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Operating instructions
furnace - which applies to all other self-contained targets - always show a reading
which is too low. Considerable errors can occur at targets with reflecting, polished
or bright surfaces, e.g. molten steel and metal without oxide layer and ceramic
materials. Exact results can only be obtained when the emissivity coefficient is
correctly adjusted on the pyrometer.
The spectral emissivity coefficient of a body does not represent an exact material
constant, but is also largely dependent on the surface properties (→ 16 Ways to
determine emissivity).
16.4 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 „background 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.
17 Ways to determine emissivity
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 emissivity 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.