Pyrometer- Handbook - Contika

Window Materials
Pyrometers permit non-contact measurement of
temperatures of materials in furnaces, vacuum chambers or
other enclosed areas. Of course, one needs a special
opening through which the pyrometer can "see" the surface
of the object to be measured. In many cases these openings
must be closed off by windows (for instance, in a vacuum,
when under pressure, when dealing with gases, liquids or
viscous masses). Depending on the temperature range and
the spectral range of the pyrometer, the correct choice of
window material is essential. Table 2 (see page 63) gives a
general picture of the most commonly used materials and
their technical data. The transmission range must be
chosen so that it will not conflict with the pyrometer's
spectral range, which is determined by temperature and the
material of the object to be measured.
Among other necessary properties are mechanical
strength, moisture & chemical resistance, and the ability to
withstand thermal shocks.
The minimum thickness of the window (d min) to ensure
stability under pressure is calculated with the formula:
d min = r .
r - radius of the window
S - safety factor (≥ 4)
c - method of window attachment
(for instance c=1.1 for loose attachment)
∆p - Differential pressure
Μ r - Break modulus (material constant, see table 2)
Glass and quartz windows (used for high temperatures) are
cost efficient, as are silicon and fluor- spar windows (to
measure lower temperatures).
Pyrometer Handbook
S . c . ∆p
Μ r
spectral range
mechanical stability
49