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Re: Check Valves & Manifolds for multiple tanks

> Date: Fri, 3 Nov 2000 09:35:17 -0500
> From: "James Purchase" <jpurch at interlog_com>
> Subject: Check Valves & Manifolds for multiple tanks
> I'm still waiting for Wright (or someone else) to explain his use of a check
> valve to "regulate" the flow of CO2. I continue to believe that his
> description was either mistaken or not fully described. 

True, James, it was quite sloppily described, at best.

> A check valve does
> not "regulate" anything - it is used purely to prevent the backflow of a
> second fluid (gas or liquid) into sensitive equipment (needle valves,
> regulators, CO2 cylinders).

Well, as I will try to show, there are some made that can actually do
another task, even if rather feebly.

Let me give it one more try. 

As I understand it, most normal gas pressure regulators do not work at all
well at outlet pressures of below a few psi. That, unfortunately is the
exact range of backpressures we see when feeding a reactor or diffuser in
our tanks. It's even lower, going into a pump Venturi, maybe even negative.

Most plant enthusiasts would like a regulation of the *volume* of gas being
injected (flow rate), not the pressure. It is measured in bubbles/second in
a small chamber dedicated to that function (often part of the reactor).

Flow regulators are easily created with tiny turbulent-flow apertures, as in
a properly-sized needle valve. Good ones cost a bunch. "Proper size" for low
volume isn't cheap and safely withstanding over 900 psi of a somewhat
corrosive gas is *really* pricey.

Fortunately inexpensive pressure regulators let us bypass the
direct-off-the-tank kind of needle valve, by putting pressure down to safer
levels, first. The regulation of pressure also makes the needle valve much
less critical as flow rate need only be constant over a much smaller range
of pressure changes.

It was discovered that some check valves have a *very* strong spring, unlike
the ones we use with normal air pumps that have weak flap valves or softer
springs pushing a diaphragm or ball-valve closed. Those with strong springs
can be used to increase back-pressure to a pressure regulator to put it up
into its normal, accurate operating-pressure range.

One such "check valve," that is supplied with the Eheim diffusers, takes
about 11-12 psi to just crack open the valve (way above any air-pump
pressure we usually use). At that level, it is acting a little like a flow
regulator (turbulent aperture) and a little like a spring-diaphragm pressure
regulator, but not doing either very well. The adjustment on the real
pressure regulator, upstream, can be set in the 13-15 psi range and is very
stable there. Fine tweaking can provide a very stable bubble rate into a
(flow-resistant) glass diffuser. As long as that input pressure stays
regulated, and the flow is tiny enough to keep the "check valve" nearly
closed, it works OK.

Those check valves are not a good flow regulator, nor are they really
pressure regulators. Rather, they are a constant pressure-dropping device
that lets the pressure regulator work up in its best range. It is thus
possible to fine tweak pressure to get a constant flow rate under stable
conditions of temperature and regulator function. 

When used with a porous-glass diffuser, with its own flow-regulator-like
properties, the system I have works surprisingly well, and I do like the
safety factor of not having any one single aperture that can cause an
explosion if clogged.

I would consider use of any normal bell-type injector, or pump Venturi,
would be pretty unstable (compared to the fine-bubble-generator in the Eheim
unit). For such systems, I would use a needle valve, for sure. The
sintered-glass diffuser can be thought of as a crude, distributed
needle-valve flow regulator, I guess, as it does give stable performance,
when used as part of a complete system.

The system breaks down at end-of-tank, when the primary regulator
experiences loss of control and output pressure increases by a few psi
(normal function). That overcomes the bare cracking of the "check valve" and
larger flow happens.

In conclusion, the high-back-pressure "check valve" is being used as a part
of a complete system, where it does some of the functions of a needle valve,
but its primary function is to drop a lot of pressure, so the pressure
regulator is stable enough to keep constant flow going through the
sintered-glass diffuser.

Used as part of a complete system, it is pretty safe, and particularly safe
against serious blow-outs. All bets are off, IMHO, if you try to mix and
match, without clear understanding of the exact role of each component, and
its behavior. Failure to notice low tank pressure can be more drastic, but
I've had, like Jeff, blowouts without hurting fish, so...??? Wonder what



Wright Huntley, Fremont CA, USA, 510 494-8679  huntleyone at home dot com

         "Ignorance creates a fertile climate for demagogues;
         maybe that's why politicians give so much support to
                the public education establishment."
                                              Walter Williams
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