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Operating instructions CellaTemp PX 41
4.4 Measurements errors
The cause of measurement errors in the use of pyrometers is often an incorrectly
determined or wrong emissivity� Another source of error is the reflected „back-
ground radiation"� If the measurement object has a low emissivity and there are
hotter objects in the surrounding area, measurement results can be affected�
These objects then have to be shaded� This effect is particularly to be observed in
the measurement of a colder object within a hot oven�
EN
5 Ways to determine emissivity (one-colour mode)
Technical literature or operating manuals often contain data on the emissivity of
various materials� This information should be used with caution, however� It is
important to know for which temperature and which wavelength the emissivity
value is applicable� Furthermore, the stated emissivity values were obtained under
ideal conditions� In actual practice, the total emissivity of the target object will vary,
depending on the amount of extraneous radiation transmitted through the object
from the background or reflected onto the object from the foreground� The emissiv-
ity can be determined using one of the following methods:
Contact measurements
Measure the temperature with a contact thermocouple and measure the surface
temperature with a pyrometer� Adjust the emissivity coefficient on the pyrometer
until both devices show the same temperature� When measuring with the thermo-
couple, make sure to have good thermal contact and low heat dissipation�
Using a reference emissivity coefficient
Apply matte black colour to a part of the surface to be measured� This part has an
emissivity of 94 %� At first, measure the temperature of the coloured part� Then
make a comparative measurement right next to the coloured part and adjust the
emissivity on the pyrometer until it displays the previous measurement reading
again�
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