A key comparison of low absolute pressure standards, organized under the auspices of the Consultative Committee for Mass and Related Quantities (CCM), was carried out at seven national metrology institutes (NMIs) between March 1998 and September 1999 in order to determine the degrees of equivalence of the standards at pressures in the range 1 Pa to 1000 Pa. The primary standards, which represent two principal measurement methods, included five liquid-column manometers and four static expansion systems. The transfer standard package consisted of four high-precision pressure transducers: two capacitance diaphragm gauges to provide high resolution at low pressures, and two resonant silicon gauges to provide the required calibration stability. Two nominally identical transfer packages were used to reduce the time required for the measurements, with Package A being circulated among laboratories in the European region (Istituto di

This report describes a CCM key comparison of absolute pressure at five National Metrology Institutes (NMIs) that was carried out from August 1998 to May 2002. The goal of the key comparison was to determine the degree of equivalence of NMI standards at pressures in the range of 3×10 -6 Pa to 9×10 -4 Pa. The primary standards were dynamic expansion standards at four of the NMIs and a series expansion standard at the fifth NMI. The transfer standard package consisted of two spinning rotor gauges (SRGs) and three Bayard-Alpert ionization gauges. Due to equipment malfunctions, only one of the ionization gauges was calibrated by all of the participants. The SRG measurements were used to compare NMIs at 9×10 -4 Pa and to normalize the ionization gauge results at that same pressure. The ionization gauge measurements were used to compare NMIs at the lower pressures. The degrees of equivalence of the NMI standards were determined in two ways: deviations from the key comparison reference value (KCRV), and pairwise differences between those deviations. The standards of four of the NMIs show equivalence to the KCRV and each other over the full range of pressures relative to the expanded uncertainties of the comparisons at the k=2 level. The standard of one NMI was equivalent to the KCRV at 3×10 -6 Pa only, and showed lack of equivalence to the standards of one or more NMIs in the range of 9×10 -6 Pa to 9×10

A preliminary phase of an international comparison of standards for high and ultrahigh vacuum was carried out by the National Institute of Standards and Technology (NIST), the National Physical Laboratory-Teddington (NPL-UK), and the Physikalisch-Technische Bundesanstalt (PTB). A spinning rotor gauge (SRG), a Bayard-Alpert gauge (BAG) and an extractor gauge (EXG) were chosen as transfer standards. The comparison was carried out in a star-like pattern with PTB as pilot laboratory. The argon pressures generated by the standards at 9×10−4 Pa were compared by measuring the accommodation coefficient of the SRG, the argon pressures from 3×10−7 Pa to 9×10−4 Pa by measuring the sensitivity of the two ionization gauges. The accommodation coefficients determined at NIST and PTB indicate a difference between the pressures generated by the NIST and PTB standards of (PNIST−PPTB)/PPTB=(0.09±0.11)% (standard or one-sigma uncertainty). For the pressures between 3×10−7 Pa and 9×10−4 Pa the results obtained at NIST and PTB had a mean difference of 〈(PNIST−PPTB)/PPTB〉=(0.24±0.12)%, with a maximum difference of (1.2±0.6) at the lowest pressure. The NIST-PTB differences are all within the combined uncertainties of the two standards. Large transfer standard instabilities and inconsistent results in a first NPL-PTB comparison prompted a repeat set of measurements. An average of the two sets of SRG measurements indicates a significant difference between NPL and PTB standards of (PNPL−PPTB)/PPTB=(1.31±0.14)%. The two sets of ionization gauge measurements effectively repeated, but the results for the individual gauges are inconsistent. The EXG results indicate no significant pressure dependence in the difference between the standards for pressures below 9×10−4 Pa. The BAG results indicate a significant increase in the difference between the standards as the pressure is reduced, with (PNPL−PPTB)/PPTB becoming as large as 5% or 6% at the lower pressures. Several potential problems with the BAG operation were identified, which might indicate that more confidence should be placed in the EXG results. However, there is no reliable evidence that the BAG comparison results were actually affected, and the results for each gauge are so internally consistent that neither can be rejected. Further work should be directed towards resolving this discrepancy.

No abstract

The Bureau International des Poids et Mesures has initiated world-wide intercomparisons of pressure standards. The intercomparison in the regime of very low pressures, i.e. 10-4 to 1 Pa, using Ar as the test gas, has now been completed with the participation of laboratories in nine countries (in alphabetical order): Czechoslovakia, Federal Republic of Germany, France, India, Italy, Japan, People's Republic of China, UK and United States of America. In addition, five of these laboratories made measurements using H2 as the test gas. Four spinning-rotor gauges were employed as transfer standards. The achieved transfer uncertainty between the pilot laboratory (Physikalisch-Technische Bundesanstalt) and a participating laboratory lies between 0.16% and 0.30% (1σ) for Ar and has allowed an intercomparison at this level of accuracy. At all investigated pressures the standards in the various laboratories lie within an interval of ±3% around the standard of the pilot laboratory. For a brief synopsis of the intercomparison results, the calibration data of each laboratory for one gas species are reduced to a single number, i.e., the average over the pressure range 10-3 to 1 Pa, although this procedure is somewhat arbitrary and obscures scatter and pressure-dependence of some calibration results. These averages deviate from their mean by not more than about 1%. Systematic deviations between some laboratories are clearly identifiable: however, for most of the laboratories it appears that these are compatible with the estimated uncertainties of the pressure generation and the transfer uncertainty.