Temperature fluctuations may greatly affect salinity measurements. Accuracies of +/- 0.001 are possible on Guildline Autosal salinometers only when the measurements are made in conditions in which the temperature is maintained within 1-2o C throughout the entire measurement run. When ambient laboratory temperature fluctuates more than 1-2o C during the analysis session, a considerable drift is noted (see Figure 1 in Stalcup, 1991, Appendix A). As mentioned in the "Preparation" section of the Processing Guide (Appendix K), the temperature of the room in which the salt samples are stored and in which the measurements are to be made should be maintained at a range of 21-23o C. Bath temperature is usually fairly consistent and remains at 24o C, but it is recommended to check the bath temperature as often as possible. In the asal program, salinity units are converted from conductivity units assuming a bath temperature of 24o C, and thus the bath temperature must be noted if it shows anything other than 24o C.
Ambient temperature is measured using a digital thermometer placed near the salinity sample boxes away from the Autosal. This prevents the thermometer from being affected by the heat of the Autosal (from the thermistor and the light bulbs keeping the bath temperature constant at 24oC) and allows the thermometer to measure the temperature of the area in which the samples have equilibrated. Figure 2 is a graphical presentation of room temperature fluctuations during salinity measurement runs of previous HOT cruises.
Figure 2. Ambient temperature fluctuations during measurement runs for HOT cruises 22 through 149
Figure 2 shows the range of ambient temperature (maximum temperature - minimum temperature within a measurement run) plotted against HOT cruise numbers. A red line is drawn at 2o C as a threshold line for relatively stable data. According to this criterion, data from HOT 22-80 are relatively stable except for two outliers at HOT-41 and HOT-47. Data seem suspect from HOT 81-106, and from HOT 116-140.
If
temperature fluctuations do affect the salinity measurement values, variability
could be detected by a drift or unusual changes in substandard measurements
throughout the measurement run. Table
4 shows the cruises marked suspect above the red line in Figure
2, and whether any adjustments were done to the respective data due
to substandard variability. Data
were "corrected" if a drift was detected in substandard measurements throughout
the salinity measurement run. Data
labeled "no corrections needed" indicates that no substandard drift was
detected, and thus the data did not need any corrections.
For measurements during HOT-91, 96, and 128, variability was apparent
in the substandard measurements but no corrections were made because the
drift was insignificant. For HOT-92
and HOT-101, the Autosal operators re-standardized the machine in the
middle of the measurement runs and still corrected for a substandard drift
after the measurement run. For
HOT-100 and HOT-106, the machine was also re-standardized in the middle
of the measurement runs but no corrections were needed at the end of the
run due to no drift in substandards detected.
| HOT
Cruise #
|
Adjustments
to Data
|
|
41, 47, 93, 97, 98, 105, 129, 140 |
Drift detected, corrected |
|
81, 83, 87, 116-126, 131, 136 |
No drift detected, no corrections needed |
|
91, 96, 128
|
Insignificant
drift detected, no correction needed
|
|
92, 101 |
Re-standardized, drift detected, corrected |
|
100, 106 |
Re-standardized, no drift detected, no corrections needed
|