low-temperature thermometry

low-temperature thermometry [′lō ‚tem·prə·chər thər′mäm·ə·trē]

(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.

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Low-temperature thermometry

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 Gas thermometry

Ranges and sensitivities of low-temperature thermometers

Thermometer	Temperature range, K
Thermocouples
300 to 700 ppm Fe in Au/Ag + 0.37 at. % Au	1–25
Chromel/300 to 700 ppm Fe in Au	1–300
Chromel/Constantan	20–1100
Resistance thermometers
Platinum (capsule)	4–500
Rhodium + 0.5 at. % Fe	0.5–300
Carbon	0.01–300
Germanium	0.01–30
Saturation vapor pressure thermometers
Hydrogen	14–21
Helium-4	1.0–5.2
Helium-3	0.5–3.3
Noise thermometers*	0.002–0.1
Magnetic thermometers
Gadolinium metaphosphate, Gd(PO3)3	2–100
Cerous magnesium nitrate (CMN),
Ce2Mg3(NO3)12 ċ 24H2O; single crystal	0.003–4
CMN powder sphere or cylinder	0.002–4
Copper (and other nuclear paramagnets)	0.001–0.01
Gamma-ray anisotropy thermometers*	0.002–0.05
60Co in hexagonal close-packed cobalt single crystal	0.002–0.04
54Mn in iron	0.003–0.03
54Mn in nickel	0.004–0.045
3He melting-curve thermometer	0.001–1
Nuclear resonance thermometer	310 nK–2K
*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^1/2. 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 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 SQUID, Temperature, Temperature measurement, Thermocouple, Thermometer