Page 298 - Instrumentation Reference Book 3E

282 Temperature measurement

14.6.2. I Total radiation therinonwter
In this type of instrument. the radiation emitted by
the body whose temperature is required is focused
on a suitable thermal-type receiving element. This
receiving element may have a variety of forms. It
may be a resistance element. which is usually in the
form of a very thin strip of blackened platinum, or a
thermocouple or thermopile. The change in tem-
perature of the receiving element is then measured
as has already been described.
In a typical radiation thermopile a number of
thermocouples made of very fine strips are con-
nected in series and arranged side by side, or
radially as in the spokes of a wheel, so that all
the hot junctions, which are blackened to increase
the energy-absorbing ability, fall within a very
small target area. The thermoelectric characteris-
tics of the thermopiles are very stable because the
hot junctions are rarely above a few hundred
degrees Celsius, and the thermocouples are not
exposed to the contaminating atmosphere of the
furnace. Stability and the fact that it produces a Figure 14.48 Thermopile for use in total radiation
measurable e.m.f. are the main advantages of the pyrometer.
thermopile as a detector. In addition, thermopiles
have the same response to incomiiig radiant
energy regardless of wavelength within the range The total radiant flux emitted by the source will
0.3 - 20pm. The main disadvantage of the be given by
thermopile is its comparatively slow speed of R = €OAT: (1 4.30)
response which depends upon the mass of the
thermocouple elements, and the rate at which where E is the total emissivity of the body, A is the
heat is transferred from the hot to the cold junc- area from which radiation is received, u is the
tions. Increase in this rate of response can only be Stefan-Boltzniann constant, and T the actual
attained by sacrificing temperature difference temperature of the body.
with a resultant loss of output. A typical indus- This flux will be equal to that emitted by a
trial thermopile of the form shown in Figure perfect blackbody at a temperature T,, the
14.48 responds to 98 percent of a step change in apparent temperature of the body:
incoming radiation in 2 seconds. Special thermo-
piles which respond within half a second are R = O ~ ~ ; (14.31)
obtainable but they have a reduced e.m.f. output. Equating the value of R in equations (14.30) and
In order to compensate for the change in the (14.3 1):
thermopile output resulting from changes in the
cold junction temperature an ambient tempera- E~AT~ AT:
=
ture sensor is mounted by the cold junctions.
Alternative thermal detectors to thermopiles are (14.32)
also used. Thermistors and pyroelectric detectors
are currently in use. The advantage of thermistors
is that they can be very small and so have a quick
speed of response. Their main disadvantage is
their non-linearity, though this is not so great a The actual correction to be applied to the appar-
ent temperature is given in Figure 14.49. Table
disadvantage as with a direct measurement of 14.16 shows the emissivity of some metals at
temperature because provision has to be made different temperatures.
to linearize the radiated energy signal anyway. The radiation from a hot object can be made to
approximate much more closely to black body
Correction for einissivitjj When the temperature radiation by placing a concave reflector on the
of a hot object in the open is being measured, due surface. If the reflectivity of the reflecting surface
regard must be given to the correction required for is r, then it can be shown that the intensity of the
the difference between the emissivity of the surface radiation which would pass out through a small
of the object and that of a perfect blackbody. hole in the reflector is given by