Comparison of the standards for high and ultrahigh vacuum at three national standards laboratories

Abstract

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.