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CO2 systems

Bob wrote:

 >Someone trying to maintain, say, a pH of 6.7 with very soft water may
 >need a closed loop system.
 >My water is very hard, with lots of carbonate and bicarbonate. (I
 >don't know kH and gH numbers, but the water company says the hardness
 >is 18 grains). So, its buffering capacity is quite high. And my CO2
 >injection rate is quite low. And I have oyster shells in my filter
 >box. If my CO2 rate were to increase tenfold, it would cause problems
 >eventually, but my system can absorb even dramatic fluctuations.

I stand corrected.  My systems maintain pH = 6.8 at kH = 4.  With much 
harder water you would have a damped system that could run stably open 
loop, as you say.  (Of course I DO know this.).

I think this essentially adds to my point, however.  If you are trying to 
maintain stable pH/CO2 open loop and are having trouble, then you do NOT 
have a heavily damped system, and a closed loop system (pH controller) is 
called for, rather than a better regulator.