Page 258 - Instrumentation Reference Book 3E

242 Temperature measurement
The temperature at which water boils is very depen- The relationship between the Kelvin and Cel-
dent on pressure. At a pressure p, N . m-2 the boil- sius scales is such that zero degrees Celsius is
ing point of water tu in degrees Celsius is given by equal to 273.15 K
fp = 100 + 2.795 x p - 1.013 x f = T - 273.15 (14.5)
- 1.334 x p - 1.013 x 105)2 where t represents the temperature in degrees
(14.4) Celsius and Tis the temperature Kelvin.
It should be noted that temperatures on the
The temperature interval of 100 "C between the Celsius scale are referred to in terms of degrees
ice point and the steam point is called the funda- Celsius, "C; temperatures on the absolute scale
mental interval. are in Kelvins, K, no degree sign being used.
For instance, the steam point is written in Celsius,
14.2.2 Kelvin, absolute, or thermodynamic 100 "C, but on the Kelvin scale 373.15 K.
temperature scale
Lord Kelvin defined a scale based on thermo- 14.2.3 International Practical Temperature
dynamic principles which does not depend on Scale of 1968 (IPTS-68)
the properties of any particular substance. Kelvin The gas thermometer, which is the final standard
divided the interval between the ice and steam of reference is, unfortunately. rather complex and
points into 100 divisions so that one Kelvin repre- cumbersome, and entirely unsuitable for indus-
sents the same temperature interval as one Celsius trial use. Temperature-measuring instruments
degree. The unit of the Kelvin or thermodynamic capable of a very high degree of repeatability are
temperature scale is the "Kelvin." The definition of available. Use of these instruments enables tem-
the Kelvin is the fraction U273.16 of the thermo- peratures to be reproduced to a very high degree
dynamic temperature of the triple point of of accuracy, although the actual value of the
water. This definition was adopted by the thir- temperature on the thermodynamic scale is not
teenth meeting of the General Conference for known with the same degree of accuracy. In order
Weights and Measures in 1967 (13th CGPM, to take advantage of the fact that temperature
1967). Note the difference between the ice point scales may be reproduced to a much higher
(0") used for the Celsius scale and the triple point degree of accuracy than they can be defined, an
of water which is 0.01 "C. International Practical Temperature Scale was
It has also been established that an ideal gas adopted in 1929 and revised in 1948. The latest
obeys the gas law P6' = RT, where Tis the tem- revision of the scale was in 1968 (IPTS-68). The
perature on the absolute or Kelvin scale and where 1948 scale is still used in many places in industry.
P is the pressure of the gas, I/ is the volume occu- The differences between temperatures on the two
pied and R is the universal gas constant. Thus, the scales are small, frequently within the accuracy of
behavior of an ideal gas forms a basis of tempera- commercial instruments. Table 14.3 shows the
ture measurement on the absolute scale. Unfortu- deviation of the 1948 scale from the 1968 revi-
nately the ideal gas does not exist, but the so-called sion.
permanent gases, such as hydrogen, nitrogen, oxy- The International Practical Temperature Scale
gen, and helium, obey the law very closely, pro- is based on a number of defining fixed points, each
vided the pressure is not too great. For other of which has been subject to reliable gas therm-
gases and for the permanent gases at greater pres- ometer or radiation thermometer observations,
sures, a known correction may be applied to allow and these are linked by interpolation using
for the departure of the behavior of the gas from instruments which have the highest degree of
that of an ideal gas. By observing the change of
pressure of a given mass of gas at constant volume,
or the change of volume of the gas at constant Table 14.3 Deviation of IPTS-68 from IPTS-48
pressure, it is possible to measure temperatures
on the absolute scale.
The constant-volume gas thermometer is sim- -200 0.022
pler in form, and is easier to use, than the con- -150 -0.013
stant-pressure gas thermometer. It is, therefore, 0 0.000
the form which is most frequently used. Nitrogen 50 0.010
has been found to be the most suitable gas to use 100 0.000
for temperature measurement between 500 and 200 0.043
1500 "C, while at temperatures below 500 "C 400 0.076
600
hydrogen is used. For very low temperatures, 1000 0.150
1.24
helium at low pressure is used.