Linearity corrections for laboratory salinometer measurements: IAPSO Standard Seawater linearity pack vs weight diluted samples

Practical Salinity (SP) measurement error made with laboratory salinometer is usually corrected with the IAPSO (International Association for the Physical Sciences of the Ocean) Standard Seawater linearity pack. Here we checked measurement errors of a Guildline AUTOSAL 8400B salinometer using a linearity pack and weight diluted seawater samples of similar salinities, respectively. The processes of the weight dilution method as well as the evaluation of uncertainties associated with calculated SP errors were described in detail, with the weight diluted samples providing slightly lower uncertainties. Measured SP and SP errors were fitted with ordinary least squares method over two independent ranges and were checked with diluted interpolation samples. Both methods presented similar SP error trends as well as satisfactory results within stated uncertainties. Significant batch-to-batch difference between 30L17 and 30L18 was observed. Main limitations of current linearity correction methods were i) they highly depended on the standard seawater used for standardization and ii) they were not able to delineate the error characteristic with enough details over the whole measurement range. It was recommended that measurement errors should be checked at specified sub-ranges with the weight dilution method when high accuracy is required.