It IS exposed to tank pressure on one side.
And lawyers.
Hah, now you are confusing me! Let's see if we can find where my thought process is wrong.
1. Take a full cylinder, no regulator attached and open the valve.
The air has to expand from the HP portion of the cylinder into ambient pressure. This happens right at the seat/orifice area of the cylinder valve. Everything after that seat/orifice of the cylinder valve will be at ambient pressure, albeit at a high velocity.
2. Take a full cylinder, with a regulator attached and open the valve.
The air from our example above now starts to flow into our regulator system. Air from the HP side of the cylinder will
"fill up" the volume between the orifice of the cylinder valve and orifice of the first stage. While it is filling up that space, some of the air escapes through the first stage orifice, as the system is in the open position on an unpressurized regulator.
As it escapes through the first stage orifice it
"fills up" the IP side of the system. Once that IP side reaches a certain value, the piston will get pushed against the orifice, closing it.
At no point should there have been an increase to HP levels at that upper piston O-Ring. Unless one would assume that the cavity where the piston head sits fills up to HP levels of air, before the air bleeds into the hoses and under the piston. I do not believe that would be correct. If one would picture it as an in-compressible liquid flowing step by step through the system, I understand how that conclusion could be drawn, but that is not the case with a compressible gas. A gas should spread out evenly, with the points closer to the HP portion at a much higher velocity, but not at a higher pressure. A gas does not fill up a certain area before moving on to the next, it spreads out.
To make matters more confusing, if my memory serves right, the area just past the orifice should be where the "vena contractor" sits. The point of the highest gas velocity and therefore the least pressure.
Writing this down makes me think that maybe pressure was not a reason for FKM in that area, but rather gas velocity.
Now where did I go wrong here?
