Determination of water density: limitations at the uncertainty level of 1 × 10−6

Abstract

Absolute measurements of water density with very small uncertainties on the order of 0.001 kg/m3 have previously been a metrological challenge, as is shown by measurements of the density of pure water performed in recent decades with different methods. However, using water as a reference liquid with a well-known density, it is possible to perform density measurements relative to this reference liquid by means of an oscillation-type density meter. Using this so-called substitution method, it is possible to obtain uncertainties of about 0.002 kg/m3 or a relative uncertainty of 2 × 10−6. The conversion from relative to absolute measurements is performed using a water density table. The uncertainty of this absolute measurement is given by the combination of the uncertainty of the relative measurement and the uncertainty given for the density table.