Keller 1110968, Operating Instructions Manual, Page: 19 / 30

19
EN
Operating instructions CellaTemp PR xx
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�
12.3 Measurements of Real Radiators
Real radiation sources are specified 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 if the emissivity is not taken into account� 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 of the target is correctly adjusted on the pyrometer�
The spectral emissivity coefficient of a body does not represent an exact material
constant, but depends on the surface properties of the object to be measured
(→ 16 Ways to determine emissivity).
12.4 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�