Since the beginning of measurement of pressure in the 17th century, the unit of pressure has been defined by the relationship of force per unit area. The present state of optical technology now offers the possibility of using a thermodynamic definition-specifically the ideal gas law-for the realization of the pressure unit, in the vacuum regime and slightly above, with an accuracy comparable to or better than the traditional methods of force per area. The changes planned for the SI in 2018 support the application of this thermodynamic definition that is based on the ideal gas law with the necessary corrections for real-gas effects. The paper reviews the theoretical and experimental foundations of those optical methods that are considered to be most promising to realize the unit of pressure at the highest level of metrology.

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A new primary standard for the realization of vacuum pressures between 10 -10 Pa and 3 ϫ 10 -2 Pa has been completed at the Physikalisch-Technische Bundesanstalt (PTB). It is based on the continuous expansion method, whereby the range is extended to lower pressures by the use of a flow divider. In this flow divider, the gas flow from the fully automated flowmeter with throughputs ranging from 10 -6 Pa l/s up to 3 Pa l/s (at 23 C) is directed via two differently sized orifices into two chambers: the XHV calibration chamber, which is designed for calibration pressures 10 -10 Pa to 10 -4 Pa, takes up 1 % of the flow, while the UHV calibration chamber, which is designed for pressures 10 -8 Pa to 3 ϫ 10 -2 Pa, takes up 99 % of the flow. Both calibration chambers are operated with cryopumps during calibration. The relative standard uncertainties of pressures generated by this new standard range from 7 ϫ 10 -3 at 10 -10 Pa, when a perfectly stable residual pressure is assumed, to 2 ϫ 10 -3 at 10 -4 Pa.

For applications in vacuum metrology, gas flowmeters have to deliver pure gas flows with well-known molar flow rates of low ( mol/s) and very low values. Accurate flowmeters are used to generate pressures in continuous expansion systems, to measure leak rates, and to measure pumping speeds of ultrahigh-vacuum pumps. At the Physikalisch-Technische Bundesanstalt (PTB) a new, fully automated, all-metal flowmeter was commissioned for use with a continuous expansion system in the pressure range Pa to Pa. It delivers gas flows with molar flow rates between mol/s and mol/s (throughputs Pa l/s to 3 Pa l/s at 23 C) with relative standard uncertainties ranging from 1.45 % at mol/s down to 0.14 % at mol/s.

Recently, results for the CO(2) R(12) line strength parameter have been reported, which differ significantly and are inconsistent with respect to quoted uncertainties. We investigate to what extent this inconsistency might be caused by the chosen data analysis methods. To this end, we assess and compare a parametric fitting procedure and a non-parametric approach. We apply the methods to simulated and measured line spectra, and we specify the conditions required for the safe application of the two procedures. For our present data, the corresponding conditions are satisfied for both methods, and consistent results are obtained. However, the simulations reveal that the fitting procedure can show shortcomings when the uncertainty in the wavenumber is large.

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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