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pH ground problems again



>Having recently set up a fully automatic CO2 system on a 110 gal. tank and
>be completely dumbfounded as to why my pH meter/controller would go crazy as
>soon as I dropped its probe into my aquarium, I can say that those aquarium
>grounding probes can be essential as long as they are connected to a good
>ground (water pipes are the best).
 > Apparently both me pumps (an eheim and a quiet one) were sending
>electricity into the aqaurium and the pH electrode was attracting quite a bit
>of it, making my meter go bonkers.

The problem you are experiencing is not due to your pumps, but due to the 
ground probe affecting your pH probe. pH probes act like a battery in the 
water. There are two probes in the water, the water acting as the electrolyte.

This can be likened to a car battery with two electrodes, one lead and the 
other zinc, the electrolyte in this case being acid. If you add a third probe,
and connect it to the negative side of the battery (the ground side), current
will flow between these two probes.

The same is happening in your tank, the ground probe (connected through
the mains wiring) is connected to your pH controller through its ground
connection and is affecting your readings.

To illustrate this point, try two things, first calibrate your probe in buffer
solutions, then take a sample of tank water in a small beaker and
test it with your probe. Now remove your ground probe and place your
pH electrode in the tank, take the readings. Remove the pumps from 
the water and again take readings. Now with the pumps removed 
from the water, add the ground probe and you should see that your
pH readings swing out.

More on this was written about 2 months ago, it'll be in the archives.

Marque APD - ANGFA(NSW) - Sydney