Unchopped Broad-band Radiation Pyrometers

About unchopped broad-band radiation pyrometers

The instrument finds wide application in industry and has a measurement accuracy which varies from 0.05% of full scale in the best instruments to 0.5% in the cheapest.


However, the level of accuracy deteriorates significantly over a period of time, and an error of 10C is common after 12 years of operation at high temperatures. As its name implies, the instrument measures radiation across the whole frequency spectrum and so uses a thermal detector. This consists of a blackened platinum disk to which a thermopile is bonded. The temperature of the detector increases until the heat gain from the incident radiation is balanced by the heat loss due to convection and radiation.


For high-temperature measurement, a two-couple thermopile gives acceptable measurement sensitivity and has a fast time constant of about 0.1 s. At lower measured temperatures, where the level of incident radiation is much less, thermopiles constructed from a greater number of thermocouples must be used to get sufficient measurement sensitivity. This increases the measurement time constant to as much as 2 s. Standard instruments of this type are available to measure temperatures between -20C and +1800C, although in theory much higher temperatures could be measured by this method.

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