14.5 Radiation thermometers
All objects emit electromagnetic
radiation as a function of their temperature above abso[1]lute
zero, and radiation thermometers (also known as radiation pyrometers) measure
this radiation in order to calculate the temperature of the object. The total
rate of radiation emission per second is given by:
E = KT4
(14.9)
The power spectral density of this
emission varies with temperature in the manner shown in Figure 14.10. The major
part of the frequency spectrum lies within the band of wavelengths between 0.3
µm and 40 µm, which corresponds to the visible (0.3–0.72 µm) and infrared
(0.72–1000 µm) ranges. As the magnitude of the radiation varies with
temperature, measurement of the emission from a body allows the temperature of
the body to be calculated. Choice of the best method of measuring the emitted
radiation depends on the temperature of the body. At low temperatures, the peak
of the power spectral density function (Figure 14.10) lies in the infrared
region, whereas at higher temperatures it moves towards the visible part of the
spectrum. This phenomenon is observed as the red glow that a body begins to emit
as its temperature is increased beyond 600°C.
Different versions of radiation
thermometers are capable of measuring temperatures between -100°C and +10 000°C
with measurement inaccuracy as low as ±0.05% (though this level of accuracy is
not obtained when measuring very high temperatures). Portable, battery-powered,
hand-held versions are also available, and these are particularly easy to use.
The important advantage that radiation thermometers have over other types of
temperature-measuring instrument is that there is no contact with the hot body
while its temperature is being measured. Thus, the measured system is not
disturbed in any way. Furthermore, there is no possibility of contamination,
which is particularly important in food and many other process industries. They
are especially suitable for measuring high temperatures that are beyond the
capabilities of contact
instruments such as thermocouples,
resistance thermometers and thermistors. They are also capable of measuring
moving bodies, for instance the temperature of steel bars in a rolling mill.
Their use is not as straightforward as the discussion so far might have
suggested, however, because the radiation from a body varies with the
composition and surface condition of the body as well as with temperature. This
dependence on surface condition is quantified by the emissivity of the body.
The use of radiation thermometers is further complicated by absorption and
scattering of the energy between the emitting body and the radiation detector.
Energy is scattered by atmospheric dust and water droplets and absorbed by
carbon dioxide, ozone and water vapour molecules. Therefore, all radiation
thermometers have to be carefully calibrated for each particular body whose
temperature they are required to monitor.
Various types of radiation
thermometer exist, as described below. The optical pyrometer can only be used
to measure high temperatures, but various types of radiation pyrometers are
available that between them cover the whole temperature spectrum. Intelligent
versions (see section 14.13) also now provide full or partial solution to many
of the problems described below for non-intelligent pyrometers.
14.5.1 Optical pyrometers
The optical pyrometer, illustrated in
Figure 14.11, is designed to measure temperatures where the peak radiation
emission is in the red part of the visible spectrum, i.e. where the measured
body glows a certain shade of red according to the temperature. This limits the
instrument to measuring temperatures above 600°C. The instrument contains a
heated tungsten filament within its optical system. The current in the filament
is increased until its colour is the same as the hot body: under these
conditions the filament apparently disappears when viewed against the
background of the hot body. Temperature measurement is therefore obtained in
terms of the current flowing in the filament. As the brightness of different
materials at any particular temperature varies according to the emissivity of
the material, the calibration of the optical pyrometer must be adjusted
according to the emissivity of the target. Manufacturers provide tables of
standard material emissivities to assist with this.
The inherent measurement inaccuracy
of an optical pyrometer is ±5°C. However, in addition to this error, there can
be a further operator-induced error of ±10°C arising out of the difficulty in
judging the moment when the filament ‘just’ disappears. Measurement accuracy
can be improved somewhat by employing an optical filter within the instrument
that passes a narrow band of frequencies of wavelength around 0.65 µm
corresponding to the red part of the visible spectrum. This also extends the
upper temperature measurable from 5000°C in unfiltered instruments up to 10
000°C.
The instrument cannot be used in
automatic temperature control schemes because the eye of the human operator is
an essential part of the measurement system. The
reading is also affected by fumes in
the sight path. Because of these difficulties and its low accuracy, hand-held
radiation pyrometers are rapidly overtaking the optical pyrometer in
popularity, although the instrument is still widely used in industry for
measuring temperatures in furnaces and similar applications at present.
14.5.2 Radiation pyrometers
All the alternative forms of
radiation pyrometer described below have an optical system that is similar to
that in the optical pyrometer and focuses the energy emitted from the measured
body. However, they differ by omitting the filament and eyepiece and having
instead an energy detector in the same focal plane as the eyepiece was, as
shown in Figure 14.12. This principle can be used to measure temperature over a
range from -100°C to +3600°C. The radiation detector is either a thermal
detector, which measures the temperature rise in a black body at the focal
point of the optical system, or a photon detector.
Thermal detectors respond equally to
all wavelengths in the frequency spectrum, and consist of either thermopiles,
resistance thermometers or thermistors. All of these typically have time
constants of several milliseconds, because of the time taken for the black body
to heat up and the temperature sensor to respond to the temperature change.
Photon detectors respond selectively
to a particular band within the full spectrum, and are usually of the
photoconductive or photovoltaic type. They respond to temperature changes very
much faster than thermal detectors because they involve atomic processes, and
typical measurement time constants are a few microseconds.
Fibre-optic technology is frequently
used in high-temperature measurement applications to collect the incoming
radiation and transmit it to a detector and processing electronics that are
located remotely. This prevents exposure of the processing electronics to
potentially damaging, high temperature. Fibre-optic cables are also used to
apply radiation pyrometer principles in very difficult applications, such as
measuring the temperature inside jet engines by collecting the radiation from
inside the engine and transmitting it outside (see section 14.9).
The size of objects measured by a
radiation pyrometer is limited by the optical resolution, which is defined as
the ratio of target size to distance. A good ratio is 1:300, and this would
allow temperature measurement of a 1 mm sized object at a range of 300 mm. With
large distance/target size ratios, accurate aiming and focusing of the
pyrometer at the target is essential. It is now common to find ‘through the
lens’ viewing provided in pyrometers, using a principle similar to SLR camera
technology,
as focusing and orientating the
instrument for visible light automatically focuses it for infrared light.
Alternatively, dual laser beams are sometimes used to ensure that the
instrument is aimed correctly towards the target.
Various forms of electrical output
are available from the radiation detector: these are functions of the incident
energy on the detector and are therefore functions of the temperature of the
measured body. Whilst this therefore makes such instruments of use in automatic
control systems, their accuracy is often inferior to optical pyrometers. This
reduced accuracy arises firstly because a radiation pyrometer is sensitive to a
wider band of frequencies than the optical instrument and the relationship
between emitted energy and temperature is less well defined. Secondly, the
magnitude of energy emission at low temperatures gets very small, according to
equation (14.9), increasing the difficulty of accurate measurement.
The forms of radiation pyrometer
described below differ mainly in the technique used to measure the emitted
radiation. They also differ in the range of energy wavelengths, and hence the
temperature range, which each is designed to measure. One further difference is
the material used to construct the energy-focusing lens. Outside the visible
part of the spectrum, glass becomes almost opaque to infrared wavelengths, and
other lens materials such as arsenic trisulphide are used.
Broad-band (unchopped) radiation
pyrometers
The broadband radiation pyrometer
finds wide application in industry and has a measurement inaccuracy that varies
from š0.05% of full scale in the best instruments to š0.5% in the cheapest.
However, their accuracy deteriorates significantly over a period of time, and
an error of 10°C is common after 1–2 years’ 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
disc to which a thermopileŁ 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.
Chopped broad-band radiation
pyrometers
The construction of this form of
pyrometer is broadly similar to that shown in Figure 14.12 except that a rotary
mechanical device is included that periodically interrupts the radiation
reaching the detector. The voltage output from the thermal detector thus
becomes an alternating quantity that 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
+20°C and +1300°C. This form of pyrometer suffers similar accuracy drift to
unchopped forms. Its life is also limited to about two years because of motor
failures.
Narrow-band radiation pyrometers
Narrow-band radiation pyrometers are
highly stable instruments that suffer a drift in accuracy that is typically
only 1°C in 10 years. They are also less sensitive to emissivity changes than
other forms of radiation pyrometer. They use photodetectors of either the
photoconductive or photovoltaic form whose performance is unaffected by either
carbon dioxide or water vapour in the path between the target object and the
instrument. A photoconductive detector exhibits a change in resistance as the
incident radiation level changes whereas a photovoltaic cell exhibits an
induced voltage across its terminals that is also a function of the incident
radiation level. All photodetectors are preferentially sensitive to a
particular narrow band of wavelengths in the range 0.5 µm–1.2 µm and all have a
form of output that varies in a highly non-linear fashion with temperature, and
thus a microcomputer inside the instrument is highly desirable. Four commonly
used materials for photodetectors are cadmium sulphide, lead sulphide, indium
antimonide and lead–tin telluride. Each of these is sensitive to a different
band of wavelengths and therefore all find application in measuring the
particular temperature ranges corresponding to each of these bands.
The output from the narrow-band
radiation pyrometer is normally chopped into an a.c. signal in the same manner
as used in the chopped broad-band pyrometer. This simplifies the amplification
of the output signal, which is necessary to achieve an acceptable measurement
resolution. The typical time constant of a photon detector is only 5 µs, which
allows high chopping frequencies up to 20 kHz. This gives such instruments an additional
advantage in being able to measure fast transients in temperature as short as
10 µs.
Two-colour pyrometer (ratio
pyrometer)
The theoretical basis of the
two-colour pyrometer is that the output is independent of emissivity because
the emissivities at the two wavelengths
This is based on the assumption that
Selected waveband pyrometer
The selected waveband pyrometer is
sensitive to one waveband only, e.g. 5 µm, and is dedicated to particular,
special situations where other forms of pyrometer are inaccur[1]ate. One example of
such a situation is measuring the temperature of steel billets that are being
heated in a furnace. If an ordinary radiation pyrometer is aimed through the
furnace door at a hot billet, it receives radiation from the furnace walls (by
reflection off the billet) as well as radiation from the billet itself. If the
temperature of the furnace walls is measured by a thermocouple, a correction
can be made for the reflected radiation, but variations in transmission losses
inside the furnace through fumes etc. make this correction inaccurate. However,
if a carefully chosen selected-waveband pyrometer is used, this transmission
loss can be minimized and the measurement accuracy is thereby greatly improved.
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