Estimates of the difference between thermodynamic temperature and the International Temperature Scale of 1990 in the range 118 K to 303 K

Abstract

Using exceptionally accurate measurements of the speed of sound in argon, we have made estimates of the difference between thermodynamic temperature, T, and the temperature estimated using the International Temperature Scale of 1990, T90, in the range 118 K to 303 K. Thermodynamic temperature was estimated using the technique of relative primary acoustic thermometry in the NPL-Cranfield combined microwave and acoustic resonator. Our values of (T-T90) agree well with most recent estimates, but because we have taken data at closely spaced temperature intervals, the data reveal previously unseen detail. Most strikingly, we see undulations in (T-T90) below 273.16 K, and the discontinuity in the slope of (T-T90) at 273.16 K appears to have the opposite sign to that previously reported.

Progress towards the determination of thermodynamic temperature with ultra-low uncertainty

“…-The AGT apparatus used at NPL is based on the resonator previously characterized for an extremely accurate determination of the Boltzmann constant k [46] with multiple modifications to the thermostatting system to cover the overall temperature range between 118 K and 303 K. The AGT was operated in relative mode, with measurements of the speed of sound in Ar recorded as a function of pressure along several (20) closely spaced isotherms, including T TPW . For this experiment, a detailed description of the separate uncertainty contributions to the determinations of T and T 90 , and a critical discussion of the results, may be found elsewhere [47]. -LNE-CNAM has designed and is currently developing a variety of apparatus and cryostats to conduct AGT experiments over the wide temperature range between 4 K and 350 K. At present, results obtained using the small volume (0.5 litre) resonator previously used for an accurate determination of k [48] and a larger volume (3.1 litre) resonator were reported at six temperatures between 77 K and 290 K [49].…”

“…Most importantly, the outstanding progress achieved by the AGT method during the last few years is well illustrated by the visual comparison of the uncertainty of the WG4 interpolation (shaded area in figure 4) with the much smaller uncertainty associated with the dense NPL dataset. By separating the contributions to the overall uncertainty of (T − T 90 ) of [47], it is evident that the uncertainty associated with the acoustic determination of T is significantly smaller than the total uncertainty which can be achieved in the realization of ITS-90 using standard platinum resistance thermometers [55]. In fact, the unprecedented precision and accuracy of NPL thermometry reveals some subtle features of the differences (T − T 90 ), previously unnoticed, and whose origin is likely to be inherent in the reference function of the ITS-90.…”

Energy accommodation coefficient extracted from acoustic resonator experiments

“…In the present work, we extract the NEAC from the literature of measurements of the acoustic resonance frequencies of gas-filled, quasi-spherical, metal-walled cavities. These well-documented measurements were performed to determine the Boltzmann constant and the thermodynamic temperature with small uncertainties using different kinds of acoustic resonators 8–24 . To attain low uncertainties, the temperature jump at the gas-surface interface is taken into account by using the model for radially-symmetric, acoustic standing waves in the cavity.…”

Linestrength ratio spectroscopy as a new primary thermometer for redefined Kelvin dissemination

“…Equally important for a proper Kelvin dissemination is to calibrate systematic discrepancies, T-T 90 between temperature points of the ITS-90 (T 90 ) and the measured T by using the new defined PTs [26,27]. Although such systematic errors seem to be small if compared with precision of ST's, they can produce measurable effects in applications that need precise temperature determinations.…”