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Re: CO2 control with FROG & needle valve

> >It simply seems to me that the pressure and flow are highly correlated.

True.

> >The package that the FROG came with states it is preset at 22SCFH. 
> >Hence, the regulator will try to keep the flow constant.

Not quite accurate. For a given pressure drop, the flow will be constant
ie. at 800 psi the flow is 22 cubic feet per hour. Reduce the pressure
drop by restricting the flow downstream.

> try tank+FROG+gauge+needle-valve in that order
> and close the needle valve.  The gauge will rise to 800PSI or so.

Also true (just not a good idea).

Don't use the needle valve to shut-off flow; instead use the valve on
the CO2 bottle. Needle valves aren't designed for on/off operation and
you can damage the valve seat by trying to do it. This makes them
non-linear flow restrictors and trickier or impossible to adjust right.

Flow through a needle valve or orifice (as in FROG) is correlated to
the pressure drop across the device. I don't have my old text books handy
so I don't have the formulae but I can make some generalizations.
(for those who really care, the manufacturer should be able to provide
pressure/flow graphs for each device)

The pressure at the outlet of the needle valve is fixed at ambient room
pressure (i.e. zero relative to ambient). The pressure at the inlet of
the FROG is also nearly constant (at a given temperature) until the liquid
CO2 in the tank is used up. Assume this to be ~800psi at 20C.

As long as there is flow through the needle valve & FROG, the sum of the
pressure drops across the two restrictions is exactly equal to the
total pressure difference, 800 psi. Assuming that the two devices have
close to the same pressure/flow constant (an engineering value which
approximates reality) the pressure in the middle will be ~400psi. The
problem with a single needle valve alone is that its too sensitive to
adjust to get the proper bubble rate. With two devices with a similar
pressure drop, you have halved the sensitivity of the needle valve.

I suspect (but don't know for sure) that in the tank+FROG+gauge+
needle-valve configuration (properly adjusted), most of the pressure
drop should be across the FROG. This will give the needle valve a nice
smooth control and is exactly what we want.

Could somebody try the following? Connect two needle valves (with
proper rating) as follows: tank+valve1+valve2 -> aquarium.
With valve2 fully open, adjust valve1 to the approximate bubble rate
needed (or a little bit more). Then use valve2 to do the fine tuning.
My bet is this will be approximately as good as a FROG+valve.

Steve