Progress of acoustic gas thermometry using a cylindrical resonator at temperatures up to 809 K

We are extending acoustic gas thermometry (AGT) to temperatures up to 809 K with low uncertainties to measure the difference between the thermodynamic temperature T and the temperature T90 on the International Temperature Scale of 1990 (ITS-90). A cylindrical resonator was machined from a Ni-Cr-Fe alloy and acoustic and microwave waveguides welded to the cavity's endplates transmitted sound and microwaves to and from the cavity. A long-stemmed standard platinum resistance thermometer (LSPRT) measured T90 at the cavity wall. Pure argon gas flows continuously through the cavity to minimize the effects of desorption from the cavity walls. The resonator was hung in a pressure vessel surrounded by a radiation shield, a heat pipe and a furnace. We measured the acoustic resonant frequencies in argon in the range 580 to 809 K with relative standard uncertainty (3-5)×10-6. We applied a second-order boundary-layer correction to these frequencies. Each microwave resonance frequency was measured with a relative standard uncertainty of 3×10-7. The inconsistency between four independent microwave modes was 2.2×10-6 at 809 K, which contributed an uncertainty of 3.6 mK to the determination of T. The inconsistencies were not temperature dependent, which showed the structural stability of the resonator from room temperature to 809 K. This article is part of the Theo Murphy meeting issue 'The redefined kelvin: progress and prospects'.