Due to its advantages, contact-free temperature measurement more and more finds its way into road construction.

From the contact-free temperature sensors at the paver (temperature measurements behind the screed, single measurements at the augers, temperature measurement in a bunker) to infrared cameras for thermographic pictures during the paving process, up to road compactors working with an infrared sensor to detect the asphalt temperature.

How does contact-free temperature measurement work?

Radiation emitted from a body penetrates the atmosphere and is focused on a detector element via input optics, transformed into an electrical signal and filtered. Based on the sensor calibration and the set emissivity, the electrical signal is converted into temperature values.

This means:

Every body with a temperature over the absolute zero point of 0 Kelvin (-273.15 °C) emits a characteristic radiation (emission) at the surface. The complete spectrum of the electromagnetical emission is between 0.7 to 1000 µm. For contact-free temperature measurement, only the wavelength range of 0.7 to 18 µm is relevant (The unit of the wavelength is micrometer [µm]). (The electromagnetical spectrum consists of radio waves, visual light, ultraviolet radiation, gamma and X-radiation).

What about the emissivity?

Not every object is a so-called ”black body*“. This means, that many bodies emit less radiation at same temperature. The emissivity is the ratio of the actual radiation and the black body (the emissivity ε is between 0 and 1).

Picsen2.JPG

Figure 2 serves to clarify.

The object to be measured emits and reflects energy that is received by the infrared sensor. The infrared measurement range of an IR sensor (0.7 to 18 µm) should be chosen according to the temperatures to be measured and the possible environmental conditions. (Measuring steel, plastics, asphalt, … with steam, rain, dust,flames, …) In longwave atmospheric environments (7 to 18 µm), the transmission (permeability) is consistently high.

*Black body

A black body absorbs radiation to 100 %. There are no reflections and transmissions.

Therefore, a black body has an emissivity of 1.

@ In Part II the single infrared detectors are introduced and shortly explained.