Acoustical Thermometer and the National Bureau of Standards Provisional Temperature Scale 2–20 (1965)

An acoustical thermometer has been developed at the National Bureau of Standards to determine values of temperature that approximate the Thermodynamic Temperature Scale. The instrument, called an ultrasonic interferometer in acoustical literature, has been used to determine isotherms of the speed of sound in helium gas as a function of pressure. Each isotherm has been extrapolated to zero pressure to approximate the condition of an ideal gas; from the resulting intercepts, values of temperature were calculated. The instrument and its operation are described and the data isotherms, which have been determined at approximately every degree from 2° - 20 °K, are listed. The isotherm data have been treated both graphically and by electronic computer analysis (by the method of least squares) to yield values of temperature that are the basis for a new temperature scale – NBS Provisional Scale 2 - 20 (1965). When the scale is compared with other existing scales, T58 and NBS (1955), in regions of overlap, the agreement with NBS (1955) is excellent but noticable departures from T58 - the helium-4 vapor pressure scale - are indicated.