Acoustic gas thermometry

We review the principles, techniques and results from primary acoustic gas thermometry
\n(AGT). Since the establishment of ITS-90, the International Temperature Scale of 1990,
\nspherical and quasi-spherical cavity resonators have been used to realize primary AGT in the
\ntemperature range 7K to 552 K. Throughout the sub-range 90K < T < 384 K, at least two
\nlaboratories measured (T − T90). (Here T is the thermodynamic temperature and T90 is the
\ntemperature on ITS-90.) With a minor exception, the resulting values of (T −T90) are mutually
\nconsistent within 3 × 10−6 T . These consistent measurements were obtained using helium and
\nargon as thermometric gases inside cavities that had radii ranging from 40mm to 90mm and
\nthat had walls made of copper or aluminium or stainless steel. The AGT values of (T − T90)
\nfall on a smooth curve that is outside ±u(T90), the estimated uncertainty of T90. Thus, the
\nAGT results imply that ITS-90 has errors that could be reduced in a future temperature scale.
\nRecently developed techniques imply that low-uncertainty AGT can be realized at
\ntemperatures up to 1350K or higher and also at temperatures in the liquid-helium range.