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 +20C and +1300C. 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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