Temperature measurement technologies

Resistance measurement

Resistance temperature measurement determines temperature based on resistance. This technology uses the property of metals to change resistance as temperature changes. For example, a platinum sensor element (Pt100) has a resistance of 100 Ω at 0 °C. As the temperature changes, the resistance changes accordingly.

Thin film tip design

The Resistive Temperature Detector (RTD) element is bonded to a thin film carrier to reduce the thermal mass of the electrical leads. A specialised assembly carrier positions the RTD element into precisely the correct location and preloads the RTD with constant force against the probe’s inner sheath wall. This creates direct and controlled contact between the RTD element and the process medium. The result is a fast and repeatable response.

ifm instruments using the thin film tip design include the TC1, TN, TR, TA Industry, TK, TV, TT and TM families.

Competing products often have the sensing element potted into the tip of the sheath tube. The potting compound acts like an insulator, slowing the heat transfer to the RTD element.

Metallic bonded tip design

This design uses a process that metallically bonds the RTD element directly onto the copper-plated inner wall of the probe tip. This creates very low thermal mass with a direct metallic bond for optimal heat transfer. This tip construction offers response speeds twice as fast as our already fast thin film design.
The metallic bond construction is ideally suited for processes where fast reaction speed and exact temperature measurement is required, such as UHT and HTST pasteurisation processes as well as SIP measurement.

The TC2, TA1 and TA2 product families for food and hygienic applications feature a metallic bonded tip design.

The TSM clamp-on temperature sensor was designed with careful consideration for the effects of ambient temperature on surface mount sensors and is likewise based on the metallic bonded tip design.

For resistance measurement, semiconductor materials are used. They are characterised by a significant change in resistance even with small temperature variations, enabling high resolution across the temperature range. Temperature-dependent semiconductor materials are classified according to their temperature coefficient: PTC (Positive Temperature Coefficient) and NTC (Negative Temperature Coefficient).

The sensors of the TCC family use this measurement technology. They feature two sensing elements that self-detect and send a warning if any signal drift is occurring. The PTC (Positive Temperature Coefficient) element increases its resistance with increasing temperature. The NTC (Negative Temperature Coefficient) element decreases its resistance with increasing temperature. Because the PTC and NTC react to temperature change in opposite directions, the microprocessor is able to measure the difference between the evaluated temperature values and warns the user in the event of drift.

Infrared temperature sensors, also called pyrometers, detect the amount of infrared radiation emitted from the object. A lens focuses the infrared radiation onto a detector, which converts the energy into an electronic signal. This technology enables temperature measurement from a distance without requiring contact to the object.

Infrared sensors are ideal for measuring the temperature of hot or cold objects as well as for detecting the presence of very hot objects (up to 2,500 °C). This is particularly advantageous in steel mills or glass plants, where conventional inline sensors would reach their limits.

All objects with a temperature above -273 °C (0 K) radiate some level of infrared energy. The object’s ability to emit this energy is known as emissivity (ε). Many factors influence the emissivity of the object including material and surface finish.

Emissivity information is available from the following table, but the values can vary in practice: