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2.5 Notes
- The zero adjustment of the instrument is very stable, the maximum deviation per month or longer
is 3mg/l.
- After replacement of the membrane and the electrolyte the instrument always has to recalibrated.
The advised procedure of calibration with sodiumbisulfite is at the 1st, 2d and 3d day, then the
15d day and afterwards once in the month.
In this way the instrument and the electrode are always well calibrated.
- For the zero adjustment a deviation of 5ppm is acceptable. It is not necessary to repeat the zero
calibration very often.
The sodiumbisulfite solution has to be prepared just before calibration with distilled water in a 2%
ratio. The storage time of this liquid is maximum one month, on the condition that the storage
bottle will be well closed and will be stored dark. However it is better to make a new calibration
liquid for every calibration by mixing 20 gr sodiumbisulfite with 1 litre distilled water.
- Mainly during calibration of the zero the instrument has to reach it's value within 45 seconds.
- When the electrode will be moved with a speed between 0,3 and 0,7 m/s in a sample, oxygen will
flow into the sample water caused by the movement of stirring.
For little sample water (±100cc) this will have a bigger influence as when much water is used (for
example 1 litre). For this we advise to use at least 1 litre sample water.
The opening of the sample cup is also important.. The bigger the surface the more oxygen will
flow into the water compared to a small opening of the sample cup.
- The best way to stir the water is with a magnetic stirrer. Manual is also possible but requires more
handling from the user.
- The membrane and the electrolyte have to be replaced when the zero adjustment is slow. The
membrane and the electrolyte normally can be used for 5 or 6 months or longer. This can be
checked as mentioned above.
- The sodiumbisulfite powder and the magnetic stirrers can be obtained from Nieuwkoop BV or your
own supplier.
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