Uncertainty evaluation of the static expansion system and its long-term stability at NMIJ

A pressure standard based on the static expansion of gases has been made available at the National Metrology Institute of Japan (NMIJ) for gauge calibrations in the range of 0.1 mPa–2 kPa over the past 30 years. The static expansion system (SES) generates calibration pressures by multiple-step gas expansions using two expansion ratios (R900≃907.3 and R50≃53.4). These expansion ratios were determined using the two reference gauges method instead of the one reference gauge method, which has been in use thus far. Relative uncertainties of 0.073% (k = 1) for R900 and 0.036% (k = 1) for R50 were obtained, and the calibration uncertainties were slightly improved compared to the current calibration and measurement capability (CMC) of the SES at NMIJ as follows:U = 0.28% (k = 2) (1 mPa–10 Pa) for spinning rotor gauges (SRGs).U = 0.23% (k = 2) (0.2 Pa–10 Pa) for capacitance diaphragm gauges (CDGs).U = 0.17% (k = 2) (10 Pa–110 Pa) for CDGs. In addition, the calibration results of the check standards of the past 12 years are reported. The data suggest that the SES was stable compared to the calibration uncertainty over the years. •Calibration uncertainty of the static expansion system at NMIJ was evaluated.•It is available for vacuum gauge calibrations in the range of 0.1 mPa–2 kPa.•Calibration uncertainty is about 0.3% for SRGs and CDGs.•Its expansion ratios were determined using the two reference gauges method.•Calibration results of the check standards of the past 12 years are reported.