probe and calibrated individually.
For the highest accuracy, special glass-sheathed standard PRTs, usually of 25 ohms at 0 °C, are calibrated at the fixed points of the International temperature scale 1990 (see above). The ITS-90 specifies equations to relate the resistance to temperature and, using these, uncertainties can be achieved of 0.001 °C or better. Standard PRTs can be used from temperatures as low as 259 °C up to 660 ºC, or even, 962 ºC, with some increase in uncertainty and of loss of reproducibility.
we calibrates standard platinum resistance thermometers, using fixed-points to provide reference temperatures over the ranges specified in the ITS-90.