COMPARISON OF THE STANDARDS FOR HIGH AND ULTRAHIGH-VACUUM AT 3 NATIONAL STANDARDS LABORATORIES

A preliminary phase of an international comparison of standards for hi gh and ultrahigh vacuum was carried out by the National Institute of S tandards and Technology (NIST), the National Physical Laboratory-Teddi ngton (NPL-UK), and the Physikalisch-Technische Bundesanstalt (PTB). A spinning rotor gauge (SRG), a Bayard-Alpert gauge (BAG) and an extrac tor gauge (EXG) were chosen as transfer standards. The comparison was carried out in a star-like pattern with PTB as pilot laboratory. The a rgon pressures generated by the standards at 9 x 10(-4) Pa were compar ed by measuring the accommodation coefficient of the SRG, the pressure s from 3 x 10(-7) Pa to 9 x 10(-4) Pa by measuring the sensitivity of the two ionization gauges. The accommodation coefficients determined a t NIST and PTB indicate a difference between the pressures generated b y the NIST and PTB standards of (P-NIST-P-PTB)/(PTB)=(0.09 +/- 0.11)% (standard or one-sigma uncertainty). For the pressures between 3 x 10( -7) Pa and 9 x 10(-4) Pa the results obtained at NIST and PTB had a me an difference of [(P-NIST-P-PTB)/P-PTB]=(0.24+/-0.12)%, With a maximum difference of (1.2+/-0.6) at the lowest pressure. The NIST-PTB differ ences are all within the combined uncertainties of the two standards. Large transfer standard instabilities and inconsistent results in a fi rst NPL-PTB comparison prompted a repeat set of measurements. An avera ge of the two sets of SRG measurements indicates a significant differe nce between NPL and PTB standards of (P-NPL-P-PTB)/P-PTB = (1.31 +/- 0 .14)%. The two sets of ionization gauge measurements effectively repea ted, but the results for the individual gauges are inconsistent. The E XG results indicate no significant pressure dependence in the differen ce between the standards for pressures below 9 x 10(-4) Pa. The BAG re sults indicate a significant increase in the difference between the st andards as the pressure is reduced, with (P-NPL-P-PTB)/P-PTB becoming as large as 5% or 6% at the lower pressures. Several potential problem s with the BAG operation were identified, which might indicate that mo re confidence should be placed in the EXG results. However, there is n o reliable evidence that the BAG comparison results were actually affe cted, and the results for each gauge are so internally consistent that neither can be rejected. Further work should be directed towards reso lving this discrepancy. (C) 1997 American Vacuum Society.