How to select the correct radiation thermometer

Radiation thermometers these recent years are remarkably cheap. This is probably due to the expansion of users and cost reduction efforts made by manufacturers. As there are many types of radiation thermometers these days, such as specification and price, selecting the correct one will be difficult. Purchasing it just by price and not by purpose will be regretting. Please refer to the explanation below to select the correct radiation thermometer. 

Important specifications when selecting the correct radiation thermometer

When selecting a thermometer, it is said that types of sensors, measuring temperature range, accuracy and etc. are important specifications to consider. In addition, the following needs to be paid attention to for radiation thermometers: target size, measuring distance and the presence of emissivity setting function.

Moreover, temperature drift and measured wavelength range are also important.

When comparing specifications with different catalogs, there seems to be slight standard differences among manufacturers. It is also difficult to determine all performance just by specifications. In order to match the purpose of use when purchasing, it is recommended to consider the following points. 

Target size / Object distance (Measuring view)

The measuring area of the radiation thermometer is shown as the target size (measuring view) of the object distance. 

Choose a target size smaller than the measuring object. 

If the target size is bigger than the measuring object, the radiation thermometer measures around the object so accurate measurement cannot be expected. It is called “visual field defect” when the measuring object does not meet the view of the radiation thermometer.

However, it does not mean that choosing the smallest target size is correct because radiation thermometers have a phenomenon known as the size-of-source effect. 

In contrast to the visual field affect, the size-of-source effect is a phenomenon which the output of the thermometer changes due to the size difference even  if the measuring object is a size that meets the target size. The size-of-source effect is a phenomenon that arises from diffraction due to the aperture diaphragm of the optical system, scattering of object lens, reflection inside the lens barrel, heating of detecting elements due to incident light, etc. 

According to the display rules made by JIS, manufacturers should include the size-of-source effect in the specifications but not many manufacturers do so. However, when the measuring object is particularly small, it is important to select not only a small target size but also a radiation thermometer that has a small size-of-source effect. Figure 5 shows an actual measurement of the size-of-source effect of HORIBA`s handheld radiation thermometers and thermometers sold at markets. 

Emissivity setting function

Radiation thermometers measure the intensity of radiant energy emitted from the surface of the measuring object and calculate the temperature. The intensity of radiant energy emitted from the object is determined not only by the object temperature but also by a specific coefficient called “emissivity”. Therefore, when measuring temperature with a radiation thermometer, knowing the temperature beforehand and setting the emissivity correction is necessary. Incorrect emissivity will cause measurement error. 

The correct way to set the emissivity will be explained later but the emissivity setting function on radiation thermometers are a necessary function for accurate measurement. 

※Measurement influenced by the size-of-source effect is carried out by setting a blackbody furnace with an opening equal to or 1.5 times bigger, or by setting an aperture size corresponding to the prescribed target size with an opening diameter of 1.4 times bigger in front of a stable heat source and then finding the difference between the indicated values of the radiation thermometer for each diaphragm. (Jan. 1998)

※Equation of size-of-source effect is below.

Temperature drift

When ambient temperature suddenly changes, such as moving the radiation thermometer from one place to another, temperature drift occurs in the measuring value due to the change in the infrared sensor and the optical system resulting as measurement error. 

Generally, the value of temperature drifts described in catalogs are calculated by converting the indicated difference of when sufficient time passes to the change of ambient temperature of the thermometer into “value per 1 degree ambient temperature change”. However, transient temperature drift are not written in catalogs as it is difficult to take countermeasures and as there are not any defined evaluation methods. 

Even radiation thermometers actually sold at markets, the characteristics differ as shown in Figure 6. Radiation thermometers have a faster responses than contact types as measurement can be completed in a few seconds but it is necessary to pay attention to sudden environmental changes. 

Measured wavelength range

At last but not least, measured wavelength range is an extremely important point when choosing a radiation thermometer. 

1.Temperature and measured wavelength range
According to Planck`s law, the higher the object temperature, shorter wavelength light is emitted and the lower the object temperature, longer wavelength light is emitted.  In other words, radiation thermometers that use short wavelengths are suitable for measuring high temperature substances and long wavelengths are suitable for measuring low temperature substances. 

2.Measuring object and measured wavelength range
As mentioned in “Emissivity setting function” that radiant energy from actual objects depend on not only temperature but also on the emissivity of the object itself, emissivity of a substance depends on the wavelength range of infrared rays.
In other words, depending on the substance, infrared rays of a specific wavelength range may not be emitted. Therefore, it is necessary to choose a radiation thermometer that matches the characteristics of the measuring object within the measured wavelength range. Please refer to the radiation characteristics of typical substances (metal, glass, plastics etc.) explained in the “Radiation characteristics of the measuring object”. 

Figure 6 Actual measurement of temperature drift
Figure 6 Actual measurement of temperature drift

 

 

 *Graph showing change in measured value after moving the radiation thermometer to a 5 degree room from a 26 degree store room and measuring the -10 degree blackbody.

(Jan 1998)

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