Siemens 7MC3040-..., Operating Manual, Page: 61 / 116

s
Operating manual PA
53
16 Theory of Non-Contact Temperature Measurements
All materials radiate thermal energy in all states of aggregation above
absolute zero. This radiation is mainly caused by atomic or molecular
oscillations. This temperature radiation is only a limited sector within the
total electromagnetic radiation spectrum. It extends from the visible
range starting at wavelengths of approx. 0.5

m to the infrared range
with wavelengths of more than 40

m. The PA radiation
pyrometers detect infrared radiation for non-contact temperature meas-
urement.
16.1 Advantages of Non-Contact Temperature Measurement
Non-contact temperature detection means cost-effective temperature
measurement because this technique only requires a single investment
in an instrument without any follow-up costs for consumables such as
thermocouples. This method enables temperature detection of moving
objects - quick temperature measurements within milliseconds - for ex-
ample at automatic welding processes. Small objects with medium and
high temperatures can also be easily and accurately measured. When
measuring materials with low specific heat, a non-contact method does
not induce heat loss which would distort the temperature reading (as is
the case with contact temperature probes). Non-contact temperature de-
tection is ideal with corrosive molten materials for which the use of ther-
mocouples is hardly feasible. Last but not least it is also possible to
measure the temperature of voltage-carrying objects.
16.2 Measurements at Black Bodies (Cavity Radiators)
A black body or a black radiator is used to calibrate radiation pyrometers.
This black body is designed in a way that its radiation does not
depend on material characteristics, but only on its temperature. A black
body emits at any wavelength the maximum energy possible for the spe-
cific temperature. Real bodies do not have this ability. In other words, a
black body completely absorbs the radiation without reflection or trans-
mission losses. The spectral emissivity coefficient
ε(

) 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.
e 
( )

M
M
S
e
(

):
Emissivity coefficient of the object’s surface (targeted
spot) at wavelengh

M: radiant energy actually emitted by a real object
M
S
:: radiant energy emitted by a black body (perfect radiator)