Two YSI Inc. probes, #13341 and #13251, were used during 1994. Sensor #13251 was used in all the cruises; new sensor #13341 was used also in a dual-sensor configuration during cruises HOT-54 through 59. These sensors were empirically calibrated following procedures described previously. Two sets of calibration parameters were usually obtained per cruise, corresponding to the casts at Station 1 and 2. In some of the 1994 cruises, there was an obvious drift in the CTD oxygen values throughout Station 2 casts, apparently due to sensor electrolyte depletion. These cruises required more than one set of calibration parameters at Station 2.

     Figure 4.7 shows the CTD oxygen data quality for every 2-dbar bin in each cast at Station ALOHA for cruises 51 through 59.

     Comparisons between sensor pairs revealed that sensor #13341 was measuring large voltages in the upper 100 dbar compared to #13251. A close inspection revealed that these values were sometimes reaching the upper bound of the sensor range (5 V), these oxygen values are invalid. The sensor was refurbished with a new head prior to cruise 59, but the problem seems to have been present during cruises HOT-54 through 58.

     Sensor #13251 performed correctly during cruises HOT-51 through 53. The oxygen traces from cruise 54 showed a mismatch with the bottle data after calibration, indicating that the sensor malfunctioned. During cruise HOT-55 the sensor failed and gave bad spikes in all the casts. A post-cruise evaluation at Sea-Bird revealed bad contacts between the sensor's head and the electronics. In light of this problem, sensor #13341 data are reported instead for cruise HOT-55, after flagging the upper 100 dbar from all casts due to the sensor's problems described above. It was also noticed that the oxygen values from this sensor were drifting during the cruise, requiring three different sets of calibration coefficients for casts at the beginning, the middle and the end of the cruise. In addition, many of the casts exhibited large spikes at various levels, which were flagged as bad (Fig. 4.7). Sensor #13251 data are reported for cruise HOT-54. Although this sensor showed a better calibration than sensor #13341, these data should be used with caution due to the mismatch with the bottle data mentioned before.

     The head of sensor #13251 was repaired on 10 August 1994, before cruise HOT-56, but the data obtained during cruises HOT-56, 57 and 58 were not reliable after calibration, which indicated further problems with the sensor. In cruise HOT-59, the oxygen trace was noisy, and after the cruise, the sensor's membrane was found torn and it was replaced. The data from sensor #13341 are reported here for cruises HOT-56, 57 and 58, after flagging suspect the upper 100 dbar of data in all casts. In addition, the sensor seemed to have drifted during station 2 casts. These casts required two sets of calibration coefficients (Table 4.6).

     Table 4.6 gives the means and standard deviations for the final calibrated CTD oxygen values minus the water sample values. Only results from the sensor's data reported here are included.

Figure 4.7. CTD oxygen data quality per cast (vertical bars) and per 2-dbar bin at Station ALOHA during HOT cruises 51 through 59. The tick marks along the x-axis separate different casts. Casts from the same cruise are separated by vertical black lines.

Table 4.6: CTD minus Bottle O2 (µmol kg-1) Comparison for Each Cruise

 

 

Station 1, Kahe Point

Station 2, ALOHA

 

0 to 1500 dbar

0 to 4700 dbar

500 to 4700 dbar

Cruise

Mean

St. Dev.

Mean

St. Dev.

Mean

St. Dev.

51

0.08

1.51

0.12

2.30

0.34

2.14

52

0.02

1.67

0.04

1.75

0.05

1.85

53

0.03

2.23

0.06

2.21

0.12

2.32

54

0.05

3.10

0.04

2.18

0.07

2.25

55

-0.01

3.45

0.09

0.09

0.00

0.96

1.83

0.13

0.10

0.14

0.00

0.95

1.44

0.14

56

0.00

1.61

0.07

0.00

1.42

0.29

0.09

0.00

1.25

0.33

57

0.00

1.42

0.00

0.00

1.26

1.76

0.06

0.31

0.61

1.34

58

0.00

2.04

0.00

0.00

0.24

1.52

0.00

0.08

0.25

1.49

59

0.07

2.55

0.03

1.38

0.07

1.53

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