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).
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.
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.
Reference: Culkin, F. and Ridout, P.S. (1998), Stability of IAPSO Standard Seawater, j.atmos. & oceanic tech., 15, 1072-1075