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>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.