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I’ve replaced the membrane and the fill solution on my DO meter and I’m still having problems getting steady, reliable readings. Any suggestions?

You’re off to a good start. Replacing the membrane and fill solution is always the first step to try when you’re having trouble with your DO meter. The fill solution loses its ability to carry oxygen over time and the membrane can become clogged or coated. Once you’ve replaced both of these items and you’re still having problems, the problem is most likely with the probe’s gold cathode or silver anode. For correct probe operation, the gold cathode must be textured properly. It can become tarnished or plated with silver after extended use. The gold cathode can be cleaned by using ultra-fine fine sandpaper or a piece of cardboard. Using the sandpaper or cardboard, wet sand the gold with a twisting motion about 3 times or until all silver deposits are removed and the gold appears to have a matte finish. Be careful not to sand off all the gold - it’s usually a very thin layer! Wipe the gold cathode thoroughly with a wet paper towel before putting on a new membrane cap.

Usually after a year or so, a thick layer of AgCl also builds up on the silver anode. The anode must be cleaned to remove this layer and restore performance and sensitivity. To clean the anode chemically, remove the membrane cap and soak the entire anode section in a 14% ammonium hydroxide solution for 3 to 4 minutes. Rinse thouroughly in cool tap water followed by a rinsing with distilled or deionized water. The anode should then be wiped with a wet paper towel to remove the residual layer from the anode. The anode can also be cleaned mechanically by sanding off the dark layer from the silver anode with 400 grit wet/dry sandpaper. Wrap the wet sandpaper around the anode and twist the probe. Rinse the anode with clean water after sanding, followed by wiping thoroughly with a wet paper towel.