Henning Wolf

Metrological challenges for measurements of key climatological observables Part 2: oceanic salinity

Salinity is a key variable in the modelling and observation of ocean circulation and ocean-atmosphere fluxes of heat and water. In this paper, we examine the climatological relevance of ocean salinity, noting fundamental deficiencies in the definition of this key observable, and its lack of a secure foundation in the International System of Units, the SI. The metrological history of salinity is reviewed, problems with its current definitions and measurement practices are analysed, and options for future improvements are discussed in conjunction with the recent seawater standard TEOS-10.

Water density measurement by a magnetic flotation apparatus

A magnetic flotation apparatus for determining the density of water over a wide temperature range is described. After a short discussion of the advantages of a magnetic flotation method against the hydrostatic weighing method, the apparatus built at the Physikalisch-Technische Bundesanstalt (PTB) is described in detail. Contributions to the measurement uncertainty are discussed. The functionality of the apparatus is demonstrated with a series of water density measurements at 20 °C.

A method to measure the density of seawater accurately to the level of 10−6

A substitution method to measure seawater density relative to pure water density using vibrating tube densimeters was realized and validated. Standard uncertainties of 1 g m−3 at atmospheric pressure, 10 g m−3 up to 10 MPa, and 20 g m−3 to 65 MPa in the temperature range of 5 °C to 35 °C and for salt contents up to 35 g kg−1 were achieved. The realization was validated by comparison measurements with a hydrostatic weighing apparatus for atmospheric pressure. For high pressures, literature values of seawater compressibility were compared with substitution measurements of the realized apparatus.

Compressibility measurements using an oscillation-type density meter

Compressibility measurements performed for certified density reference liquids, for liquids commonly used in hydrostatic weighing apparatuses for measuring the density or the volume of solids and for those liquids used in the international density comparisons CCM.D-K2 and EUROMET 627 are reported.

The density–salinity relation of standard seawater

The determination of salinity by means of electrical conductivity relies on stable salt proportions in the North Atlantic Ocean, because standard seawater, which is required for salinometer calibration, is produced from water of the North Atlantic. To verify the long-term stability of the standard seawater composition, it was proposed to perform measurements of the standard seawater density. Since the density is sensitive to all salt components, a density measurement can detect any change in the composition. A conversion of the density values to salinity can be performed by means of a density–salinity relation. To use such a relation with a target uncertainty in salinity comparable to that in salinity obtained from conductivity measurements, a density measurement with an uncertainty of 2 g m−3 is mandatory. We present a new density–salinity relation based on such accurate density measurements. The substitution measurement method used is described and density corrections for uniform isotopic and chemical compositions are reported. The comparison of densities calculated using the new relation with those calculated using the present reference equations of state TEOS-10 suggests that the density accuracy of TEOS-10 (as well as that of EOS-80) has been overestimated, as the accuracy of some of its underlying density measurements had been overestimated. The new density–salinity relation may be used to verify the stable composition of standard seawater by means of routine density measurements.

Density reference liquids certified by the Physikalisch-Technische Bundesanstalt

The improvement of modern oscillation-type density meters requires density reference liquids with low uncertainty. The new apparatus for the hydrostatic measurement of density at PTB was improved to reach measurement uncertainties smaller than 0.005 kg m?3. Thus, it is now possible to deliver density reference liquids with an uncertainty of 0.01 kg m?3.