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2.1. Beckman sensor
The first Sea-Bird SBE-13 CTD oxygen sensor used during HOT cruises incorporated a Beckman polarographic sensor element manufactured by Sensor-Medics. This sensor consists of a Teflon membrane covering a layer of potassium chloride gel. A constant voltage applied across two electrodes results in a current nearly proportional to the activity of oxygen diffusing across the membrane. This current and the temperature of the cell are measured, and dissolved oxygen is calculated using an algorithm based on that given by Owens and Millard (1985, see Sect. 3 below). Calibration of the sensor is conducted by comparing the CTD values against discrete water samples taken from bottles closed during CTD upcasts (Section 3). Because the oxygen sensor shows considerable hysteresis (upcast oxygen values are very different from the downcast values) the calibration is made using the downcast CTD values at the density levels where the bottles were closed on the upcast.
The oxygen sensor was calibrated using the ensemble bottle data from station 2 (ALOHA) (see Section 3)—akin to assuming that the sensor did not drift during the 3-4 days on station. In fact, during some cruises, the drift was large between casts, and calibrated dissolved oxygen traces for casts where no bottle samples were taken showed obvious deviations from the bottle data taken during other casts.
The Beckman sensor was notoriously difficult to calibrate, primarily because of its nonlinear sensitivity to pressure and temperature (Section 3). Difficulty in calibration also occurred because the response characteristics were sensitive to fouling of the membrane, and because the electrolyte in the cartridge could become depleted or breakdown with time.
In addition, the behavior of the Beckman sensor in the top 100 m or so of profiles appeared to be poor, because the shape of the O2 profile often did not resemble the distribution suggested by the bottles. Experiments with the version of this sensor produced by YSI Inc, showed that this was a particular characteristic of the Beckman version of the membrane and cartridge (see Sections 4.2 and 4.3).
The Beckman sensor (SN 13-0186) was used during HOT cruises 1 through 32.
2.2. YSI sensor
The YSI sensor is a version of the SBE-13 oxygen sensor that uses a modified YSI 5739 oxygen probe produced by Yellow Springs Instruments Incorporated. This sensor replaced the Beckman sensor, for the reasons given in the previous section. These two sensors were cross-calibrated for several cruises before we began to use the YSI as the primary sensor. Up to cruise 24, only the Beckman sensor was used for continuous oxygen profiling. On cruises 25 through 32 the YSI sensor (SN 13-0233) was paired with the Beckman and the results obtained from both sensors were compared. During this period, the YSI sensor provided slightly less resolution than the Beckman. However, consistent with our previous findings, the results demonstrated that the YSI sensor was superior to the Beckman especially in the upper 100 m of the water column (Section 4.3, Figure 4.1). Therefore, we used the YSI as the primary sensor beginning with HOT-31. Beginning with HOT-32 the gain on the YSI sensor was increased to double the resolution of this sensor. This change increased the precision of the YSI to approximately that obtainable with the Beckman sensor.
YSI sensor SN 13-0233 had problems during HOT-40, showing spikes in the deep section of the deep cast (see Sect. 4.4). The sensor was replaced by YSI sensor SN 13-0251 before HOT-41.
A CTD SBE 911plus with dual temperature, conductivity and oxygen sensors was put into service in 1994, after HOT-53, and a second YSI sensor SN 13-0341 was acquired. Oxygen sensor pairs were routinely used after this cruise, which prevented complete data loss despite failure in one of the sensors. In addition, the monitoring of the sensors’ measurement differences permitted detection of sensor problems during cruises. A second YSI sensor SN 13-0434 was acquired in 1999 as a backup, to be used as a replacement in case of failures with either of the other two sensors.
Revised procedures to check for possible sensor problems were implemented in late 1995, and included a close inspection of the sensor’s membrane for wrinkles or tears, for air bubbles in the electrolyte reservoir, and in-house checks of atmospheric oxygen level as recommended by Sea-Bird. These pre-cruise inspections often resulted in the in-house replacement of the sensor’s membrane, or refilling of the electrolyte.
Although the YSI sensor was superior to the Beckman sensor, the YSI units used during cruises suffered numerous failures and drift due to deteriorated or ruptured membranes, depletion of the electrolyte, or due to major problems such as sensor head cracks. Details of membrane replacements and electrolyte replenishments, as well as major sensor problems are in subsections 4.4 through 4.12.
2.3. Sea-Bird SBE 43 sensor
The SBE 43 sensor also uses a polarographic membrane oxygen detector in its oxygen sensor. In comparison with the YSI sensor, the SBE 43 membrane and electrolyte can only be replaced at Sea-Bird. The SBE 43 minimizes the temperature differences between the water and oxygen sensor, as it calculates temperature compensation using a temperature measured very near the active surface of the sensor. As a result, the SBE 43 is less susceptible to error when profiling through areas of high temperature gradients than previous oxygen sensors. The data from HOT cruises indicate that this sensor is more stable than previous sensors, it has less pressure hysteresis, and it is easier to calibrate.
Sea-Bird started testing various developmental versions of the SBE 43 oxygen sensor during the 1999 HOT cruises, paired with our YSI sensors. Production line versions of the sensor were used during 2001, although improved versions of the sensor are still being tested as of the date of this report. Since February 2002, only SBE 43 sensors have been used during HOT cruises. Sensor problems were frequent but not unexpected with the developmental versions of the SBE 43 sensor. However, most of the production line sensors have also suffered recurrent failures. On the other hand, one of these production line sensors (SN 43134) has been frequently used for more than one and a half years without any failure. The following table 2.1 shows the history of our SBE 43 production line sensors, indicating problems that required attention at Sea-Bird. Sensors SN 43019 and 43020 belong to the UH Shipboard Technical Assistance Group (STAG) and were only used during a couple of HOT cruises each.
Table
2.1. History of repairs on SBE 43 oxygen sensors. All repairs were conducted
at Sea-Bird
Sensor SN Date 43019 26 Apr
01 New Sensor
acquired from Sea-Bird. This sensor belongs to STAG " 26 Oct
01 Sensor
showed problems during HOT-131 and 132. The sensor was sent back
to Sea-Bird for inspection/repair. 43020 26 Apr
01 New Sensor
acquired from Sea-Bird. This sensor belongs to STAG " 13 Jul
01 Sensor
showed problems during HOT-128 and 129. The sensor was sent back
to Sea-Bird for inspection/repair. " 13 Nov
01 Sensor
was repaired by Sea-Bird. A completely new membrane and electronics
were installed. 43166 13 Nov
01 New Sensor
acquired from Sea-Bird. " 15 Dec
01 Sensor
showed problems during HOT-133. The sensor was sent back to Sea-Bird
for inspection/repair. " 16 Jan
02 Sensor
was found to have a slow response. Replaced electrolyte and membrane.
Re-backfilled the sensor's electrolyte reservoir. Installed a new
style DO plenum " 20 Feb
02 Sensor
failed at the bottom of the deep cast during HOT-135 " 7 Mar 02 The unit's
sensor head assembly was found to be defective. Replaced sensor
head assembly " 13 Aug
02 Sensor
drifted during HOT-138 (6/2002). Sensor's head assembly was found
to have failed. Replaced sensor's head assembly. " 15 Oct
02 Sensor
had an offset in the Kahe cast during HOT-140. Sensor was sent to
Sea-Bird for evaluation. " 16 Oct
02 Given the
history of failures of this sensor, Sea-Bird gave the option to
replace the sensor. The sensor was replaced by SN 43325 43325 22 Oct
02 New Sensor.
This sensor was given by Sea-Bird as a replacement for SN 43166. " 31 Mar
03 Sensor
gave large values in the deep section of the deep cast compared
to other sensor during HOT-146. Sensor was sent to Sea-Bird for
evaluation. " 4 June
03 Pressure
compensation bag was found to be leaking.
Replaced at Sea-Bird " 23 July
03 Sensor
trace showed steps (jumps) in the deep section of the deep cast
compared to the other sensor during HOT-150.
Sensor was sent to Sea-Bird for evaluation. " 22 Aug
03 Sea-Bird's
inspection found that the sensor's SPAR assembly had failed. Replaced
the sensor's SPAR assembly. 43262 20 Mar
02 New Sensor
acquired from Sea-Bird " 21 Dec
02 Sensor
showed offset with respect to secondary sensor and large hysteresis
during HOT-143. Sensor sent to Sea-Bird for evaluation. " 18 Feb
03 Sensor's
pressure compensation bag found to have failed. Replaced sensor
lid and membrane assembly and re-backfilled the sensor's electrolyte
reservoir. " 24 Feb
03 Sensor
used during HOT-145, insignificant drift observed. Sensor performed
well in subsequent cruises. 43134 8 Jan 02 New SBE
Sensor 430134 acquired from Sea-Bird. No problems with this sensor
as of the date of this report.
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