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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.