Radiation Thermometers
About radiation thermometers
All bodies emit electromagnetic
radiation as a function of their temperature above absolute zero. The
total rate of radiation emitted per second is given by:
E = KT4
The power spectral density of this emission varies with temperature.
The major part of the frequency spectrum lies within the band of
wavelengths between 0.3 µm and 40 µm, which corresponds to the visible
(0.3-0.72 µm) and infrared (0.72—1000 µm) ranges. As the magnitude of
the radiation varies with temperature, measurement of the emission
from a body allows the temperature of the body to be calculated.
Choice of the best method of measuring the emitted radiation depends
on the temperature of the body. At low temperatures, the peak of the
power spectral density function lies in the infrared region, whereas
at higher temperatures it moves towards the visible part of the
spectrum. This phenomenon is observed as the red glow which a body
begins to emit as its temperature is increased beyond 600°C.
Radiation thermometers have one major advantage in that they do not
require to be in contact with the hot body in order to measure its
temperature. Thus, there is no disturbance at all of the measured
system. Furthermore, there is no possibility of contamination, which
is particularly important in the food and many other process
industries. They are especially suitable for measuring high
temperatures that are beyond the capabilities of contact instruments
such as thermocouples, resistance thermometers and thermistors. They
are also capable of measuring moving bodies, for instance the
temperature of steel bars in a rolling mill. Their use is not as
straightforward as the discussion so far might have suggested,
however, because the radiation from a body varies with the composition
and surface condition of the body as well as with temperature. This
dependence on surface condition is quantified by the emissivity of the
body. The use of radiation thermometers is further complicated by
absorption and scattering of the energy between the emitting body and
the radiation detector. This energy is scattered by atmospheric dust
and water droplets and absorbed by carbon dioxide, ozone and water
vapor molecules. Therefore, all radiation thermometers have to be
carefully calibrated for each particular body whose temperature they
are required to monitor.
Various types of radiation thermometer exist. The optical pyrometer
can only be used to measure high temperatures, but various types of
radiation pyrometers are available which between them cover the whole
temperature spectrum.
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Temperature Measurements
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