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Re: flexible DIY reactors.

> I was thinking about my DIY CO2 setup, particularly about another post 
> about using rigid bottles. I made my container out of a flexible walled 
> pop bottle. I don't get a steady bubble flow, but about 7 bubbles or so 
> come out rapidly a couple of times a minute. I think this is what's 
> happening: after a stream comes out, the flexible bottle is deflated. As 
> the pressure builds up again, the bottle starts expanding. Once a certain 
> pressure is reached, the energy in the bottle walls is released, and a 
> spurt of bubbles comes out.
> I would guess that rigid bottles result in a steady pressure.

The sides of most clear plastic bottles are flexible but they don't seem to
be very elastic, and they would have to be stretchy to store up energy
that way.  The volume of gas in the container is "stretchy" because 
the gas expands and contracts as the pressure changes - that does store 

My DIY systems used to act like this, and here's what I figure caused 
the problem:

The CO2 system has to supply enough pressure to get gas to the outlet of
the system - that pressure is pretty much determined by the depth of water
over the outlet.  Given that pressure, the yeast then have to supply a
little additional pressure to balance the surface tension on the bubble it
forms.  The surface tension is higher in small bubbles so more pressure is
needed to make smaller bubbles.

When the pressure gets high enough, a bubble is formed and released, and
there is a sudden drop in pressure in the CO2 system.  That pressure drop
travels back through the gas line and gas in your system expands as the
pressure drops.  There is a brief moment before the surface at the opening
reforms and reestablishes the surface tension.  In that instant the
expanding gas can escape as a stream of bubbles.  The size of the stream
depends on the size of the outlet (smaller outlet -> bigger pressure drop
-> longer stream) and the volume of gas in the system (bigger total volume
-> larger volume change on pressure drop -> longer stream). 

If your tubing or bottle really is stretchy (instead of just flexible)
then replace it with something that isn't elastic.  Reduce the gas volume
in the system by reducing the headspace in the reactor and using the
shortest, smallest tubing that will do the job.  Finally, put a valve on
the gas line, as close to the outlet as you can and adjust the valve down
to a point where it's almost closed.  The nearly-closed valve keeps a
sudden pressure drop at the outlet from passing back through the system. 
Of course, exactly how nearly closed it has to be depends on details of
your system. 

I use an in-line valve for drip irrigation systems and get a very 
regular stream of bubbles through a 1/32" outlet.  Of course, the valve 
does increase the total pressure in the reactor, and if you have any leak 
you will lose more CO2.  Also you have to be careful not to close the 
valve completely for very long, or the pressure will build up until 
something gives.

Roger Miller