Page 260 - Instrumentation Reference Book 3E

244 Temperature measurement

14.2.4 Fahrenheit and Rankine scales industry. However, in laboratories or under special
industrial conditions, a wider range of instruments
These two temperature scales are now obsolete in is available. In Table 14.7 is a summary of the more
Britain and the United States. but as a great deal usually used measuring instruments in the range
of engineering data, steam tables, etc., have been quoted. All measuring instruments require to be
published using the Fahrenheit and Rankine tem- calibrated against standards. In the case of
perature a short note for reference purposes is temperature the standards are the defining
relevant. fixed points on the IPTS-68. These fixed points
are not particularly easy to achieve in workshop
Fahrenheit This scale was proposed in 1714. Its conditions. Although the secondary points
original fixed points were the lowest temperature are intended as workshop standards it is
obtainable using ice and water with added salts more usual, in most instrument workshops. to
(ammonium chloride) which was taken as zero. calibrate against high grade instruments
Human blood heat was made 96 degrees (98.4 on whose calibration is traceable to the IPTS-68 fixed
the modern scale). On this scale the ice point is at points.
32°F and the steam point at 212°F. There does
not appear to be any formal definition of the
scale. 14.3 Measurement techniques:
To convert from the Fahrenheit to Celsius direct effects
scale. if t is the temperature in Celsius and f the
temperature in Fahrenheit
Instruments for measuring temperature described
in this section are classified according to the nat-
t = $( f - 32) (14.7) ure of the change in the measurement probe pro-
duced by the change of temperature. They have
Rankiize The Rankine scale is the thermo- been divided into four classes: liquid expansion,
dynamic temperature corresponding to gas expansion, change of state, and solid expan-
Fahrenheit. Zero in Rankine isi of course, the sion.
same as zero Kelvin. On the Rankine scale the
ice point is at 491.67"R. Zero Fahrenheit is
459.67 "R. To convert temperature from Fahr- 14.3.1 Liquid-in-glass thermometers
enheit to Rankine, where R is the Rankine
temperature The glass thermometer must be the most familiar
of all thermometers. Apart from its industrial and
laboratory use it finds application in both domes-
R = f + 459.67 (14.8)
tic and medical fields.
Table 14.6 illustrates the relationship between the
four temperature scales. 14.3. I. 1 hferczrry-filled glass tlzeumometer
The coefficient of cubical expansion of mercury is
14.2.5 Realization of temperature measurement about eight times greater than that of glass. If,
Techniques for temperature measurement are very therefore, a glass container holding mercury is
varied. Almost any temperature-dependent effect heated, the mercury will expand more than the
may be used. Sections 14.3-14.6 describe the main container. At a high temperature, the mercury
techniques for temperature measurement used in will occupy a greater fraction of the volume of
the container than at a low temperature. If, then,
the container is made in the form of a bulb with
a capillary tube attached, it can be so arranged
that the surface of the mercury is in the capillary
Table14.6 Comparison of temprature scales
tube, its position along the tube will change with
K "C "F "R temperature and the assembly used to indicate
temperature. This is the principle of the mercury-
Absolute zero 0 -273.15 -523.67 0 in-glass thermometer.
Boiling point 90.19 -182.96 -361.33 162.34 The thermometer. therefore. consists simply of
02 a stem of suitable glass tubing having a very
Zero Fahrenheit 255.37 -17.78 0 459.67 small; but uniform, bore. At the bottom of this
Ice point 273.15 0 32 491.67 stem there is a thin-walled glass bulb. The bulb
Steam point 373.15 100 212 671.67 may be cylindrical or spherical in shape and has a
Freezing point 1235.08 961.93 1763.47 2223.14
of silver capacity very many times larger than that of
the bore of the stem. The bulb and bore are