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Check Valves for CO2

One little piece of equipment which is usually included in CO2 systems is a
Check Valve. Some manufacturers of CO2 equipment for aquariums either make a
separate one (Dupla, Dennerle) or encorporate it into things like Bubble
Counters (Dupla) or CO2 Diffusors (Dupla, Eheim).

I suppose that they are concerned about water from an aquarium getting
sucked backwards and into things like Needle/Regulating Valves and/or
Regulators. I just don't know how "secure" these little bits of plastic
truly are, and when considering a set-up that might contain several hundreds
of dollars worth of valves and regulators, I'd like to make sure that my
investment is protected from water damage.

The APD archives and the KRIB contain scant information on Check Valves.
There are numerous posts regarding the failure of cheap valves, probably not
compatible with CO2.

When I look at my Dupla Check Valves, I notice that they are plastic bodied
and are "spring-loaded" internally. They are described by Dupla as "Type
4.5", with no explanation of what this means. Anyone know? They are rated
for a Maximum Pressure of 3.0 bar (44.1 psig), which I assume means that
they should never be exposed to anything higher than this. The Minimum
Pressure is listed as 0.15 bar (2.2 psig) and I assume that they mean that
below this level they will close, preventing flow in either direction. Am I
thinking correctly here?

When I look at various valve manufacturer's web sites, I can see that there
are lots of folks making Check Valves. Most of the manufacturers give a bit
more info than Dupla does, and in different formats.

For example, Swagelok makes several different series of Check Valves, and
they list properties such as Working Pressure, Nominal Spring Size, Cracking
Pressure and Reseal Pressure. Swagelok defines Cracking Pressure as the
pressure at which the first indication of flow occurs and Reseal Pressure as
the pressure at which there is no indication of flow. I'm stumped as to
figure out what an "appropriate" figures for these things would be for an
aquarium situation.

Working  Nominal   Cracking  Reseal
Pressure Spring    Pressure  Pressure
psi      Size psi  psi       psi
C Series
3000     1/3       Up to 3   6 back pressure
3000     1         Up to 4   6 back pressure
3000     10        7-15      3 back pressure
3000     25        20-30     17 back pressure
CH Series
6000     1/3       0-3       6 back pressure
6000     1         0-4       5 back pressure
6000     5         3-9       1
6000     25        20-30     17

Since the two most common ways of feeding CO2 into an aquarium (sintered
ceramic disk, requiring 10-20 psig for optimal use, and a reactor requiring
only 1-2 psig) I'm wondering how ONE type of check Valve can be used for

Why do some valves with really low cracking pressure require higher reseal
pressures than other valves with higher cracking pressure?

What kind of "back pressure" is generated within a CO2 line when the
positive flow of CO2 is cut off upstream of the Check Valve?

How do you select a Check Valve to suit our needs from looking at the
Cracking Pressure and Reseal Pressure values? Which combo would work for
aquarium applications?

From the Working Pressure rating of these two series of valves, I doubt that
they could ever be damaged by overpressure with CO2. These valves are also
available with a variety of seal materials to ensure compatibility with CO2.

If someone can provide some insight here, I'd appreciate it (and I can think
of a few others on the list who would as well).


James Purchase