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Re: DIY CO2 bubble size

Broken Tiger at admagee at teleport_com wrote:

> The instructions on the KRIB say I should use about or two bubbles per minute.

I think they say 1-2 bubbles per second, but I'm not sure which instructions
you are referring to.

> I am using a glass bead, low back pressure air stone on the air line, but
> cannot get "one or two bubbles per minute." Without an in-line air valve, I
> get a nearly steady stream of small, not tiny bubbles. With various
> adjustments, I got bursts of bigger bubbles every 2 or three seconds, streams
> of tiny bubbles, or very slow, but steady, streams of small(vs tiny) bubbles.
> The instructions don't say what size bubble should escape once or twice a
> minute, or if a burst of bubbles from an air stone counts as one whole bubble
> or several bubbles.  Iam confused and concerned.

You will need to put a bubble counter in between your diffusor and your CO2
supply. The 1-2 bubbles/sec thing is just a general starting point for
"roughing in" your CO2 injection rate. Once you get it going, you will then
monitor the CO2 content of the water a couple of times a day or so, and
adjust the flow rate to maintain the desired CO2 content of the water. This
could be anywhere from a few bubbles/minute to 2 or more bubbles/sec,
depending on specifics of your tank.

As far as the bubble-size issue, I also was curious about this when I
started learning about CO2 fertilization. A physicist friend of mine thinks
it is possible to calculate the size a bubble of CO2 will be in water, but
he started to get a headache while thinking about it. :) The diameter of the
bubble would be dependent upon the moles of gas, the temperature of the
system, the equilibrium pressure, the viscosity of the fluid, etc. You would
have to figure out how many moles of gas it takes to balance the fluid
pressure, and then the number of moles can then be converted into a volume
by dividing through by the density of the gas.  The diameter of the bubble
is then solved by multiplying the volume by 2/Pi then taking the cube root.
This would only be valid at a given depth in the water column, because as
the bubble rises to the top it would get larger and larger.

Of course, even if you knew what size someone's bubbles were, it would be
kind of tricky to adjust your bubbles to match it.  :)

> The Krib article also mentions upping the efficiency and diffusion by sticking
> the line in an intake or a diffuser.  I am using a Rio 200 powered with
> prefilter and a small Aquaclear.  CAn I just stick the airline in the box of
> the Aquaclear or run it into the Venturi input on the Rio?

The diffusion efficiency is increased by the mixing action of an impeller in
a filter, which will break up the bubbles and also provide longer contact
time with the water. This is usually done by feeding the CO2 into the intake
of powerheads or canister filters. I'm not familiar with the Rio 200 or the
Aquaclear powerheads (if that's what you have).

> am I ok on a forty gal. currently lightly planted tank with just the clumsy
> air stone? Should I go to a higher backpressure?

Measure the CO2 content of the water.

Dan Dixon