Metrological traceability of oceanographic salinity measurement results

Abstract

Consistency of observed oceanographic salinity data is discussed with respect to contemporary metrological concepts. The claimed small uncertainty of salinity measurement results traceable to the conductivity ratio of a certified IAPSO Standard Seawater reference is not metrologically justified if results are compared on climatic time scales. This applies in particular to Practical Salinity <i>S</i><sub>P</sub>, Reference Salinity <i>S</i><sub>R</sub>, and the latest estimates of Absolute Salinity using the TEOS-10 formalism. In climate time scales an additional contribution to the uncertainty that is related to unknown property changes of the reference material must be accounted for. Moreover, when any of these measured or calculated quantity values is used to estimate Absolute Salinity of a seawater sample under investigation, another uncertainty contribution is required to quantify the accuracy of the equations relating the actually measured quantity to the Absolute Salinity. Without accounting for these additional uncertainties, such results cannot be used to estimate Absolute Salinity with respect to the International System of Units (SI), i.e. to the unit chosen for the mass fraction of dissolved material in the sample, which is "g/kg". From a metrological point of view, such deficiencies in the calculations involving other quantities will produce SI-incompatible results. We outline how these problems can be overcome by linking salinity to primary SI measurement standards

Thermodynamic properties of seawater, ice and humid air: TEOS-10, before and beyond

“…During the development of TEOS-10, much attention was paid to the question of how salinity should be defined in oceanography Seitz et al, 2011;Wright et al, 2011;Pawlowicz et al, 2016). In current practice, salinity measurement is performed by electronic sensors for pressure, temperature and conductivity.…”

“…These chains of calibration procedures establish the socalled metrological traceability of measurement results (de Bievre and Günzler, 2005;Seitz et al, 2011;VIM3, 2012). Traceability to the same primary reference is a necessary condition for the comparability of measured values from different devices at different locations or times.…”

The density–salinity relation of standard seawater

“…Both of these deficiencies entail a risk of inconsistent long-term salinity val-H. Schmidt et al: The density-salinity relation of standard seawater ues. To remedy the deficiencies, Seitz et al (2011) proposed to perform routine measurements of the standard seawater density. In practice, this would be achieved by determining the salinity of a standard seawater batch not only by conductivity measurement, but also by density measurement; the conversion to salinity is carried out in this second approach by means of a density-salinity relation.…”

Absolute Salinity, ''Density Salinity'' and the Reference-Composition Salinity Scale: present and future use in the seawater standard TEOS-10

“…Any extension must agree precisely with the Reference Salinity when the dissolved material has the composition assigned to Standard Seawater. In addition, it is desirable to introduce a measure of salinity that is traceable to the SI IOC et al, 2010) which is not achieved by the introduction of Reference Salinity (Seitz, 2010b). We shall argue that the introduction of "Density Salinity" S dens A addresses both of these issues.…”

“…An important advantage of any approach using density to specify salinity is that density measurement results that are obtained by the substitution method (for example) are traceable to the SI (Seitz et al, 2010b). Although at present this linkage is still immature, the introduction of Density Salinity should facilitate future attempts to bring ocean salinity measurements within the general framework of physical standards.…”