Abstract. This report summarizes the results of a regional key comparison (APMP-IC-2-97) under the aegis of Asia Pacific Metrology Program (APMP) for pressure measurements in gas media and in gauge mode from 20 kPa to 105 kPa. The transfer standard was a pressure-balance with a piston–cylinder assembly with nominal effective area 335.7 mm² (TL-391) and was supplied by the National Metrology Institute of Japan [NMIJ]. Nine standards laboratories from the APMP region with one specially invited laboratory from the EUROMET region, namely Physikalisch-Technische Bundesanstalt (PTB), Germany, participated in this comparison. The comparison started in October 1998 and was completed in May 2001. The pilot laboratory prepared the calibration procedure as per the guidelines of APMP and International Bureau of Weights and Measures (BIPM) [1–4]. Detailed instructions for performing this key comparison were provided in the calibration protocol and the required data were described in: (1) Annex 3 – characteristics of the laboratory standards, (2) Annex 4 – the effective area (A'_p'/mm²) (where the prime indicates values based on measured quantities) at 23 °C of the travelling standard as a function of nominal pressure (p'/kPa) (five cycles both increasing and decreasing pressures at five predetermined pressure points) and (3) Annex 5 – the average effective area at 23 °C (A'_p'/mm²) obtained for each pressure p'/kPa with all uncertainty statements. The pilot laboratory processed the information and the data provided by the participants for these three annexes, starting with the information about the standards as provided in Annex 3. Based on this information, the participating laboratories are classified into two categories: (I) laboratories that are maintaining primary standards, and (II) laboratories that are maintaining standards loosely classified as secondary standards with a clear traceability from other established national metrology institutes (NMI) as per norm of the BIPM. It is observed that out of these ten laboratories, six laboratories have primary standards [Category (I)]; the remaining four laboratories are placed in Category (II).

During the process of comparison, there was a loss of mass of the piston TL-391 by an amount of roughly 23.4 mg in two instances, originally noticed at NMIJ and then at NMISA within the first year of circulation. Unfortunately, we do not have any information where it had happened and when. But after that, there was no change of mass of the piston for nearly two years till the end of this comparison. NMIJ reported that it happened due to the damage of the pin attached to the piston. Interestingly, loss of mass of the piston by an amount of 7.6 mg was reported in the transit from NMIJ to NMISA and within one month. Since the damage and subsequent loss of mass occurs instantaneously, we believe that the first damage had happened close to NMIJ and the second at NMISA where the necessary corrections have been already made. Therefore, we have not introduced any correction to any datum provided by the participating laboratories in both the phase A and phase B loops.

The obtained data were compiled and processed under the same program as per the Consultative Committee for Mass and Related Quantities (CCM)/BIPM guidelines [5]. Following the approach of Elster et al [6], we have evaluated these results and establish a link with CCM.P-K6 through the link laboratory PTB (Germany). From the CCM.P-K6 key comparison reference value (KCRV), we have estimated the relative deviation of A'₀/mm² from the reference value for all ten laboratories and compared this with their estimated expanded uncertainties at k = 2. The bilateral degree of equivalence of the participating laboratories in the APMP.M.P-K6 comparison has been estimated from the degree of equivalence between two laboratories. These results show an excellent agreement of all participating laboratories within the estimated expanded uncertainties using a coverage factor k = 2.

Main text. To reach the main text of this Paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.

The final report has been peer-reviewed and approved for publication by the CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).