Steffen Seitz

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.

Uncertainty of empirical correlation equations

The International Association for the Properties of Water and Steam (IAPWS) has published a set of empirical reference equations of state, forming the basis of the 2010 Thermodynamic Equation of Seawater (TEOS-10), from which all thermodynamic properties of seawater, ice, and humid air can be derived in a thermodynamically consistent manner. For each of the equations of state, the parameters have been found by simultaneously fitting equations for a range of different derived quantities using large sets of measurements of these quantities. In some cases, uncertainties in these fitted equations have been assigned based on the uncertainties of the measurement results. However, because uncertainties in the parameter values have not been determined, it is not possible to estimate the uncertainty in many of the useful quantities that can be calculated using the parameters. In this paper we demonstrate how the method of generalised least squares (GLS), in which the covariance of the input data is propagated into the values calculated by the fitted equation, and in particular into the covariance matrix of the fitted parameters, can be applied to one of the TEOS-10 equations of state, namely IAPWS-95 for fluid pure water. Using the calculated parameter covariance matrix, we provide some preliminary estimates of the uncertainties in derived quantities, namely the second and third virial coefficients for water. We recommend further investigation of the GLS method for use as a standard method for calculating and propagating the uncertainties of values computed from empirical equations.

Metrology for pH Measurements in Brackish Waters—Part 1: Extending Electrochemical pHT Measurements of TRIS Buffers to Salinities 5–20

Harned cell pHT measurements were performed on 2-amino-2-hydroxymethyl-1,3-propanediol (TRIS) buffered artificial seawater solutions in the salinity range 5 20, at three equimolal buffer concentrations (0.01, 0.025, 0.04 mol·kg H2O 1), and in the temperature range 278.15 – 318.15 K. Measurement uncertainties were assigned to the pHT values of the buffer solutions and ranged from 0.002 to 0.004 over the investigated salinity and temperature ranges. The pHT values were combined with previous results from literature covering salinities from 20 to 40. A model function expressing pHT as a function of salinity, temperature and TRIS/TRIS·H+ molality was fitted to the combined data set. The results can be used to reliably calibrate pH instruments traceable to primary standards and over the salinity range 5 to 40, in particular, covering the low salinity range of brackish water for the first time. At salinities 5-20 and 35, the measured dependence of pHT on the TRIS/TRIS·H+ molality enables to extrapolate quantities calibrated against the pHT values, e.g. the dissociation constants of pH indicator dyes, to be extrapolated to zero TRIS molality. Extrapolated quantities then refer to pure synthetic seawater conditions and define a true hydrogen ion concentration scale in seawater media.

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.

EURAMET.QM-S7. Comparison meaurement of electrolytic conductivity at pure water level

In this comparison the equivalence of conductivity measurement results has been investigated in the conductivity range from ultrapure water level (5.5 μS m-1) up to 1500 μS m-1). The measurements were performed in a closed pure water loop with primary methods and with commercial devices. To this end the conductivity measurement cells of the participating institutes were integrated in series with the primary cell of PTB, which in this way linked the results. In general the primary methods of PTB and DFM are consistent within an expanded uncertainty of 0.5 %. A slight inconsistency at the ultrapure water level resulted from a small leakage during that measurement. SP, using a commercial device, measured consistent values. CMI, also using a commercial device, underestimated its measurement uncertainty. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Metrological Traceability Concept for Electrolytic Conductivity and pH

The metrological traceability concept links measurement results by a chain of calibrations to the quantity values of primary standards, which are realised by primary measurement procedures. These procedures undergo periodical international comparison measurements, in order to guarantee worldwide comparability of measurement results. In this article we demonstrate how the metrological traceability concept applies to electrolytic conductivity and pH measurements. Furthermore we will outline promising activities in current metrological research to extend traceability of electrolytic conductivity measurements down to the low μS cm -1 level.