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 10°C is common after 1–2 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 -20°C and +1800°C, although in theory much higher temperatures could be measured by this method.