Description

The DO-25 is the DO Filling Solution used by the Galvanic DO Electrode in the AP-700 and AP-800 sensors. The AP-700 and AP-800 may be fitted with either an Optical DO or Galvanic DO electrode. Refer to the photograph on the right to identify your DO electrode type. The Galvanic DO electrode consists of an electrolyte filled DO sensor cap with a clear, gas-permeable membrane stretched across a silver electrode. The Galvanic probes requires flowing water over the sensor as Oxygen near the electrode will be consumed during operation.

Replacing a Galvanic DO Electrode Membrane Cap

The Galvanic DO electrode membrane is a very thin sheet of special plastic, which is permeable to oxygen. Oxygen molecules pass through this membrane into the Oxygen sensor. The membrane is extremely delicate and is factory fitted into the DO Membrane Cap. To ensure optimum performance, the DO Membrane Cap must be replaced every 1-2
months.

Never touch the plastic membrane as the oils in your skin will block the pores in the membrane and stop it from working correctly.

To replace the DO Membrane Cap, follow these simple steps.

1. Remove the Probe sleeve.
2. Unscrew the DO Cap from the end of the DO electrode by rotating it anti-clockwise.
3. Gently scrape away any soft grey or white deposits from the zinc shaft of the electrode taking care not to damage the electrode in the process. If the deposits are hard and dry, soak
   the electrode in DO filling solution until the deposits soften up, then remove them.
4. Use fine grade sandpaper (1500 grade) to remove any stubborn deposits from the zinc shaft, and if necessary, the tip of the electrode.
5. After removing the deposits, rinse the electrode with DO Electrode Filling Solution.
6. Rinse a new DO Membrane Cap with DO Electrode Filling Solution then tap it out so that it is completely empty.
7. Using the DO Electrode Filling Solution dropper bottle, half fill the DO Membrane Cap. Gently tap the cap to ensure any trapped air bubbles are released.
8. Holding the Probe so that the DO electrode is facing downwards, slowly screw the half-filled DO Membrane Cap back onto the DO electrode then tighten the cap. Do not over-tighten. Finger tight is fine. Some solution will overflow. This is normal.
9. Wash the DO electrode with fresh water then replace the lower Probe sleeve.
10.  Wait at least six hours (preferably over-night) to allow any oxygen dissolved in the filling solution to be consumed.
11. Carry out both Zero point and 100% point DO calibration as described earlier.

NEVER re-install a Galvanic DO Membrane Cap once it has been fully tightened. The membrane will be stretched and will not seal properly over the silver cathode a second time. If the membrane does not create a proper seal over the silver cathode, the DO sensor will not operate correctly and any readings given will be erroneous.

Please note: After a period of use and dependant upon how previous cleaning operations have been performed, the surface of the zinc shaft will become rugged and apparently corroded. This is entirely normal and will not affect the performance of the electrode.