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Re: CO2 Regulators

Thanks to Casey Huang for pointing out the difference between a diaphram
failure and the sort of "dump" that people here seem worried about (maybe we
should call it "End of Tank Syndrome").

"As to why inlet pressure decreases, the outlet pressure increases, I do
not have an explanation now and I do not doubt your observation."

Cylinder Pressure (or Inlet Pressure to the regulator) will decrease as the
contents of the cylinder are used up. In the case of materials which are
stored in the cylinder as LIQUIDS, and CO2 is one such material, the
Cylinder Pressure will stay at or very close to maximum (approx. 900 psig)
until all or most of the LIQUID phase CO2 in the cylinder has evaporated and
been released from the cylinder via the regulator.

As the Cylinder Pressure falls, the Output Pressure (from the regulator)
will rise (Remember, I made a mistake when I posted my "table" the first
time around. I subtracted when I should have added..... Just be thankful I'm
not your tax accountant....<g>. This mistake was corrected in my follow up
post.)

This "rise" in regulator output pressure as the cylinder pressure falls
happens with ALL single and two stage cylinder regulators (the types that we
would commonly use), but the magnitude of the "rise" is a function both of
regulator design and construction level of the regulator (not necessarily
price). Two stage resulators will always experience much less of a rise than
single stage regulators (refer back to the table I posted with some typical
numbers direct from both BOC and Air Products). But in single stage
regulators (the type most commonly used by aquarium hobbyists), the
magnitude of the rise can also vary dramatically from model to model. Only
the user can decide how much of a rise he/she is willing to tolerate/cope
with when making a buying decision.

For OUR purposes, and in MOST aquarium-related situations where the hobbyist
is careful to always swap out the near empty cylinder for a full one before
the cylinder runs totally empty, this discussion is really moot or at best a
"worst case scenario". Again, how important this is to the individual
hobbyist is a matter between them and their check books.

Never the less, it is a fact and worthy of understanding. Quoting from the
Air Products web site (my comments are in square brackets):

--------
-- How Regulators Work

- Single-Stage Regulators

High-pressure media enters the regulator through the inlet into the
high-pressure chamber. When the adjusting knob is turned clockwise, it
compresses the range spring and exerts a force on the diaphragm, which
pushes the valve stem open. This releases gas into the low-pressure chamber,
exerting an opposing force on the diaphragm. An equilibrium is reached when
the spring force on the diaphragm is equal to the opposing force of the gas
in the low-pressure chamber.

In a single-stage regulator, delivery pressure increases as cylinder
pressure decays, because there is less gas pressure exerted on the valve
stem. Thus, frequent adjustment of the control knob is required to maintain
constant delivery pressure. This does not pose a problem, however, with
pipelines and liquefied gas products where inlet pressure is maintained
relatively constant.

[NOTE: CO2 is a "liquified gas product", so this rise in output pressure is
not a factor of prime importance to us unless the cylinder is nearly empty.
JP]

- Two-Stage Regulators

A two-stage regulator functions similarly to two, single-stage regulators in
series. The first stage reduces inlet pressure to a preset intermediate
pressure, typically 350 to 500 psig. By adjusting the control knob, the
second stage reduces the intermediate pressure to the desired delivery
pressure.

Like the single-stage regulator, outlet pressure from the first stage of the
two-stage regulator rises as cylinder pressure decreases. However, instead
of passing out of the regulator, the gas flows into the second stage where
the pressure is moderated. Thus, delivery pressure remains constant even as
cylinder pressure decays, eliminating the need for frequent control knob
adjustment.

-- Selecting the Proper Regulator

- Line and Cylinder Regulators

Line regulators are typically point-of-use regulators serving low-pressure
pipelines. They are also used in conjunction with high-pressure cylinder
regulators that limit the inlet pressure to 250 to 400 psig.

Cylinder regulators are available in either single-stage or two-stage models
for high-purity, general purpose, or special service applications.

[NOTE: This is the type of regulator we commonly use for CO2 injection. JP]

- High-Purity Regulators [Note: This term can be used for either Line or
Cylinder models. JP]

High-purity regulators are designed and constructed to provide diffusion
resistance and easy cleanup. Metal diaphragms and high-purity seats and
seals minimize or eliminate outgassing and inboard diffusion.

[NOTE: For the "picky-types", or for special situations, some High Purity
Single Stage Cylinder Regulators have excellent decay characteristics and
can rival two stage regulators. They are usually more expensive but can make
excellent choices for use in "deluxe" installations. JP]

- General Purpose Regulators [Note: This term can be used for either Line or
Cylinder models. JP]

General purpose regulators are designed for economy and longevity. They are
recommended for noncorrosive general plant, pilot plant, and maintenance
shop applications where diffusion resistance is not required.

[NOTE: Most manufacturers recommend one of their General Purpose Cylinder
Regulators for CO2 use. Make sure that the dealer KNOWS that it will be used
for CO2 service to ensure that the construction materials are appropriate
for use with CO2. JP]

- Special Service Regulators [Note: This term can be used for either Line or
Cylinder models. JP]

Special service regulators are specifically constructed for special
applications including oxygen, acetylene, and fluorine service and
high-pressure, ultrahigh-pressure, and corrosion service.

[Note: We would not likely be using this type of regulator. JP]

To make your selection easier, this catalog lists the proper regulator for
almost every gas, pressure, and situation. Simply look up the gas or mixture
for your application and you will find the appropriate regulator listed
under "Recommended Equipment." CGA valve outlets are also noted for each gas
and gas mixture. The regulator must be equipped with the appropriate CGA
connection for the cylinder valve outlet.

[NOTE: The North American standard for CO2 cylinder valves is CGA 320. Some
European regulators might be outfitted with a different valve...JP]

[NOTE: This material is from the Air Products web site -
http://www3.airproducts.com/specgas/equipment/regulatorselection.asp JP]
---------

James Purchase
Toronto