Experimental results from four recent research reports on the determination of the density/temperature relationship of Standard Mean Ocean Water (SMOW) under a pressure of 101 325 Pa are analysed and a new formula is recommended for metrological applications. This paper determines the formulae of density and relative density, with their uncertainties, in the temperature range 0 °C to 40 °C. The uncertainty estimation of one of the reports included in the analysis has been re-evaluated. Effects on water density due to isotopic mixtures other than SMOW, ambient pressures different from 101 325 Pa, and the presence of dissolved air, are also reviewed.

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

An experiment to measure the change in density of water which has been saturated with air is described and the results given. The 80 points taken in the range 4°C to 20°C have been fitted to the equation Ap = -4.612 t 0.106 t w i t h d e n ~i t y i n l o -~ kgm-' andtin O C , and confidence limits have been given. Other results are discussed.

To determine a new value of the Avogadro constant with a relative combined standard uncertainty of 2 × 10 −8 , the mass determination of a 1 kg 28 Si sphere is crucial and should be determined with the highest level of accuracy. In the next two years the laboratories involved in the International Avogadro Project should be able to determine the mass of a 1 kg silicon sphere under vacuum with a combined standard uncertainty within 5 µg. To obtain such a target it is essential to gain experience and to promote cooperation in mass measurement on silicon spheres among the laboratories involved. For this purpose an international comparison has been performed to evaluate the weighing procedure and to reveal the difficulties encountered in the mass determination of a silicon sphere.This particular comparison, which is the first international mass comparison under vacuum conditions, has demonstrated that the reference value for the mass of a 1 kg silicon sphere can be determined traceable to the International Prototype with a standard uncertainty of 4.0 µg.

The change in the density of water when nitrogen, oxygen, and argon are dissolved has been measured from 3 to 21 °C and the weighted sum of the results compared with the result for air. Good agreement was found, contrary to earlier reports. The partial molar volumes for the three gases have been calculated. These results are discussed in the light of current theories of liquid water.

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