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IR-pyrometers

The IR-pyrometer is the perfect thermometer. It measures the temperature of the target when ordinary thermometers measure their own temperature. The IR-pyrometer measures radiated heat neither touching nor disturbing the object thermally.

Traditionally the pyrometer is associated to high temperature measurements, but it has advantages also in normal temperature ranges.

Pyrometer-collection-w200.jpg

 

Principle of function

Heat is kinetic energy in the world of atoms. It is spread to the environment in different ways, as radiation among other things.

The pyrometer senses the change of radiative intensity in the infrared wavelength range, about 0.7 to 20 µm. Pyrometry should be like reading in an open book. Unfortunately the laws of nature put obstacles in its way.

 

Laws of radiation

Planck, Wien, Kirchhoff and others settled laws for radiation. One of the most wellknown formulas is E+T+R=1. It means that heat radiation associated to a body contain three components:

  1. Emission (E) is the own heat radiated from the body, i e which we want to measure.
  2. Transmission (T) is heat from other sources passing through the body.
  3. Reflexion (R) is heat from other bodies that is reflected like in a mirror.

Very simplified, the pyrometer measures the sum of those three components. The emission part of the total radiation varies hugely depending on material properties of the body.

In fact, transmission can be neglected in most cases. Then emission and reflection are what are left entering the pyrometer. The relation E + R = 1 is what remains as absorbed and emitted energy of a body, and is constant at stationary conditions when the body is in thermal balance with its environment. E is called the emissivity coefficient or emission factor and is normally written like the Greek letter Epsilon. The emissivity varies 0 < e < 1. A so called blackbody must have e > 0.99.

IR-reflexer-w200.jpg

Watch out for reflexes!

 

Wave length

Radiated heat contain different combinations of wave lengths depening on materials. One material attenuates a certain wave length which is optimal for temperature measurments on others. Hence, one type of pyrometer can not measure the temperature of all materials.

An example is transparent plastic film which transmits light visible for the human eye but stops certain other wavelengths of the infrared range. With the correct choice of pyrometer you can reduce the influence of the transmission on the measuring result.

 

Optics

In principle the pyrometer is like a camera that ”photographs” the heat radiation. A usual mistake is that your picture shows more than you want it to do. Impertinent heat radiation can disturb the measurement.

For this reason you have to install the pyrometer carefully in a way that no impertinent radiation enters the pyrometer via the lens. This can be achieved by using the proper optics. As is the case with cameras, top grade optics is a condition for good measuring results. Material of the lens and choice of IR-filters determine in which wave length range the measuring can be done.

IR-spot-w200.jpg

Alternative 1 (left) is correct.
Alternative 2 will monitor a
"mixture" of T1 and T2.

 

Atmosphere

At e g high temperature measuring the pyrometer normally is positioned several metres from the heat source. With the proper optics it is possible to focus on the right spot, but still there is a problem left to be solved.

The atmosphere between the pyrometer and the measuring object, e g furnace gases, filters the heat radiation. Different atmospheres have separate properties depending on their compositions. As an example the moisture of the air blocks certain wave lengths within the IR range.

There are gaps in narrow wave length bands which let through the heat radiation without losses. If you limit the working range to these gaps it is possible to measure accurately also at a distance.

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Transmittance windows in air (blue).

The higher th temperature the shorter
the wave length to be used.

 

Calibration

A pyrometer is to be calibrated according to the same principles as all other temperature measuring equipment. You measure an object with known temperature and correct the pyrometer accordingly. In the field of pyrometry a reference object called black body is used. Its emission must be  0.99 <  < 1.00. See laws of radiation above.

Pentronic offers accredited calibration of IR-pyrometers within -10 till +550 °C.

 

Correct equipment and know-how

The described natural laws are just a few of the factors influencing pyrometer temperature measurements. The conclusion is that a good measuring result can only be achieved using the proper pyrometer and a good portion of know-how.

Under Instrumentation > IR-pyrometers you will find a selection of Pentronic’s programme of IR-pyrometers. They will cover the most common measurements required. We also market special pyrometers for unusual and challenging measurement tasks. Please contact us for more information.

 

Optimal wave length ranges for different materials

Wave length[µm] Min. temp.
[°C]
Material
2.0 - 2.7 300 Metal. glass. ceramic
2.0 - 4.5 200 Metal
3.43 ± 0.15 100 Plastic film
5.7 ± 0.1 40 Plastic film
6.8 ± 0.15 0 Plastic film (PE etc)
7.93 ± 0.15 0 Plastic film (Polyester, PVC etc)
8.05 ± 0.15 0 Plastic film and ceramic
3.9 ± 0.1 400 Glass, measuring through gases
4.9 - 5.5 100 Glass
7.5 - 8.2 0 Glass and ceramic
4.26 ± 0.13 400 CO2 gas
4.5 ± 0.1 400 CO and CO2 gas
4.66 ± 0.1 400 CO gas
5.3 ± 0.1 400 NO gas
8 - 10 0 Thick film, ceramic
8 - 14 -50 General measurements
8 - 20 -50 Low temperature, high resolution
9.6 - 11.5   -25 Measuring through atmospheres at a long distance