Metrological traceability of oceanographic salinity measurement results

Abstract

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 S P , Reference Salinity S R , and the latest estimates of Absolute Salinity using the TEOS-10 formalism. On 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 −1 ". 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.

Metrological challenges for measurements of key climatological observables: oceanic salinity and pH, and atmospheric humidity. Part 1: overview

“…Such links may consist of equations (or “conventions”), such as the TEOS-10 equation for the density of standard seawater which permits the calculation of Absolute Salinity from measurements of temperature, pressure and density, all of the latter traceable to the SI (Seitz et al, 2011). A similar approach is possible for the relative fugacity making use of the TEOS-10 equation of state of humid air (Feistel, 2012; Feistel et al, 2015).…”

“…An uncertainty level of 0.002 g kg −1 in dissolved mass fraction (i.e., a relative uncertainty of 6 × 10 −5 for typical seawater with a dissolved mass fraction of about 35 g kg −1 ) is required for routine research and monitoring purposes (SUN, 1985; Seitz et al, 2011). Significant efforts have been made to ensure consistency of salinity measurements to this level over the past century; unfortunately, no robust link has yet been established between any of the salinity definitions and the International System of Units (SI) despite the fact that Practical Salinity was recommended for oceanography in the context of SI units (SUN, 1985; Siedler, 1998).…”

“…A proposed new concept that takes advantage of currently available density measurement technology and at the same time leaves established oceanographic practice largely unaffected is a combination of conductivity and SI-traceable density measurement (Seitz et al, 2011). In this concept, the salinity of SSW samples can be additionally certified (or at least checked) by density measurements in combination with the TEOS-10 equation of state.…”

Best Practices for the Ocean Moored Observatories

“…IAPSO Standard Seawater is used as a primary standard for the conductivity calibration (Seitz et al, 2011;Integrated Ocean Observing System, 2016). The practical salinity of IAPSO is obtained by a series of conductance measurements at OSIL relative to KCL standard solutions, prepared using precise weights of KCL crystals (Bacon et al, 2007).…”

Preface "An historical perspective on the development of the Thermodynamic Equation of Seawater – 2010"

“…However, a widely accepted international standard for temperature does exist -the International Temperature Scale of 1990(ITS-90, Preston-Thomas, 1990)and the technical effort required in maintaining this standard occurs outside the oceanographic community. More problematic is the fact that absolute measurements of electrical conductivity, traceable to the seven base units of the International System of Units (SI), can still not be made accurately enough to support the needs of oceanographers (Seitz et al, 2011). Physical oceanographers must then grapple with fundamental measurement issues in a way that does not occur in (and is not easily appreciated by) other areas of earth science.…”