pH Swings

Paul Sears wrote:

>	A change of 0.2 in the pH means that the CO2 concentration has
>changed by a _factor_ of 1.58.  (log(1.58) = 0.2).  If you increase the
>KH, you will have to increase the CO2 concentrations if you want to
>keep the pH where it was.  If you can do this, you will reduce the 
>effect of a change of x ppm in the CO2 concentration, but only because
>it is a smaller fraction of the CO2 concentration.  It is the higher
>CO2 concentration that has done it. 

I think I understand what you've said Paul, but let me try an example to be
sure:  Situation A:  Assume a pH of 7.0, dKH of 2 and CO2 level of 6 ppm. 
If the plants pull out 3 ppm CO2 during the day, the pH will rise to a
little over 7.3 (interpolating from the KH/pH/CO2 chart).  Situation B: 
Assume a pH of 7.0, dKH of 5 and CO2 level of 15 ppm.  If the plants are
pulling out 3 ppm CO2, the pH will rise only to about 7.1.

I recently went from Situation A to Situation B by first cranking up my CO2
reactor to 15 ppm and higher.  Per the chart, the pH fell to 6.6 and lower,
so I added bicarbonate to get back to pH 7.0.  I noticed that my daily pH
swings were significantly reduced.  (My CO2 consumption has probably
increased to 6 ppms or so, because the pH rise is actually about .2.  The
plants are growing wonderfully!)  The reduction in pH swing is because the
CO2 consumption is a smaller percentage of the higher CO2 levels and NOT
because of the increase in KH.  I can see this from the chart because if I
had left the pH at 6.6 (with dKH 2 and CO2 15 ppm), a 3 ppm CO2 usage would
have increased the pH by .l to 6.7--the same amount as in Situation B. 

Regards, Steve Dixon