Resistance thermometer sensors use a wire, film, chip or bead whose electrical resistance changes with temperature. The resistor is connected to a measuring bridge or voltmeter, whose output is processed to obtain the temperature. The thermometer must first be calibrated, and this may be done using fixed points, if appropriate, or by comparison with known standard thermometers.

Standard Platinum Resistance Thermometers (SPRTs) are the thermometers specified for calibration at the fixed points of the ITS-90. There are three main types.

• The helium-filled capsule-type, cSPRT, is suitable for use at very low temperatures. A fine platinum wire is held in a strain-free manner and sealed in a short (typically 60 mm long by 5 mm diameter) capsule with four leads emerging through the seal for measurement of the resistance.
• The long-stem SPRT is suitable for insertion into furnaces at temperatures up to the freezing point of aluminium (660 °C), and also at temperature down to about – 200 °C. In this case the sensor is mounted on a support in dry air at the end of a long (>450 mm) silica tube, and the four connecting wires are led out through a seal which is kept close to room temperature. Both these types of SPRT are commonly made with sensing resistors of about 25.5 Ω at 0.01 °C, and the sensitivity is then about 0.1 Ω/°C.
• Special high-temperature SPRTs are used for measurements up to the freezing point of silver, 962 °C. They have lower resistances, 2.5 Ω or 0.25 Ω at 0.01 °C, to overcome problems with the reduced insulation resistance of the silica supports at high temperatures.

NPL calibrates SPRTs with uncertainties of 0.001 °C or less, in ranges of the ITS-90 up to 420 °C, the freezing point of zinc. See our measurement services for details.

Industrial platinum resistance thermometer sensors are usually made with wires or films of 100 Ω at 0 °C. They are commonly known as IPRTs, Pt100s, RTDs (resistance temperature detectors), etc. They are made to meet the specifications of IEC 60751 (BS EN 60751), and are designed to survive more robust treatment than the laboratory SPRTs. The sensors are usually protected inside a steel probe, from which copper cable leads to the measuring instrument.

Using a typical 1 mA measuring current, a Pt100 sensor would have a sensitivity of approximately 0.4 mV/°C, and a resolution of 0.001 °C can be readily achieved using modern voltmeters or resistance bridges. The accuracy of a measurement is limited by the calibration and stability of both the sensing probe and the instrument, and on how they are used. Dedicated instruments can display the output either as a resistance or directly in temperature, and the probe and instrument can be calibrated together.

Thermistors are temperature sensitive resistors made from small beads of various semiconductive oxides. In the more common NTC (negative temperature coefficient) types, the resistance increases very strongly as the temperature falls. They are well suited for use in small probes with fast response, eg in medical thermometry, where good sensitivity is achieved over useful temperature ranges. Thermistors are not standardised, and manufacturers' specifications must be referred to.