On each station, seawater salinity samples of ~225 mL are drawn from all CTD-rosette bottles closed during the cast. A Guildline Instruments Portasal™ Salinometer (8410A) measures the seawater sample conductivity precisely, comparing it to a reference seawater standard. From these comparisons, salinities are calculated and logged using Windows-PC based software that averages data that meet replicate criteria. Concurrent with the water sampling, a Sea-Bird Electronics CTD equipped with dual conductivity meters profiles in situ data. Data processing software is then used to compare the bottle salts to in situ CTD measurements. These are used to confirm bottle closures at target depths, monitor CTD sensor performance, and helps select the best salinity data for future comparison and publication.
Salinity bottles are collected in ~250 ml borosilicate KIMAX-35 bottles. These square cross-sectioned bottles have screw tops with separate plastic thimbles to prevent leakage and evaporation. Numbered, stackable divider boxes hold 24 numbered bottles to match the 24 Niskins on the rosette. Bottles are left filled with previously unused sample water to limit salt crystallization and to pre-leech silicate from the glass.
Numbered salt bottles are matched to the corresponding Niskin bottles and filled to the shoulder after three ~40 ml rinses. The last fill is done without interruption until overflowing; then ~10 ml is poured out over the thimble and the bottle is stoppered and capped. Samples are left to equilibrate to room temperature for at least 8 hours prior to analysis. Before being placed on the salinometer, samples are: gently inverted 3 times to remove any possible stratification; after being uncapped, seawater trapped around and under the thimble is 'wicked' away using a Kimwipe; if the thimble falls off before the removal of trapped seawater or 'champagnes' off the sample bottle, ~10-20 ml of the sample is dumped into a collection bucket.
Guildline Portasal™ Salinometer (8410A)
Guildline Portasal Salinometer model 8410A is used to make the precise conductivity comparisons between the water samples and reference water standards. Two of these instruments are taken on each cruise. Guildline specifications state an accuracy of ±0.003 PSU (same set point temperature as standardization and within -2°C and +4°C of ambient), and a precision of 0.0003 PSU.