Salinometer calibration for measurement of oceanic samples is traditionally carried out using IAPSO Standard Seawater (salinity = 35). For measurement of samples of higher and lower salinity, OSIL produce standards at salinities of 10, 30 and 38. Salinometer linearity checks provide a useful diagnostic tool in assessing the function of a salinometer and are routinely carried out at the OSIL Service Centre. Offset values can be used to improve the accuracy of measurements made away from 35 salinity. A large offset away from the 35 salinity point can be indicative of electronics misalignment, cell ageing or temperature bath malfunction.
A typical procedure for linearity correction would be:
1) Standardise the salinometer with P-series Normal Standard Seawater (Salinity 35)
2) Measure the salinities for 38H-series, 30L-series and 10L-series Standard Seawaters
3) Calculate the difference between the measured value and the label value for each of these standards i.e. measured value-label value = salinometer offset
4) Plot a graph of offset at each salinity (a linearity curve) |
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5) Use the plotted offset data to correct salinity measurements away from salinity 35 e.g. in the example below at salinity 10 the offset is -0.004 in salinity, therefore add 0.004 to your measured values around 10.
NOTES
Guildline Autosal accuracy specified as better than +/- 0.002 in salinity over the working range (2-42)
Guildline Portasal accuracy specified as better than +/- 0.003 in salinity over the working range (2-42)
Shelf-life : Our laboratory studies have shown that IAPSO Standard Seawater, in bottles, remains in calibration for at least three years after the calibration date. Culkin & Ridout (1998) showed changes equivalent to less than 0.001 in salinity in the label value, for these new-style borosilicate bottles, after 158 weeks storage. The older style soda glass ampoules stayed within specification for at least 18 months which was the time-limit of the experiments.
REFERENCES
Culkin, F. and Ridout, P.S. (1998), Stability of IAPSO Standard Seawater, j.atmos. & oceanic tech., 15, 1072-1075