Which thermocouples are the best


“There are many applications for non-contact temperature measurement. If the process-specific temperature changes are below ± 30 K, and high accuracy and reproducibility are crucial, then this infrared technology is particularly suitable. "

Conventional infrared probes provide good measurement results for industrial applications when accuracy is not particularly important and large temperature differences (> ± 50 K) are the rule. If the temperature fluctuations are kept within narrow limits (<± 30 K), great accuracy, precision and repeatability are required, infrared thermocouples (T / C) could be the best solution.

Conventional infrared measuring devices work with internal signal processing and external power supply and provide an image of the measured variable temperature as a voltage or current signal. The infrared sensors that we are discussing here work without auxiliary voltage and the output signal is based on the Seebeck effect. The sensor produces a signal proportional to the temperature difference between the measuring point and the reference junction (reference temperature). Common materials for thermocouples and external temperature displays are used.

In this article we will first consider the influence of different emissivities and different ambient temperatures on both measurement methods with infrared probes and infrared thermocouples. Then we show typical applications for which the latter technology is best suited.

Black radiators versus gray radiators versus natural radiators

The emissivity is a measure of how large the proportion of a surface is that radiates heat instead of reflecting it. For example, an ideal mirror would reflect all light or infrared radiation - and not emit anything itself. This would mean that the emissivity would be zero. However, only when we see the reflecting mirror surface ourselves do we realize that we are dealing with a real mirror. If 90% of the mirror is reflecting, then 10% of the surface is emitting. The emissivity would then be 0.1. For every non-transparent material, the rule applies that emission (ε) and reflection (ρ) add up to one.

ε + ρ = 1

A body that does not reflect any radiation (ρ = 0), i.e. has an emissivity of ε = 1, is called a black body. The basically mathematical concept of the black body is extremely useful for the physics of infrared radiation and has had a solid theoretical background since the time of Max Planck.

However, in the real world of control and monitoring tasks, infrared measuring devices do not measure on black bodies. You always measure on surfaces with an emissivity ε <1 and a reflectance ρ> 0. For example, shiny metals with a reflective surface show emissivities in the range between 0.05… 0.2. Their temperature is very difficult to measure using infrared methods.