Low-temperature thermometry

(cryogenics) The assignment of numbers on the Kelvin absolute temperature scale to achievable and reproducible low-temperature states, and the choice and calibration of suitable instruments for the practical measurement of low temperatures, such as thermocouples, and resistance, vapor-pressure, gas, and magnetic thermometers.

The measurement of temperature below 32°F (0°C). Very few thermometers are truly wide-range, and hence most of the conventional methods of thermometry tend to fail the further one moves below room temperature (see table). The defining instrument for the lower regions of the International Practical Temperature Scale, the platinum resistance thermometer, rapidly loses sensitivity below −405°F (30 K), and its official limit is set at −434.81°F (13.81 K), the triple point of equilibrium hydrogen. This scale is based upon measurements of thermodynamic temperature made with the gas thermometer; the gas thermometer may be used down to about −456°F (2 K). See also Gas thermometry.

*Primary thermometer.

The low-temperature region is unique in having available several different types of primary thermometers, all of which are quite practical. The least practical, perhaps, is the acoustic thermometer, which uses the property that, extrapolated to zero pressure, the speed of sound in a gas is proportional to T^½. This has been used in the range −456 to −424°F (2–20 K) as an alternative to, and check upon, the gas thermometer. The Johnson noise in a resistor can be used with particular advantage at low temperatures when allied with SQUID detector technology. See also Electrical noise; Sound.

In suitable systems it is possible to spatially orient atomic nuclei at very low temperatures, and if these nuclei are emitters of gamma rays, the emission pattern is anisotropic to a degree which is a measure of the thermodynamic temperature, Finally the magnetic susceptibility of suitable atomic nuclei may also be employed via the Curie law. Nuclear magnetic resonance or static SQUID-based techniques may be employed, but nuclear magnetic resonance is preferable in being unaffected by magnetic impurities, to which the second method falls hostage. See also Nuclear magnetic resonance (NMR); Squid.