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Traceability of practical salinity to the SI

"Salinity" designates the salt content in seawater, is an important measurand in oceanography and is stored in databases for the global recording and monitoring of oceanographic and climatic parameters. Salinity is, in particular, needed for the calculation of density changes in ocean waters which are the driving force behind the large ocean currents and have a significant influence on climate. Therefore, measurement results must be comparable over long periods of time with measurement uncertainties as small as possible.

The chemical analysis of ocean water is, however, time-consuming and requires experienced staff. It is therefore unsuitable for practical application when it comes to producing a global salinity profile of the ocean over long periods of time. Since the conductivity of seawater depends on the concentration of the salts dissolved in the water, it can be used as a measurand of the amount of dissolved salts. The measuring devices used for these measurements are relatively easy to operate and can, for example, be fitted into buoys and depth probes for automated operation. The conversion of a measured conductance value into the salt content is done in accordance with the Practical Salinity Scale of 1978 (PSS-78) (Opens external link in new windowUNESCO Technical Papers in Marine Science 37). The salt content determined in this way is called "practical salinity".

 



This map shows the seawater salinity distribution in the oceans according to the Practical Salinity Scale. The picture also shows the thermohaline circulation. The warm surface currents are shown in red.

Image courtesy of Richard Simmon/NASA

Measuring devices used to determine practical salinity are calibrated with so-called Standard Sea Water (SSW). This is natural seawater taken from the North Atlantic Ocean which has been diluted until a measuring device for conductance value measurements displays the same conductance value as for a KCl solution that is precisely defined in PSS-78. The ratio of the two conductance values is noted on the SSW bottles by the manufacturer. Devices for measuring practical salinity are then adjusted in such a way that they indicate the conductance value ratio that is indicated on the SSW bottle during a measurement of SSW. If the conductance value of a seawater sample with unknown salinity is subsequently measured with this device, the result received is the conductance value ratio of this sample in relation to the SSW or in relation to the defined KCl solution, respectively. From the conductance value ratio determined in this way, the practical salinity of the sample is finally calculated according to PSS-78. SSW thus serves as a primary standard for the measurand "conductance value ratio" or for the practical salinity calculated from it.

In contrast to what is usual in conductivity measurements, here the measured conductance value is not adapted to the conductivity of a reference solution whose conductivity is measured traceable to the SI in a metrological institute by means of a primary procedure. This is due to the fact that the relative measurement uncertainties of the reference solutions (approximately 0.02 %) are by roughly one order of magnitude greater than when using conductance value ratios because systematic measurement errors are largely cancelled out by the ratio formation. Due to the ratio formation, the relation to the standards of the SI gets, on the other hand, lost and it is impossible to verify if the different SSW batches are really identical over a long period of time. In order to ensure the comparability of the measurement results of practical salinity over a long period of time, the conductance value ratio must repeatedly be replicated within the narrow uncertainty limits required. To examine these replications, comparison measurements are conducted with SSW samples from different batches. SSW, though, has a limited life, which implies that this type of analysis can only be reliably conducted for a period of approximately 5 years. In addition, minor variations in the production of SSW can add up in an unfavorable way over decades.

Within the scope of the European research project Opens external link in new windowEMRP ENV05 "Ocean", PTB has measured the density and conductance value ratios of SSW under pressure and temperature conditions that are typical of oceanography. This made it possible to establish an empirical relationship between the conductance value ratio of SSW and its density that can be measured in relation to the SI with a sufficiently small measurement uncertainty (approximately 2–3 ppm). By means of a high-precision density measurement, the conductance value ratio of a new SSW batch which has been determined according to PSS-78 can, in future, be calibrated at PTB. To this end, the conductance value ratio will be compared with the conductance ratio calculated from density, so that traceability to SI standards which remain stable in the long run can be established.

While the first project was primarily devoted to the characterization of SSW, conductance sensors that will be deployed in the deep sea are now to be characterized within the scope of a new metrological research project. For this purpose, the measuring set-up will be extended to enable the integration of commercial conductance sensors. Then, the measurement results of the sensors obtained by means of the currently used non-SI-traced calibration practice will be compared with those obtained by means of the SI-traced conductance value ratio/density relation and evaluated. If the measuring set-up proves effective, it will be used in the medium term for calibrating sensors of this type.