Chopped Broad-band Radiation Pyrometers
About chopped broad-band radiation
pyrometers
The construction of this form of pyrometer is broadly similar to that
of unchopped
broad-band radiation pyrometer except that a rotary mechanical
device is included which periodically interrupts the radiation
reaching the detector. The voltage output from the thermal detector
thus becomes an alternating quantity which switches between two
levels. This form of a.c. output can be amplified much more readily
than the d.c. output coming from an unchopped instrument.
This is particularly important when
amplification is necessary to achieve an acceptable measurement
resolution in situations where the level of incident radiation from
the measured body is low. For this reason, this form of instrument is
the more common when measuring body temperatures associated with peak
emission in the infrared part of the frequency spectrum.
For such chopped systems, the time
constant of thermopiles is too long. Instead, thermistors are
generally used, giving a time constant of 0.01 s. Standard instruments
of this type are available to measure temperatures between +20°C and
+1300°C. This form of pyrometer suffers a similar drift in accuracy to
unchopped forms. Its life is also limited to about 2 years because of
motor failures.
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Temperature Measurements
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