TwoColor Pyrometer (Ratio Pyrometer)
About the twocolor pyrometer (Ratio
pyrometer)
As we know, the emitted
radiation–temperature relationship for a body depends on the
emissivity. This is very difficult to calculate, and therefore in
practice all pyrometers have to be calibrated to the particular body
they are measuring.
The twocolor pyrometer (alternatively
known as a ratio pyrometer) is a system which largely overcomes this
problem by using a specific arrangement. Radiation from the body is
split equally into two parts which are applied to separate narrowband
filters. The outputs from the filters consist of radiation within two
narrow bands of wavelengths. Detectors sensitive to these frequencies
produce output voltages V1 and V2 respectively. The ratio of these
outputs, V1/V2, can be shown to be a function of temperature and to be
independent of the emissivity provided that the two wavelengths are
close together.
The theoretical basis of the twocolor pyrometer is that the output is
independent of emissivity because the emissivities at the two
wavelengths are equal. This is based on the assumption that the two
wavelengths are very close together. In practice, this assumption does
not hold and therefore the accuracy of the twocolor pyrometer tends
to be relatively poor. However, the instrument is still of great use
in conditions where the target is obscured by fumes or dust, which is
a common problem in the cement and mineral processing industries.
Twocolor pyrometers typically cost 50–100% more than other types of
pyrometer.
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Temperature Measurements
