Hudson, Ralph P. Formerly, Bureau International des Poids et Mesures, Sèvres, France.
- Thermometry below 1 K
- Primary thermometers
- Revision of IPTS-68
- Links to Primary Literature
- Additional Readings
The measurement of temperature below 0°C (32°F). Very few thermometers are truly wide-range, and hence most of the conventional methods of thermometry tend to fail the further the temperature drops below room temperature (see table). The defining instrument for a great part of the International Temperature Scale, the platinum resistance thermometer, rapidly loses sensitivity below 30 K, and its official limit is set at 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 2 K, for which purpose it is usually filled at, and “piggybacked” upon, a temperature in the region of the normal boiling point of equilibrium hydrogen, about 20.28 K. Liquid-in-glass thermometers may be used down to about −39°C or −38°F (mercury), about −100°C or −148°F (ethyl alcohol), and almost down to 70 K with various organic fluids. A liquid becomes sluggish at the lower end of its range, and also, with the exception of mercury, it presents problems of wetting the capillary wall and draining. Thermocouples are quite commonly encountered as temperature sensors down to 77 K and below, and thermistors and semiconductor diode sensors are used down to the liquid helium region, all three being available commercially in conjunction with linearizing electronics. Certain thermocouples have been developed especially for use in the 1–20 K region, and others with enough sensitivity for use in the millikelvin region. The electronic thermometer based upon the thermal expansion of a quartz frequency-controlling element may be used down to −40°C (−40°F). See also: Gas thermometry; Thermistor; Thermocouple
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