Appendix E – Calibration
95
Appendix E – Calibration
Calibration Procedures
Normal good practices should be observed when calibrating DO systems. When
the instrument is first connected to the oxygen sensor, i.e. when the unit is first
installed, or whenever the oxygen sensor is changed or the membrane replaced,
the user should perform a zero check and span calibration of the system using the
following procedure. If necessary the user can use a span calibration other than
100% by simply setting the span calibration level in the “Set Span Level” item in
the “Calibration” menu.
Notes.
• As an aid to stable air calibration, a partially covered bucket can be used to
shield the sensor from the temperature variations which arise from exposure to
the wind and sunlight.
• It is recommended that because the OE15 oxygen cartridge has a finite life, a
spare cartridge should be stocked where a significant down time is not
acceptable to the application.
• The OE15 sensor requires a minimum fluid flow of 0.5ms
-1
to refresh the
depletion layer which forms around the sensor membrane. This applies to both
air and solution readings.
• All calibration is done in % Saturation, even if the required operating mode is
concentration.
• Approximate sensor current is 700uA = 100% Sat with Galvanic sensor and
60nA = 100% Sat with Polargraphic sensor.
Zero Check in a de-oxygenated solution
• Prepare a fresh solution of approximately 2% wt/vol. of sodium sulphite in de-
mineralised water.
• Wash off any process chemicals or water from the sensor, which may
contaminate the solution. Use de-mineralised water or follow the
manufacturers cleaning instruction as necessary.
• Set the instrument to read Current.. Select % saturation as the secondary
reading. Allow the output to settle in air at (or close to) 100% saturation. Place
the sensor in the sodium sulphite solution and observe the current reading.
The reading should drop below 10% of the air saturated reading within 35
seconds.
• If this time is exceeded, cycle the sensor between the free air and the solution
to improve the speed of the response. If cycling it 3 or 4 times does not
improve the response significantly, store the sensor overnight in the solution
and then re-test it with a fresh solution the following day. If it still does not
respond within the specified time the cartridge membrane should be checked
and replaced if necessary, otherwise the electrolyte will have to be replaced
(this must be done at the factory for OE15 probes).
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