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...APPENDIX A
A3 DO Calibration
Note. Both the oxygen and the temperature sensors
must be exposed to the calibration medium.
A3.1 Zero Calibration
A 5% sodium sulphite solution is required that must be prepared
well in advance by dissolving 5.0g of anhydrous sodium sulphite
in 100ml of demineralized water. It must be stored in a tightly
closed bottle. Ideally, this bottle should have a sufficiently wide
neck to allow direct insertion of the oxygen and temperature
sensors. Do not store the solution for more than one week.
When the oxygen sensor is dipped into the solution, ensure that
no air bubbles are trapped on, or close to, the membrane and
that the sensor is supported so that the membrane cannot be
damaged by contact with the bottom of the bottle.
When the sensors are withdrawn, all traces of sodium sulphite
must be removed by rinsing them thoroughly with demineralized
water.
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A3.2 Span Calibration
Either air or air-saturated water may be used. Air calibration is
more convenient and is likely, in practice, to be at least as
accurate as calibration in air-saturated water.
A3.2.1 Air Calibration
The air must be saturated with water vapour. This is conveniently
achieved by suspending the sensors inside a bottle containing a
few drops of water. Alternatively, the sensors can be suspended
close (within a few centimetres) to the surface of a body of water.
The operation of the oxygen sensor is such that the output in air
is slightly higher than in air-saturated water at the same
temperature. This difference is reproducible, allowing calibration
in air by adjusting the instrument reading to 108% saturation (or
the equivalent concentration) rather than 100%. This adjustment
is made automatically in the calibration procedure.
A3.2.2 Air-saturated Water Calibration
The air-saturated water must be prepared, as described below,
well in advance. Using an aeration stone, or a sintered glass
diffuser, aerate approximately 1 litre (0.22 galls.) of demineralized
water, either continuously for at least five minutes with a small
pump, or intermittently for at least 15 minutes with hand bellows.
These techniques are adequate for many applications provided
the ambient temperature is constant. However, to obtain an
accurate 100% saturation solution, the water must be
maintained at constant temperature and stirred gently, without
forced aeration, using a magnetic stirrer set to provide
continuous agitation without breaking the liquid surface. This
process must be continued for at least two hours to attain
complete equilibrium. For calibration, the sensors must be
suspended in the air-saturated water, which must be stirred
continuously so that the flow velocity at the membrane of the
oxygen sensor is at least 30cm/s (9.8 ft/s).
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