Duplicate salinity measurements

Duplicate salinity samples are taken on each cruise as a backup for the WOCE deep cast samples and to continue to monitor the Autosal and Autosal operator performance. These duplicates are carefully logged on separate log sheets. In addition to the WOCE deep cast, duplicates are sometimes taken from at least one other cast during the cruise. The additional duplicate samples usually come from the second deep cast (if there is one) or another cast conducted near the middle or end of the cruise.

The duplicate salinity samples should be taken from the Niskin bottles in conjunction with the regular samples. A strict order must be followed. From each Niskin, the same person should take the regular sample first, immediatly followed by the duplicate sample. This regimented order reduces error which could be induced by sampling error caused by the person taking the sample, as well as errors resulting from the seawater sitting in the bottles for too long of a time between samples. After both the regular and duplicate samples are taken from a Niskin, the next Niskin can be sampled.

The WOCE salinity technician should ensure that a sufficient number of glass bottles are brought aboard the ship to store the duplicate samples.

If plastic bottles are used to store salinity samples, extra steps must be taken after sampling. Plastic bottles must be sealed, with parafilm, immediatly after sampling in order to prevent evaporation. Only one piece of parafilm is needed for each bottle. The parafilm should be stretched and wrapped around the bottle at the point where the cap meets the bottle. The plastic bottles are subject to evaporation until they are properly sealed so they should be sealed immediately after all the samples are taken.

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