Pyrometer- Handbook - Contika

5. Determining the Emissivity of an Object
Because the emissivity factor is so important in the
calculations that determine temperature, it is essential to
establish its value accurately for a given material.
There are several ways that this can be done. Tables may
be consulted to look up the values of the emission
coefficient for many different materials (see Table 1). For
metals, however, the values are mostly qualitative.
The temperature of the object is first determined by
measuring with a contact thermometer. Then the pyrometer
is aimed at the object. Finally, the emissivity adjustment
knob is turned until both devices indicate the same
temperature. In order to use this method the object must be
sufficiently large and accessible.
Part of the object's surface is blackened with special
lacquer or soot whose emission coefficient is close to 1, is
accurately known and is stable up to the temperature to be
measured. The pyrometer measures the temperature of the
blackened surface, then it measures the untouched part of
the surface. Then the emissivity adjustement knob is set so
that the temperature value of the previous measurement is
shown.
The object is drilled to a depth of at least six times the drill
hole's diameter. The diameter must be greater than the spot
size diameter of the pyrometer. Like a black body source,
the drill hole is considered to have an emission coefficient
of nearly 1. First, the temperature in the drill hole will be
measured, then the pyrometer measures the surface, and by
correct adjustment the temperature of the drill hole will be
determined.
The emissivity of a sample object can be determined by
spectrometer analysis. The manufacturer of your
pyrometer will arrange for this analysis to be carried out.
Pyrometer Handbook
tables
comparison
with contactthermometers
partial blackening of
the surface
drilling into
the object
analysis with
spectrometer
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