Temperature Sensors

Temperature is one of the most frequently measured physical quantities, and a wide range of sensors exists to detect it. Each type relies on a property of a material that changes predictably with temperature, and each offers a particular balance of accuracy, range, cost and ease of use. The main families are thermocouples, resistance temperature detectors, thermistors, semiconductor sensors and non-contact infrared devices.

A thermocouple consists of two dissimilar metal wires joined at one end; the junction produces a small voltage that depends on temperature, an effect known as the Seebeck effect. Thermocouples cover very wide temperature ranges and are rugged, making them common in industrial furnaces. Resistance temperature detectors (RTDs), such as the platinum Pt100, exploit the predictable rise in the resistance of a metal with temperature; the Pt100 has a resistance of about 100 ohms at 0 degrees Celsius and offers excellent stability and accuracy. Thermistors are made from semiconductor material whose resistance changes strongly with temperature, giving high sensitivity over a narrower range.

Semiconductor temperature sensors integrate the sensing element with signal conditioning, often providing a calibrated analogue or digital output, as in the LM75 digital sensor. Infrared pyrometers measure temperature without contact by sensing the thermal radiation an object emits, which is useful for moving, hot or hazardous targets. Selecting a temperature sensor means matching its range, accuracy, response time and output type to the measurement task.

Frequently asked questions

How does a thermocouple measure temperature?

Two dissimilar metals joined at a junction generate a small temperature-dependent voltage through the Seebeck effect, which is read to infer the temperature.

What does Pt100 mean?

It is a platinum resistance temperature detector with a nominal resistance of about 100 ohms at 0 degrees Celsius, prized for its stability and accuracy.

When is an infrared pyrometer used?

When contact is impractical, such as for moving, very hot or hazardous objects, since it senses emitted thermal radiation from a distance.