Though the frequency in which the standardization knob was adjusted has not been consistent throughout the years (see previous Section 3.1.1), it is possible to determine the drift of the Autosal electronics throughout the HOT salinity history. By plotting the changes in standardization knob setting (SKS) against the changes in salinity values shown on the Autosal at the time of standardization, the equivalent change in salinity units per knob setting units for Autosals 8400A and 8400B were determined (Figure 3).

Figure 3. Changes in the Autosal's standarization knob setting and their corresponding change in salinity, for cruises 33 through 80 (a), 81 through 107 (b), and 108 through 148 (c).

 

The plots are divided into three categories: HOT 33-80 (Fig. 3a), HOT 81-107 (Fig. 3b), and HOT 108-148 (Fig. 3c). HOT-33 is the first cruise in which changes in SKS along with changes in salinity units were noted in the salinity measurement sheets. During HOT-80, the Autosal 8400A had to be readjusted because the standardization knob reached its minimum setting (0/00) and could not turn any further (see Section 1.1.1). After the knob was readjusted, the group resumed using the Autosal 8400A for HOT-81 but started standardizing more than once during a measurement run (see Section 3.1.1 above). This continued until HOT-108, when the group started using the Autosal 8400B and resumed practice of standardizing the machine only during initial standardization. Regardless of the readjustment and the change in standardization methods, the standardization knob adjustments for the Autosal 8400A (from HOT 33-107) equaled 1.42 x 10-4 psu/knob setting unit (Figures 3A and 3B). For the Autosal 8400B, the standardization knob adjustments equaled 2.04 x 10-4 psu/knob setting unit (Figure 3C).

From these calculations, drift of the Autosal electronics was calculated (Figure 4).

Figure 4. Electronics drift for Autosals 8400A (a), and 8400B (b) during HOT cruises 33 through 148.

 

Figures 4A and 4B show the equivalent change in salinity each time the SKS was adjusted in Autosals 8400A and 8400B, plotted over time. Each data point was recovered from Autosal log sheets and represents the equivalent change in salinity at time of one of the following: SKS change at the beginning of the salinity session, SKS change after initial standardization, SKS change if the knob was adjusted during the salinity session, or SKS change from standardization at the beginning of the duplicate session. The solid red lines indicate the drift rates estimated for each category. Apparently, when Autosal 4800A standarization knob was adjusted (after cruise 80), something else was adjusted that reduced its drift rate. However, there is no documentation describing what type of adjustment was made to the Autosal. The maximum drift in the Autosal electronics throughout history occurred during HOT 33-80 on the Autosal 8400A, at -5.3426 x 10-5 psu/day (Figure 4A, left). Assuming a maximum measurement run of five days, the maximum drift would be approximately 0.27 mpsu during the salinity measurement run. As this drift is very small, we can conclude that Autosal electronics drift has not affected significantly the HOT salinity measurements.

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