Page 520 - Mechanical Engineers' Handbook (Volume 4)

6 Special Problems in Low-Temperature Instrumentation 509

6.1 Temperature Measurement
Temperature may be measured using liquid-in-glass thermometers down to about 40 C,
using thermocouples down to about liquid H temperature, and using resistance thermometers
2
and thermistors down to about 1 K. Although these are the usual devices of engineering
measurement laboratory measurements have been done at all temperatures using gas ther-
mometers and vapor pressure thermometers.
Table 10 lists the defining fixed points of the International Practical Temperature Scale
of 1968. This scale does not define fixed points below the triple point of equilibrium He. 36
Below that range the NBS has defined a temperature scale to 1 K using gas thermometry. 37
At still lower temperatures measurement must be based on the fundamental theories of solids
such as paramagnetic and superconducting phenomena. 38
The usefulness of vapor pressure thermometry is limited by the properties of available
fluids. This is evident from Table 11. For example, in the temperature range from 20.4 to
24.5 K there is no usable material. Despite this, vapor pressure thermometers are accurate
and convenient. The major problem in their use is that the hydraulic head represented by
the vapor line between point of measurement and the readout point must be taken into
account. Also, the measurement point must be the coldest point experienced by the device.
If not, pockets of liquid will form in the line between the point of measurement and the
readout point greatly affecting the reading accuracy.
Standard thermocouples may be used through most of the cryogenic range, but, as shown
in Fig. 37 for copper–constantan, the sensitivity with which they measure temperature drops
as the temperature decreases. At low temperatures heat transfer down the thermocouple wire
may markedly affect the junction temperature. This is especially dangerous with copper
wires, as can be seen from Fig. 36. Also, some thermocouple materials, for example, iron,
become brittle as temperature decreases. To overcome these difficulties special thermocouple
pairs have been used. These usually involve alloys of the noble metals. Figure 37 shows the
thermoelectric power, and hence sensitivity of three of these thermocouple pairs.
Resistance thermometers are also very commonly used for cryogenic temperature mea-
surement. Metal resistors, especially platinum, can be used from ambient to liquid He tem-
peratures. They are extremely stable and can be read to high accuracy. However, expensive
instrumentation is required because resistance differences are small requiring precise bridge
circuitry. Resistance as a function of temperature for platinum is well known. 36

Table 10 Defining Fixed Points of the International Practical Temperature Scale, 1968
Equilibrium Point T (K)
13.81
Triple point of equilibrium H 2
Boiling point of equilibrium H 2 (P 33330.6 N/m ) 17.042
2
Boiling point of equilibrium H 2 (P 1 atm) 20.28
Boiling point of neon (P 1 atm) 27.102
54.361
Triple point of O 2
Boiling point of O 2 (P 1 atm) 90.188
Triple point of H 2 O(P 1 atm) 273.16
Freezing point of Zn (P 1 atm) 692.73
Freezing point of Ag (P 1 atm) 1235.08
Freezing point of Au (P 1 atm) 1337.58