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CO2 Regulators mystery solved

Hello James,

>Date: Fri, 29 Sep 2000 11:55:23 -0500
>From: "James Purchase" <jpurch at interlog_com>
>Subject: Re: CO2 Regulators

>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.

The explanation given by Air Products is not very clear.

If your working pressure (outlet) is set at 20 psi (which is the
pressure required by the diaphragm to over come the spring force),
starting cylinder pressure (inlet) at 800psi for example. Even when
the pressure drops to 50psi it is still more than enough (as long as
it is higher than 20 psi) to over come the spring force to close the
valve. It does not matter to the diaphragm whether the tank pressure
is 800 psi or 100 psi, what it needs is still only 20 psi in the low
pressure chamber for the valve to work correctly. Right? Not so.

After studying the CASHCO design, I have just realised that there is
an explanation for "inlet pressure decreases, outlet pressure increases"

There are actually two forces working togther for closing the valve.
One is the diaphragm and another is the piston which is resting on
the the piston spring. As the piston is connected to the high pressure
side, it create a force similar to the diaphragm (but much smaller
because of relatively smaller effective surface area) contributing
to the total closing force of the valve (counter acting against the
spring force). So when the inlet pressure decreases, the force
genereted by the piston also decreases, contributing less towards
the total closing force which means the difference must come from
the diaphragm. Therefore the outlet pressure will increase to a point
that the total closing force is again equal to the spring force.

Casey Huang