A key comparison of low absolute pressure standards, organized under the auspices of the Consultative Committee for Mass and Related Quantities (CCM), was carried out at seven national metrology institutes (NMIs) between March 1998 and September 1999 in order to determine the degrees of equivalence of the standards at pressures in the range 1 Pa to 1000 Pa. The primary standards, which represent two principal measurement methods, included five liquid-column manometers and four static expansion systems. The transfer standard package consisted of four high-precision pressure transducers: two capacitance diaphragm gauges to provide high resolution at low pressures, and two resonant silicon gauges to provide the required calibration stability. Two nominally identical transfer packages were used to reduce the time required for the measurements, with Package A being circulated among laboratories in the European region (Istituto di

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Within the Euromet region a regional key comparison (Euromet.M.P-K1.b) was carried out in order to compare national vacuum standards in the pressure range from 3 × 10-4 Pa to 0.9 Pa. The participants were the BNM-LNE (France), CEM (Spain), IMGC-CNR (Italy), IMT (Slovenia), NPL (United Kingdom), UME (Turkey), and the PTB (Germany) as pilot laboratory. The measurements were carried out from April 2000 to February 2002.Two spinning rotor gauges served as transfer standards and showed a good transport stability. The effective accommodation coefficients of the rotors had to be determined at eight target points at and between 3 × 10-4 Pa and 0.9 Pa. The uncertainty of the generated pressure in the calibration standard was reported as part of the calibration report by each laboratory. All additional uncertainties that were related to the transfer standard were evaluated by the pilot laboratory in order to have a uniform uncertainty analysis for all participants and in order to emphasize the importance of the reported uncertainty of the generated pressure.From the available data a Euromet reference value was calculated at each target pressure. The results from most of the laboratories showed a good agreement with the reference value within the combined uncertainties. A few values of three of the laboratories were significantly off the reference value.At the highest target pressure of 0.9 Pa a linkage to the lowest target pressure at 1 Pa of the key comparison CCM.P-K4 was possible by means of the results of three laboratories that took part in both comparisons.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 Mutual Recognition Arrangement (MRA).

A homodyne Michelson interferometer was developed to realize a dynamic vacuum standard. The interferometer measures variation in optical path due to refractive index changes related to the pressure of the gas. The measurement arm of the interferometer is formed by two quasi-parallel mirrors which act as a multiplication set-up to allow an increment of the optical path and consequently of the sensitivity. The interference signal is detected by a high speed camera: starting from the recorded interference pattern, two quadrature regions are identified and analyzed by custom software to obtain the quadrature phase signals. The dynamic vacuum system is mainly composed of a large low-pressure chamber VA (about 800 L) connected by a valve and a replaceable orifice to a high pressure chamber VB of about 2 L, hosting the interferometer. The fast pressure drop from 100 kPa to 100 Pa is obtained by a gas expansion from VB to VA. The velocity of the expansion process can be easily varied by substituting the orifice connecting the two chambers. The response of the system was first tested with a slow process of about 40 s at different gains of the measurement arm of the interferometer. Subsequently, a fast process (< 3 s) was considered and the result of the optical device was compared to the measurements performed by two capacitance diaphragm gauges (133 kPa and 1.33 kPa full scale). The gauges are equipped with special electronics to give each nominal reading every 0.7 ms. The two measurements performed by the dynamic vacuum standard and capacitance diaphragm gauges showed an agreement better than 12%.

A primary system based on the so-called static expansion of gases has been made available at IMGC-CNR for gauge calibration in the range 0.1 Pa to 1000 Pa.As is known, in such systems the generated pressure is calculated by application of the ideal gas law and is a function of the inlet pressure, the temperatures of the various volumes and their ratios.The uncertainty evaluation of the pressure measured with such system is the main subject of this paper.At present, the system is operated with three volumes (≈0.01 litre, ≈0.5 litre and ≈50 litre) and two expansion ratios (R 4 ∼ = 4761 and R 3 ∼ = 95) determined by application of the multiple expansion method through pressure measurements performed with the best transfer gauges available at IMGC-CNR. The characteristics of such transfer gauges are discussed with respect to two different conditions of use: that is, for determination of the expansion ratio, R j , and for measurements of the inlet pressure.The combined expanded uncertainty of the generated pressure (p xi ) is then given byPa in the pressure range 0.1 Pa to 10 Pa and U(p xi )/Pa = 1.7 × 10 −2 + 8.2 × 10 −4 p xi /Pa in the range 10 Pa to 1000 Pa.Considerations on participation in comparisons are also presented.