Page 308 - Instrumentation Reference Book 3E

292 Temperature measurement

Figure 14.61 Radiation thermometer transmitter. Courtesy Sirius Instruments Ltd.

With the growing use of microprocessors in siderations govern the choice of instrument? The
instrumentation several manufacturers are intro- obvious first choice to most people is a liquid-in-
ducing instruments where the linearization and glass thermometer. However, this requires that
compensation are performed by a microcomputer. one must be able to get close enough to read the
instrument accurately. A better solution is a dial
14.6.2.7 Radiation thermometer applications thermometer. The type of instrument chosen will
of course depend upon the accuracy and repeat-
Infrared thermometers are currently used in a ability required. In general, and especially on
wide range of laboratory and industrial tempera- bigger plants, local temperature measurement is
ture control applications. A few low-temperature for general surveillance purposes only; the meas-
examples include extrusion, lamination and dry- urement is probably not essential but is provided
ing of plastics, paper and rubber, curing of resins, as a cross-check on the control instruments to
adhesives and paints, and cold rolling and form- provide operator confidence. An inexpensive
ing of metals. bimetal thermometer is probably adequate. If
Some high temperature examples include form- greater accuracy is required then a capillary-type
ing, tempering, and annealing of glass, smelting, thermometer (see Sections 14.3.2- 14.3.4) with
casting, rolling, forging, and heat treating of metals, a short capillary can be used, or where high
and calcining and firing of ceramics and cement. accuracy is necessary an electrical technique
In short, the infrared thermometer can be used
in almost any application in the range 0 to may be specified. In the case of furnaces a port-
able radiation instrument may be the best choice.
3600°C where its unique capabilities can turn a
Of course, on small plants not controlled from
seemingly impossible measurement and control a separate control room all measurements will
problem into a practical working process. Many probably be local measurements. It is mainly in
processes now controlled manually can be con- this situation that the higher accuracy local read-
verted into continuous, automated systems.
out is required.
14.7 Temperature me as u re m e n t 14.7.1.2 Remote reading thermometers
considerations The first question to ask in the selection of remote
reading instruments is: what is the distance
14.7.1 Readout between the measurement point and the readout
location? If that distance is less than, say, 100
14.7.1.1 Local readout
meters, capillary instruments may well be the best
If temperature requires to be measured at a par- solution. However, if the distance is near the top
ticular point on, say, a chemical plant, what con- limit vapor pressure instruments will probably be