Correction of NPL-2013 estimate of the Boltzmann constant for argon isotopic composition and thermal conductivity

• Published in: Metrologia Vol. 52; no. 5; pp. S353 - S363
• Main Authors: de Podesta, Michael, Yang, Inseok, Mark, Darren F, Underwood, Robin, Sutton, Gavin, Machin, Graham
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 01.10.2015
• Subjects: Argon; Argon isotopes; argon molar mass; Atmospherics; Boltzmann constant; Calibration; Constants; Estimates; Estimating techniques; Heat conductivity; Heat transfer; Isotopes; Jargon; Measurement; Thermal conductivity
• ISSN: 0026-1394; 1681-7575
• DOI: 10.1088/0026-1394/52/5/S353

In 2013, a team from NPL, Cranfield University and SUERC published an estimate of the Boltzmann constant based on precision measurements of the speed of sound in argon. A key component of our results was an estimate of the molar mass of the argon gas used in our measurements. To achieve this we made precision comparison measurements of the isotope ratios found in our experimental argon against the ratios of argon isotopes found in atmospheric air. We then used a previous measurement of the atmospheric argon isotope ratios to calibrate the relative sensitivity of the mass spectrometer to different argon isotopes. The previous measurement of the atmospheric argon isotope ratios was carried out at KRISS using a mass spectrometer calibrated using argon samples of known isotopic composition, which had been prepared gravimetrically. We report here a new measurement made at KRISS in October 2014, which directly compared a sample of our experimental gas against the same gravimetrically-prepared argon samples. We consider that this direct comparison has to take precedence over our previous more indirect comparison. This measurement implies a molar mass which is 2.73(60) parts in 106 lighter than our 2013 estimate, a shift which is seven times our 2013 estimate of the uncertainty in the molar mass. In this paper we review the procedures used in our 2013 estimate of molar mass; describe the 2014 measurement; highlight some questions raised by the large change in our estimate of molar mass; and describe how we intend to address the inconsistencies between them. We also consider the effect of a new estimate of the low pressure thermal conductivity of argon at 273.16 K. Finally we report our new best estimate of the Boltzmann constant with revised uncertainty, taking account of the new estimates for the molar mass and the thermal conductivity of the argon.