Page 24 - Instrumentation and Measurement

FIGURE 2.16 Disappearing filament pyrometer.

The disappearing filament pyrometer has a range of about 600 ͦ C to 3000 ͦ C, an accuracy of about ±
0.5% of the reading and involves no physical contact with the hot object. It can thus be used for
moving or distant objects.

The total radiation pyrometer involves the radiation from the hot object being focused onto a
radiation detector. Figure 2.17 shows the basic form of an instrument which uses a mirror to focus
the radiation onto the detector. Some forms use a lens to focus the radiation. The detector is typically
a thermopile with often up to 20 or 30 thermocouple junctions, a resistance element or a thermistor.
The detector is said to be broad band since it detects radiation over a wide band of frequencies and
so the output is the summation of the power emitted at every wavelength. It is proportional to the
fourth power of the temperature (the Stefan-Boltzmann law). The accuracy of broad band total
radiation pyrometers is typically about ± 0.5% and ranges are available within the region 0 ͦ C to
3000 ͦ C. The time constant (a measure of how fast the system responds to a change in temperature
and is the time taken to reach about 63% of the final value) for the instrument varies from about 0.1
s when the detector is just one thermocouple or small bead thermistor to a few seconds with a
thermopile involving many thermocouples. Some instruments use a rotating mechanical chopper to
chop the radiation before it impinges on the detector. The aim is to obtain an alternating output from
the detector, since amplification is easier with an alternating voltage. It is thus of particular benefit
when the level of radiation is low. However, choppers can only be used with detectors which have a
very small time constant and thus tend to be mainly used with small bead thermistor detectors.

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