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Operating instructions CellaTemp PK / PKF / PKL xx
coefficient e(*) of a black body is equal to 1 or 100 %� The emissivity coefficient
indicates the ratio of radiation of a real 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: radiant energy actually emitted by a real object
M
S
: 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�
15.3 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 (→ 16 Ways to
determine emissivity)�
15.4 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�
