Purpose -A prototype gas prover was constructed to serve as the Italian primary standard for gas flow rates in the range 0.1 ml/min to 2 l/min. The new prover is used to calibrate high-quality industrial standards, as well as the MFCs used in microelectronic fabrications and preparation of reference gas mixtures. Design/methodology/approach -The prover measures gas volume transfers caused by displacements of a 120 mm dia. motor-operated piston, which is introduced into a temperature-controlled chamber containing up to 3 l of the required working gas at near ambient conditions. Gas delivery is made at constant rate, whereas possibly variable incoming flows are measured at constant pressure. Displacements of the piston are measured by an optical interferometer. Findings -The analysis shows that standard uncertainty ranges between 0.013 and 0.03 percent. Owing to the very accurate control and measurement of both pressures and temperatures, these figures refer equally to volume and mass flowrate. Experimental comparisons with similar national standards at LNE-France and NIST-USA confirmed the consistency of measurement results in the three Nations.Research limitations/implications -The gas prover should be used with inert gases only. Practical implications -The national industrial gas standards and the best flow transducers can now be calibrated accurately down to unprecedented flowrate values. Originality/value -The need for measurement of extremely low gas flows is quite recent, therefore possibly less than ten primary national standards are available today worldwide. Several completely different principles and designs have been developed; description of design and performance of each instrument is important to assess their respective merits. The described apparatus is innovative as regards measurement range, accuracy and control techniques.

A key comparison of low differential pressure standards, organized under the auspices of the Consultative Committee for Mass and Related Quantities (CCM), was carried out at four national metrology institutes (NMIs) between July 1998 and May 1999 in order to determine the degrees of equivalence of the standards at pressures in the range 1 Pa to 1000 Pa. The differential pressures were superimposed on a line pressure of nominally 100 kPa. The primary standards, which represent two principal measurement methods, included three liquid-column manometers and one double pressure balance. The transfer standard package consisted of four high-precision pressure transducers: two capacitance diaphragm gauges to provide high resolution at low differential 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 Metrologia G. Colonnetti, Italy; National Physical Laboratory, UK) and Package B in the Asia-Pacific region (Measurement Standards Laboratory of New Zealand). The results obtained were normalized using data obtained from simultaneous calibrations of the two packages at the pilot laboratory (National Institute of Standards and Technology, USA). The degrees of equivalence of the measurement standards were determined in two ways: deviations from key comparison reference values and pairwise differences between these deviations. The differential pressure standards of the participants were generally found to be equivalent and the results revealed no significant relative bias between the two principal methods tested.

Mercury manometers have been designed and constructed at the Istituto di Metrologia "G. Colonnetti" (IMGC) since 1986, their main characteristic being the adoption of cube-corner reflectors, carried by lightweight floats, to reflect the beams of a laser interferometer that measures the vertical separation between the two mercury menisci. The paper describes the most recent instrument, designated HG5, designed to measure up to 120 kPa in either absolute or relative mode. The major improvements introduced in HG5 over its prototype (HG-3) are: the adoption of larger-bore columns; the installation of a single-beam laser interferometer that allows the use of smaller and less intrusive reflecting floats; the improvement of temperature control and measurement; and the development of cat's-eye floats of a novel design allowing the direct reflection of laser beams on mercury menisci. The uncertainty budget and the equations giving the HG5 standard uncertainties (43 mPa to 314 mPa) under all conditions are included.

We compared the IMGC and NIST standards for small gas flows at the IMGC. The IMGC standard is a recently developed primary flow meter that extracts a large piston out of a temperature-controlled chamber. Controlling the piston's speed holds the pressure in the chamber constant. The NIST standard is a recently developed transfer standard that was calibrated against two primary standards at NIST. The first NIST primary extracts a large piston out of an oil-filled chamber in which a metal bellows is suspended. Controlling the piston's speed holds the pressure in the bellows constant. The second NIST primary is a static gravimetry method.The results include 49 nitrogen flow rates from 0.22 µmol s−1 to 770 µmol s−1 (1 µmol s−1 = 1.3448 cm3 min−1 of an ideal gas at the 'standard' conditions of 0 °C and 101 325 Pa). For all but one of the comparisons, the agreement between IMGC and NIST is better than 0.06%.