PT100 (Platinum Resistance Thermometers) can be used in temperature range of -200°C, + 850°C, they have much more reliability than the thermocouples have. They are used in machines, tanks, gas or liquid environments and surface measurements.

Principle of the measurement of temperature is to measure the resistance of platinum element. PT100, which is the most common resistance thermometer, has 100Ohm at 0°C and 138.4 Ohm at 100°C.

The relation between temperature and resistance in small range of temperatures is almost linear. For example, error in 50°C is about 0.4°C if we assume the range from 0°C to 100°C is linear.

The latest definition of the relation between temperature and resistance can be found International Temperature Standards 90 (ITS-90).