Final report of key comparison CCM.P-K3 absolute pressure measurements in gas from 3×10-6 Pa to 9×10-4 Pa

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 th