Page 296 - Instrumentation Reference Book 3E

280 Temperature measurement

property for the non-contact and non-intrusive
measurement of temperature. Instruments for
temperature measurement by radiation are called
radiation thermometers. The terms pyrometer or
radiation pyrometer were formerly used.
There are four principal techniques for the
measurement of temperature by the radiation
from a hot body: total radiation, pyroelectric.
photo-electric, and optical.
Instruments using the first three of these tech-
niques are normally constructed in the same gen-
eral physical form. Figure 14.44 shows the
general format of one of these instruments. It
consists of a cylindrical metal body made of alu-
minum alloy, brass, or plastic. One end of the
body carries a lens, which, depending on the
wavelength range required, consists of germa-
nium, zinc sulfide, quartz, glass, or sapphire. The
opposite end carries the electrical terminations
for connecting the sensing head to its signal con-
ditioning module. A typical size of such a sensing
head is 250mm long by 60mm diameter. A dia-
grammatic sketch of the construction of the
instrument is shown in Figure 14.45. Infrared
cncrgy from a target area on the object whose
temperature is to be measured is focused by the
lens onto the surface of the detector. This energy
is converted to an electrical signal which may be
amplified by a head amplifier on the circuit
Wavelength IIrn - 4 I , board. Power is supplied to the instrument and
I
I
3
1
2
the output transmitted down a cable which is
connected to terminals in the termination box.
Figure 14.43 Spectral energydistribution with In instruments working in the near-infrared
temperature. region where the lens is transparent to visible
light a telescope can be provided, built into the
instrument, so that it can be focused and aligned
In 1900 Planck obtained from theoretical con- by looking through the lens.
siderations, based on his quantum theory, the A primary advantage of radiation therm-
expression ometers, especially when used to measure high
(14.29)

where the symbols have the same meaning, and
C? = 0.014 388 m . K.
These laws also enable the correction to be
calculated for the presence of an absorbing
medium such as glass in the optical pyrometer,
and also the correction required for changes in
the spectral emissive power of the radiating surface.
The variation of spectral radiance with wave-
length and temperature of a black body source is
given by Figure 14.43.

14.6.2 Radiation thermometer types
Since the energy radiated by an object is a func- Figure 14.44 General-purpose radiation thermometer.
tion of its absolute temperature this is a suitable Courtesy Land Infrared Ltd.