Page 261 - Instrumentation Reference Book 3E

Measurement techniques: direct effects 245

Table 14.7 Temperature measurement techniques
Range (K) Technique Application Resolution (K)

0.01-1.5 Magnetic susceptance of paramagnetic salt Laboratory 0.001
0.1-50 Velocity of sound in acoustic cavity Laboratory standard 0.0001
0.2-2 Vapor pressure Laboratory standard 0.001
1.5-100 Germanium resistance thermometer Laboratory standard 0.0001
1.5-100 Carbon resitance thermometer Laboratory 0.001
1.5-1400 Gas thermometer Laboratory 0.002
Industrial 1 .0
210430 Silicon P-N junction Laboratory 0.1
Inudstrial
4-500 Thermistor Laboratory 0.001
Industrial 0.1
11-550 Quartz crystal oscillator Laboratory 0.0c1
Industrial
1 5-1 000 Platinum resistance thermometer Standard 0.000 01
Industrial 0.1
20-2700 Thermocouple General-purpose 1 .0
30-3000 Sound velocity in metal rod Laboratory 1 Yo
130-950 Liquid-in-glass General-purpose 0.1
130-700 Bimetal Industrial 1-2
270-5000 Total radiation thermometer Industrial 10
270-5000 Spectrally selective radiation thermometer Industrial 2

completely filled with mercury, and the open end
of the bore sealed off either at a high temperature, Construction of
or under vacuum, so that no air is included in liquid-in-glass
the system. The thermometer is then calibrated by thermometer
comparing it with a standard thermometer in
a bath of liquid whose temperature is carefully Scale
controlled.
When the standard thermometer and the therm-
ometer to be calibrated have reached equilib- -
rium with the bath at a definite temperature,
the point on the glass of the thermometer oppo-
site the top of the mercury mensicus is marked.
The process is repeated for several temperatures.
The intervais between these marks are then
divided off by a dividing machine. In the case of
industrial thermometers, the points obtained by
calibration are transferred to a metal or plastic
plate, which is then fixed with the tube into a -
suitable protecting case to complete the instru-
ment.
The stem of the thermometer is usually shaped
in such a way that it acts as a lens, magnifying the
width of the mercury column. The mercury is
usually viewed against a background of glass (a1
which has been enamelled white. Figure 14.2 Cross section of
shows the typical arrangement for a liquid- thermometer stern
in-glass thermometer. White vitreous Front of stem
Mercury-in-glass thermometers are available in shaped as lens to
three grades: A and B are specified in BS 1041: Part broaden apparent size
of column
2.1: 1958; grade C is a commercial grade of therm-
ometer and no limits of accuracy are specified. (bl
Whenever possible, thermometers should be cali-
brated, standardized and used immersed up to the Figure 14.2 Mercury-in-glass thermometer.
(a) thermometer and scale, (b) cross-section of
reading, Le., totally immersed, to avoid errors due thermometer stem.