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.