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 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.
More
Temperature Measurements
|