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[APD] RE: Gauging CO2 by bubble rate

jebrady asked:
> So Steve, (or anyone else who may know) do you know of any
> documentation that cross-references bubble rates with CO2
> concentration in given tank sizes?  I would like to set a
> bubble rate in my 65 that will maintain the recommended
> 25-30ppm.  This is critical for me right now.  Something is
> buffering my pH down .9 points on a 65g tank, and the tank is
> new, so I have no way to figure how much CO2 is in solution.

I did some experiments many years ago and determined that my cube 75 gal
tank had about 20ppm of CO2 if I injected CO2 at about 1 bubble every 8
seconds (if I recall correctly). George Booth did more exhaustive
experiments although I don't know if he correlated bubble rate to
concentration. He did determine factors that affected concentration
included surface disturbance and surface water movement.

Strong water circulation greatly increases the mixing of water within
the aquarium and the benefits are generally believed to outweigh the
increased surface losses. If the water flow over the leaf surfaces is
strong enough, it reduces the thin boundary layer on the leaves that is
depleted of CO2. This subject was discussed at length many years ago but
has not resurfaced until now. Search the actwin archives using [CO2
boundary layer] as search terms. Google may give better results than the
actwin search engine. [CO2 boundary-layer site:fins.actwin.com]
<http://fins.actwin.com/aquatic-plants/month.9605/msg00391.html> is an
early posting by yours truly almost 10 years ago.

Currently I'm using bubble rates of 1 bubble every 1-2 seconds in most
of my tanks and I use submerged power heads to increase water
circulation. There is a modest improvement in growth rates over a slower
bubble rate but only if NO3 is regularly dosed. If you keep the light
levels down, and have good circulation, minimal surface disturbance and
keep glass covers, a bubble rate of 1 bubble / 5 seconds is probably
sufficient. I think Tom would want it a little higher but I don't think
he goes by bubble rate.

Use Google search [CO2 bubble-rate per-second site:actwin.com] to get an
idea for the bubble rates that people are using. Most seem to suggest
higher rates than I do as high as 3 bubbles per second for large tanks.

Tom gives a compelling argument for using a fixed CO2 bubble rate
(without pH controller) in message
<http://fins.actwin.com/aquatic-plants/month.200201/msg00209.html> Once
you have established the steady state CO2 for a given pH and KH, you can
then keep that CO2 injection rate constant and ignore pH and KH from
that point on!!!

Most of the people who test for CO2 levels insist that there are too
many confounds to rely upon bubble rates. For those who consistently
achieve 20-40ppm CO2 by testing, what bubble rates do you need to
achieve your nominal CO2 level? What is your nominal CO2 level? For
those who use pH controllers, what bubble rate do you have during the ON
cycle and do you know the ratio of ON to OFF during the daytime hours?

Its a good pragmatic question JEB. I CC'd George because this is just
the type of question that he might know the answer to.


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