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Bishop Controller


     Gary claimed a + or - range of .05 in the ph level.  He did mention in
his article
that is was slow to respond.  How much surface area do you think it needs?
He said that this design is based upon the Tetra CO2 Depot system that uses
a tiny diving bell.  I am not familiar with that kit as I have never used it. 
From what he said about it, it does have a form of feedback using a reagent-
tube that is interfaced with the aquarium water in some way.  How well does
particular setup work?  If it works well, then we should simply copy the
design of
the Tetra diving bell.  Specific details on the internal design of the diving
bell would be greatly appreciated, if you can provide any.  

     If I build Jim Hurley's circuit, and use the 'Silver' probe from Pet
how can I be sure this probe will work?  The probe Hurley used changed its
output voltage 50 mv per 1 unit of ph.  (I think)  
Is this a standard value for ph probes?  The Silver probe's rate of change
would need to be the same as the one Hurley used.  
One thing I like about Hurley's circuit is that PH indication can be added
If I remember correctly, the first stage OP amp is designed to give exactly 7
volts output at ph 7.  This could easily be read on a DVM or analog panel
(Maybe another unity-gain op amp should be used to provide some current gain
to drive the volt meter and help prevent circuit loading?)  A continuous PH
would be nice.  You could observe the PH swings throughout the day and night,
and have an idea if something isn't quite right.