We used a quasi-spherical cavity as an acoustic and microwave resonator to measure the thermodynamic temperatures, T , of the triple points of equilibrium hydrogen, neon, argon and mercury and to measure the difference T − T 90 , in the range 7 K to 273 K. (T 90 is the temperature on the International Temperature Scale of 1990 (ITS-90).) In the range 7 K to 24.5 K, our preliminary values of T − T 90 agree with recent results from dielectric-constant gas thermometry and achieve uncertainties that are comparable to or smaller than those achievable using the interpolating constant volume gas thermometer as currently defined on the ITS-90. In the range 90 K to 273 K, the present results for T − T 90 obtained using a helium-filled, copper-walled, quasi-spherical cavity agree with earlier results obtained using argon-filled, steel-walled or aluminium-walled, spherical cavities. The agreement confirms our understanding of both acoustic and microwave cavity resonators and demonstrates that resonators function as primary thermometers spanning wide temperature ranges. The mutually consistent acoustic thermometry data from several laboratories imply that the values of (T − T 90)/T 90 are 5 times larger than the uncertainty of T /T 90 near 150 K and near 400 K. They also imply that the derivative dT /dT 90 is too large by approximately 10 −4 near 273.16 K and that dT /dT 90 has a discontinuity of 4 × 10 −5 at 273.16 K.

We review the principles, techniques and results from primary acoustic gas thermometry (AGT). Since the establishment of ITS-90, the International Temperature Scale of 1990, spherical and quasi-spherical cavity resonators have been used to realize primary AGT in the temperature range 7 K to 552 K. Throughout the sub-range 90 K < T < 384 K, at least two laboratories measured (T − T 90 ). (Here T is the thermodynamic temperature and T 90 is the temperature on ITS-90.) With a minor exception, the resulting values of (T − T 90 ) are mutually consistent within 3 × 10 −6 T . These consistent measurements were obtained using helium and argon as thermometric gases inside cavities that had radii ranging from 40 mm to 90 mm and that had walls made of copper or aluminium or stainless steel. The AGT values of (T − T 90 ) fall on a smooth curve that is outside ±u(T 90 ), the estimated uncertainty of T 90 . Thus, the AGT results imply that ITS-90 has errors that could be reduced in a future temperature scale. Recently developed techniques imply that low-uncertainty AGT can be realized at temperatures up to 1350 K or higher and also at temperatures in the liquid-helium range.