idronaut Ocean Seven 304Plus Operator's Manual page 66

APPENDIX A – DISSOLVED OXYGEN SENSOR
If, after replacing the membrane cap and cleaning the sensor tip with filter paper, the oxygen sensor
current is too low during calibration, it is necessary to polish the sensor tip with the abrasive paper
(which must be wet) included in the maintenance kit. It is sufficient to slightly rub the tip over the
paper two or three times without applying an excessive pressure. Wash the sensor tip with distilled
water, or with a few drops of electrolyte, to remove residues. If the silver anode appears completely
black or covered with foreign materials, it is necessary to clean it with the abrasive paper. Wrap the
paper around the silver body and rotate it to obtain original silver brightness. Wash the sensor with
tap water or, if possible, use distilled water to remove residuals. The anode cleaning procedure is
required every 2 or 3 years only. After these operations, the oxygen current, during calibration, will be
higher than normal and will drop during the first few hours to reach the normal stability level of 0.1 to
0.3 ppm/week.
A.7.6 Oxygen sensor check in the absence of oxygen
To guarantee maximum accuracy in results, it is a good practice to test the response of the sensor once
a month in the absence of oxygen. Nitrogen is recommended for this check; should Nitrogen not be
available, an aqueous solution, chemically reduced, can be alternatively used.
To carry out this test, it is important that the membrane cap should have been replaced for at least 15
minutes, thus allowing a complete sensor polarization.
Procedure
 Connect a cuvette (body of a syringe) to a Nitrogen supply using a flexible tube.
 Purge the line and adjust the gas flow rate at 200 ml/min. approx.
 Calibrate the oxygen channel by exposing the sensor to room air.
 Insert the sensor completely into the cuvette. The reading should rapidly decrease and within a
few seconds to one minute, ;it should be less than 0.2 ppm. If the reading is more than 0.2 ppm, re-
expose the sensor to room air and repeat the operation.
Should the inconvenience persist, replace the membrane cap and/or the electrolyte. Due to the high
quality construction of this oxygen sensor, which reduces to insignificant the background current,
no electronic zero calibration is necessary and possible.
IDRONAUT – Brugherio (MB) OCEAN SEVEN 304Plus CTD
58
12-2015