IAPSO standard seawater has been used from the beginning of the HOT project. When salinity samples were measured by Ted Walsh on the Minisal (HOT 1-26), IAPSO water was used to determine the correction value which was to be applied to the data for standardization (see Section 2.3.2). When the group started using Guildline Autosal salinometers (8400A and 8400B), IAPSO ampoules were used before the measurement run to standardize the machine with the standardization knob and after the measurement run to check for drift in Autosal electronics.

Standardization is the process of adjusting the Autosal electronics by turning the standardization knob until the Autosal measures the salinity of a sample with a "known" salinity value: an IAPSO sample. Standardization is done at the beginning of a HOT salinity measurement run which usually lasts about 3-5 days. The standardization knob is adjusted only if the observed reading of an IAPSO exceeds the expected reading by +/- 0.00003 conductivity units. The amount of standardization knob adjustment (initial and final knob settings) and the Autosal conductivity readings associated with the knob adjustment at the time of standardization must be noted in the Salinity Measurement Sheets. At the end of the session in which the last primary salinity samples are measured, another IAPSO is measured to check the Autosal for drift during the measurement sessions. The standardization knob must NOT be adjusted after initial standardization. Another IAPSO is measured not at the beginning of the duplicate session but at the END of the duplicate session. This is also to check for Autosal drift during the duplicate session, and the standardization knob must NOT be adjusted.

In the past, the process of initial standardization was incorrectly repeated during salinity measurement sessions when the Autosal drifted for various reasons such as sharp changes in room or bath temperature, leaks in the Autosal plumbing, unstable readings, or bad substandard measurements that may have suggested a drift in the Autosal. In these events, an IAPSO was measured in order to determine the correction value for the set of data affected by the event, and the standardization knob was adjusted. This was the case during HOT 67-106, when an IAPSO was measured and the standardization knob was adjusted at the beginning and at the end of each session and the duplicate session. This way, the machine was standardized each time a session was started or when the machine seemed to drift more than the predetermined threshold value. We do not recommend following this practice, because if a constant autosal drift existed during the measurement run, it would be corrected in steps marked by each standardization event. The appropriate correction would consist of calculating the autosal drift from the substandard measurements, and then correcting each sample by this drift. It is important to maintain consistent methods so that the drift of the Autosal electronics can be detected in a uniform context.

We recommend following the proper procedures of standardization using IAPSO seawater ampoules concerning initial standardization, standardization knob adjustment, and IAPSO frequency.


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