Previous salinity reports and Autosal logs suggest that duplicate salinity samples were not always taken in order to cross-check the quality of the measurements made by a new or inexperienced Autosal operator. Before HOT-33, duplicate samples were taken and measured in order to compare the performances of the Autosal to the Minisal. From HOT-33 to present, duplicate salinity samples were taken from the first deep cast at Station ALOHA for cross-reference between primary and duplicate samples to check the quality of the Autosal operator's measurements. Duplicate samples are also important as they are back-up samples of the deep cast in case of problems during primary sample measuring and analysis.

Duplicate samples should always be drawn from the rosette in conjunction with primary samples when sampling out at sea. Duplicate samples for HOT-70 were saltier than the primary samples by as much as 1.6 mpsu, likely due to the fact that the duplicate samples were drawn from the rosette approximately 3-6 hours after primary samples were drawn. This type of incidents are preventable by following the regular procedure of drawing the duplicate sample from each Niskin bottle immediately after the primary.

After each HOT cruise, salinity samples should not be left for more than a week before measuring (except when there is limited time between consecutive cruises, or if there are problems with the Autosal). Evaporation and salt crystal formation in the sample bottles are both important factors that develop with time, and may significantly affect salinity values. Duplicate samples should be measured at a date close to when primary samples are measured in order that they be measured in similar environments. The best time to measure duplicate samples is after the primary salinity operator is done with all primary samples (one or two days in between is fine).   This way, if there are any problems in the measurement session that are indicated by substandard IAPSO values, the data are more easily corrected.

In the past, duplicate and sometimes even triplicate samples were taken from selected casts either for training purposes for a new Autosal operator (Kennan et al. 1992, "Summary of Training S. Asghar and R. Domokos on the Guildline Autosal," Appendix L) or for comparison between glass and plastic bottles. Also, due to insufficient number of glass bottles, plastic bottles were used for some duplicate samples before HOT-84 (see Section 1.1.2). Evaporation during the use of plastic bottles, especially if measured 15 days or more after the cruise, seemed to affect the data quite significantly. Thus, triplicate samples were sometimes taken with some glass bottles for comparison.

For HOT-70 to HOT-86, duplicate samples were taken from both deep casts. For example, during HOT-78, duplicate samples were taken from both S2c1 and S2c12 (deep casts at Station ALOHA). Molly Lucas measured the primary samples while Matt Cochran measured the duplicate samples for S2c1, and Matt Cochran measured the primary samples while Craig Nosse measured the duplicate samples for S2c12. In this manner, the data allowed cross-checking of the quality of measurements by three Autosal operators. In more common instances, such as for HOT-80 through HOT-86, duplicate samples were taken from two casts (e.g. S2c1 and S2c9), and while the primary Autosal operator measured primary samples from both casts, one duplicate operator measured duplicate samples from S2c1 and another duplicate operator measured duplicate samples from S2c9. These methods were often employed to familiarize a new Autosal operator with measurement methods. Sometimes, duplicate samples were taken from shallow casts such that the trainee would not affect primary salt measurements. Also, in several isolated cases such as HOT-73, HOT-83, and HOT-149, triplicate samples were taken from one of the deep casts in order to have two duplicate operators for one cast. This way, they were able to compare the data by the two duplicate operators to each other as well as with the data by the primary operator.

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