About uncertainties in practical salinity calculations

• Main Author: M. Le Menn
• Format: Article
• Language: English
• Published: Copernicus Publications 2011-10-01
• Series: Ocean Science
• Online Access: http://www.ocean-sci.net/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.