Study on the traceability of salinity measurements in seawater
Project Description
Salinity is one of the most important input quantities of oceanographic models. The distribution of density inside the ocean is directly related to the distribution of ocean currents. Temperature, salinity, and pressure are used to calculate density.
It is not possible to measure the absolute salinit ,SA, defined as the total proportion of dissolved material in a sea water sample, g/kg, directly. The Practical Salinity Scale PSS-78, endorsed by UNESCO/IAPSO, forms the basis of an internationally accepted method for calculating salinity from conductivity measurements. The practical salinity, S, of seawater is now defined in terms of the ratio of electrolytic conductivity of seawater to that of a potassium chloride solution. A standard seawater of S = 35 has by definition a conductivity ratio of unity at 15 °C and standard pressure with a KCl solution of mass fraction 32.4356 g/kg [UNESCO Technical Papers in Marine Science Vol 37, 1981 P.16].
Instruments for measuring salinity are calibrated using so-called Standard Seawater. This standard is made from filtered, natural sea water from the north Atlantic, diluted by distilled water. The main standard (P-series) has a salinity of ca 35. At a salinity of 35 the conductivity equals per definition 42.896 mS/cm at 15 °C and atmospheric pressure. Salinity expressed in the PSS-78 is therefore not traceable to the SI.
The objective of the proposed study is to assess the degree of equivalence in conductivity results of practical salinity measurements. The samples are standard seawater and potassium chloride solution of about 50 mS/cm at 20 °C). The measurement temperature will be 21.0 °C. The participating NMIs are asked to use primary or secondary conductivity measurement methods. Other participants should use their standard measurement procedures. The results should be expressed as conductivity in mS/cm. All participants asked to deliver a complete uncertainty budget for their results. This will allow estimating the measurement uncertainty and not only the accuracy of the measurement results.
This project is supported by the WG 127 on Thermodynamics and Equation of State of Seawater of the Scientific Committee on Oceanic Research (SCOR) and the International Association for the Physical Science of the Oceans (IAPSO).
Final Report 2007-07-19
The goal of this first pilot study was to investigate the agreement in conductivity measurements for the practical salinity determination. Against this background the comparison has led to an ambivalent result. On the one hand most of the values reported and calculated respectively, have shown a very good agreement. The relative deviation of the weighted mean of salinity from the nominal value of standard seawater P-series is 4.6x104-4 % and.1.31x10-2 % for the 10L-series.
Looking at the conductivity and salinity values of the different laboratories, most of them group closely around the weighted mean. If one excludes fhe outliners, the single values of the measurements scatter around the weighted mean within approximately 01% (relative standard deviation). The relative expanded uncertainties (k = 2) of those values close to the mean are comparable to some extend, reaching from 0.05% to 0.3%
On the other hand the indicated uncertainties have to be assessed with care for some reasons:
- Some of the reported uncertainty budgets are incomplete or estimated to optimistic
- Primary and secondary methods to determine conductivity usually refer the uncertainty budget to SI units,while the uncertain§ of salinometers is referred to a calibration measurement with a SSW sample of given K15 value, without accounting for its uncertainty. Thus, these uncertainties are not comparable
- Data which had to be corrected for temperature obviously depend on the temperature coefficient. The temperature coefficients to correct the data were taken from DFM measurements. Focussing on the results of just one lab may inkoduce a systematic error or underestimate the uncertain§ of ths temperature correction respectively
To conclude this pilot study, with the constraint of having excluded a few outliers in the assessment the nominal values of the saliniff of SSW and l(rs values respectively, have beenreproduced very well. The agreement of conductivity and salinity values is good. This indicates that in a future CCQM key comparison a degree of equivalence in the order 10-4 could be reached for SSW-P-series. But in order to come to more reliable results, more attention has to be paid to the treatment of the uncertain§. In particular all uncertainties should be traceable to SI units and temperature correction needs to be put on more solid grounds.The EUROMET Project was supported by the WG 127 on Thermodynamics and Equation of State of Seawater of the Scientific Committee on Oceanic Research (SCOR) and the International Association for the Physical Science of the Oceans [APSO).The complete report can be received from the coordinator.