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Re: [APD] CO2 regulator output pressure

Clint Brearley at clint_brearley at telstra_com wrote:

> I finally got my pressurised CO2 system up and running <GRIN> and I am
> currently fiddling around with regulator and needle valve trying to achieve
> the optimal bubble rate for my setup. Just this morning, pH 6.8, KH 4.5
> degrees, CO2 21ppm, bubble rate 19 bubbles/minute, 50gal tank. So far so good.
> My question is, what kind of output pressure should I set on the regulator?
> What are the advantages/disadvantages of setting higher/lower output
> pressures, if any? I currently have a low pressure setting around 0.4 bar (5.8
> psi). Is the outlet pressure really that important? Thanks.

If your system is working well, I'd say the regulator pressure is important
in that it's set where it works. IOW, if it ain't broke, don't fix it. :-)

That said, I'll throw out some thoughts on the matter....

In my experience, the most important issue is the stability of the system,
i.e., the consistency of the flow rate over time. In low pressure systems
(where the output pressure is very low) this is usually affected most by
room temperature changes, which affect gas density and cause the rig's metal
plumbing parts to expand or contract. The second most important issue is
adjustability, although once you get your system dialed in, it's unlikely to
need much adjusting.

Generally, your regulator valve is your coarse adjustment, and your needle
valve is your fine adjustment, since the turn resolution is usually greater
on the needle valve than the regulator. Theoretically, most needle valves
are more stable and controllable in the low-to-mid portion of their turn
range. For example, if your needle valve has a range from closed to
full-open of 10 turns, it'll probably be most stable and adjustable in the
1- to 5-turn range. Therefore, a regulator pressure that allows this part of
the needle valve's range to be used is desired.

BTW,  "theoretically" also means "I don't really know for sure." ;-) My
assumptions are based on the notion that the output flow of a needle valve
is less susceptible to input pressure fluctuations (caused by room
temperature changes) when its orifice is smaller, since the ratio of input
to output pressure is proportionately smaller than with a larger orifice.
Also, if you look at flow rate charts for needle valves, the flow rates
typically increase faster with fewer turns in the upper end of the turn
range than in the lower end. This translates into higher resolution in the
lower turn range. Theoretically. :)

One method of setting the regulator pressure is to open the needle valve to
about 2 or 3 turns, then "rough in" the gas flow using the regulator valve
to get approximately the bubble count you need. Then use the needle valve to
fine-tune the flow. In my experience, this has to be done over a period of
time, since it takes my system a few hours to stabilize between adjustments.
On my rig, I end up with a regulator pressure of around 20 psig, and a
metering valve setting of around 2.5 turns.

Of course, if your needle valve's resolution is very coarse, you might have
to use a relatively low regulator pressure in order to achieve decent
adjustability, which may trade off some stability. (I haven't seen any
complaints of metering valves having too much resolution. ;-)

Dan Dixon

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