OS Ocean Science OS Ocean Sci. 1812-0792 Copernicus Publications Göttingen, Germany 10.5194/os-7-651-2011 About uncertainties in practical salinity calculations Le Menn M. 1 French Hydrographic and Oceanographic Service (SHOM), SHOM, CS 92803, 29228 Brest Cedex 2, France 17 10 2011 7 5 651 659 Copyright: © 2011 M. Le Menn 2011 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://os.copernicus.org/articles/7/651/2011/os-7-651-2011.html The full text article is available as a PDF file from https://os.copernicus.org/articles/7/651/2011/os-7-651-2011.pdf

In the current state of the art, salinity is a quantity computed from conductivity ratio measurements, with temperature and pressure known at the time of the measurement, and using the Practical Salinity Scale algorithm of 1978 (PSS-78). This calculation gives practical salinity values <I>S</I>. The uncertainty expected in PSS-78 values is ±0.002, but no details have ever been given on the method used to work out this uncertainty, and the error sources to include in this calculation. Following a guide published by the Bureau International des Poids et Mesures (BIPM), using two independent methods, this paper assesses the uncertainties of salinity values obtained from a laboratory salinometer and Conductivity-Temperature-Depth (CTD) measurements after laboratory calibration of a conductivity cell. The results show that the part due to the PSS-78 relations fits is sometimes as significant as the instrument's. This is particularly the case with CTD measurements where correlations between variables contribute mainly to decreasing the uncertainty of <I>S</I>, even when expanded uncertainties of conductivity cell calibrations are for the most part in the order of 0.002 mS cm<sup>−1</sup>. The relations given here, and obtained with the normalized GUM method, allow a real analysis of the uncertainties' sources and they can be used in a more general way, with instruments having different specifications.

 References Bacon, S., Culkin, F., Higgs, N., and Ridout, P.: IAPSO Standard Seawater: definition of the uncertainty in the calibration procedure and stability of recent batches, J. Atmos. Oceanic Technol., 24, 1785–1799, 2007. BIPM: Evaluation of measurement data – Supplement 1 to the Guide to the expression of uncertainty in measurement – Propagation of distributions using a Monte Carlo method, JCGM YYY:2006, 2006. BIPM: Evaluation of measurement data – Guide to the expression of uncertainty in measurement, JCGM 100:2008, GUM 1995 with minor corrections, 2008. Culkin, F. and Ridout, P. S.: Stability of IAPSO Standard Seawater, J. Atmos. Oceanic Technol., 15, 1072–1075, 1998. Culkin, F. and Smith, N.: Determination of the concentration of potassium chloride solution having the same electrical conductivity, at 15 °C and infinite frequency, as standard seawater of salinity 35,000 \permil, (Chlorinity 19.37394 {%}), IEEE J. Oceanic Eng., OE-5, no. 1, 22–23, 1980. Fellmuth, B., Fisher, J., and Tegeler, E.: Uncertainty budgets for characteristics of SPRTs calibrated according to the ITS-90, BIPM, CCT/01-02, 2002. Fofonoff, N. P. and Millard, R. C.: Algorithms for computation of fundamental properties of seawater, Unesco technical paper in marine science 44, 1983. Jackett, D. R. and McDougall, T. J.: Algorithms for density, potential temperature, conservative temperature and the freezing temperature of seawater, J. Atmos. Oceanic Technol., 23, 1709–1728, 2006. Kawano, T., Aoyama, M., and Takatsuki, M.: Inconsistency in the conductivity of standard potassium chloride solutions made from different high-quality reagents, Deep Sea Res. Part. I, 52, 389–396, 2005. Lueck, R. G.: Thermal inertia of conductivity cells: theory, J. Atmos. Oceanic Technol., 7, 741–755, 1990. Mensah, V., Le Menn, M., and Morel, Y.: Thermal mass correction for the evaluation of salinity, J. Atmos. Oceanic Technol., 26, 665–672, 2009. Perkin, R. G. and Lewis, E. L.: The Practical Salinity Scale 1978: Fitting the Data, IEEE J. Oceanic Eng., OE-5, no. 1, 9–16, 1980. Poisson, A.: Conductivity/Salinity/Temperature relationship of diluted and concentrated standard seawater, IEEE J. Oceanic Eng., OE-5, no. 1, 41–50, 1980. Saunders, P. M., Mahrt, K.-H., and Williams, R. T.: Standard and Laboratory Calibration, WHP Operations and Methods, 1991. Seitz, S., Spitzer, P., and Brown, R. J. C.: Consistency of practical salinity measurements traceable to primary conductivity standards: Euromet project 918, Accreditation and Quality Assurance, 13, 601–605, 2008. Seitz, S., Spitzer, P., and Brown, R. J. C.: CCGM-P111 study on traceable determination of practical salinity and mass fraction of major seawater components, Accred. Qual. Assur., 15, 9–17, 2010. Uschida, H., Kawano, T., and Fukasawa, M.: In situ calibration of moored CTDs used for monitoring abyssal water, J. Atmos. Oceanic Technol., 25, 1695–1702, 2008.