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pH Probe End-of-Service
I have recently received a few questions regarding the lifetime of pH probes and
how you can tell when the end is near. This is important if you use CO2
controllers because, in our experience, a faulty probe returns (higher or
lower?) voltages than it should, causing the controller to display a higher pH
than it should. Thus your controller may read 7.0 when the actual pH may be 6.0!
And the controller will be happy to inject enough CO2 to keep the pH at what it
thinks is 7.0; enough to create CO2 concentrations about 100 ppm. Enough to
severely kill your fish!
So, how do you tell if your probe is pooping out?
We check pH with a LaMotte narrow range pH test kit at water changes. We trust
the test kit much more than the pH controller and probe. When you notice the
two pH values diverging by more than 0.1, it's time to clean and calibrate the
The calibration routine will use two standard solutions -- pH 7.0 and either pH
4.0 or 10.0. Never stick the probe in the bottle of standard solutions - pour a
little of each into small containers and dispose of it when you are done! Note
that when you are calibrating, even a bad probe will probably be able to read
the right pH almost instantly. The cal solutions are "high ionic strength" --
lots of ions cause the measuring junction to react quickly.
However, when you put the newly calibrated probe into the low ionic strength
aquarium water, you may notice that it slowly drifts to the correct reading.
This slow drifting is a sign the probe should be replaced soon. It's hard to
say exactly what "slow" would be but if it takes more than 30 seconds to return
the right pH, we think it's time to replace it.
BTW, we like the Broadley-James "Silver" probes sold by Pet Warehouse for around
$35. We have used more expensive probes but these seem to work the best in low
ionic strength freshwater. If you are keeping marine plants, saltwater is high
ionic strength and probe selection is much less critical.
One may be tempted to tweak the controller to match the pH test kit reading.
This is OK as a short term correction but you will not be getting any
indications of how bad the probe may be getting. There is evidence that a probe
at the end of its useful life can, in a matter of hours, go from being merely
off a little bit to completely off. In a CO2 controller situation, this means
the controller will inject beaucoup CO2 to maintain what it thinks is the right
As always, when using CO2, it pays to be careful and diligent.