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[APD] Correct pH readings??

Does anyone here use a pH controller? 


I ask because I'm a bit puzzled with two things. I have a Dennerle pH
controller with a CO2 system that I bought in December 2003 and I have been
calibrating regularly once a month. It is currently set to a target pH of
6.6 so the pH varies between 6.5 and 6.7. 


The two issues: 


1. Every monthly calibration of the pH electrode has shown a correction of
between pH 0.2 and 0.3. Isn't this a lot of variance after only one month
since the last calibration? Should I think something is not right? 


2. After the last calibration the day before yesterday, the electrode was
measuring a pH of 6.35 in the tank (6.55 before calibration). Out of
curiosity, I checked the tank pH manually with a JBL pH 6.0-7.6 test kit and
got a reading of pH 6.7 (a difference of 0.35...). So which is right, which
do I trust more? 


Any comments/feedback welcome! 


Thanks, Laith



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