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Operating instructions CellaTemp PX 4x 5x 6x
The emissivity coefficient indicates the ratio of radiation of a real body (target) to
the radiation of an ideal black body (target) to the radiation of an ideal black body�
ε(λ): Emissivity coefficient of the object’s surface (targeted spot) at wavelengh λ
M: Emissivity coefficient of the object’s surface (targeted spot) at wavelength
MS: radiant energy emitted by a black body (perfect radiator)
Most burning, annealing and hardening furnaces emit a radiation of nearly ‚1‘
which corresponds to the conditions of a black body if the aperture through which
the measurement is made is relatively small�
4.3 Measurements of Real Radiators
Real radiation sources are characterized by the relation of the emitted radiation to
the radiation of a black body with the same temperature� Measurements outside a
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 (→ Ways to determine emissivity).
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�