Intermediate pressure theory/question/debate

[MR12111]

No, the ease of breathing is related to the difference between the IP and the spring tension, not the absolute value of either one.

But that's what I described in my statement .. the "difference" would be lower as the spring doesn't need t hold back as much pressure ..Am I missing something here ? It's all just for debate ..You set a reg to factory specs and either it's a good breather or not .. I always just tried to get the cracking pressure as low as I could ...

 

[tursiops]

Am I missing something here ?

Yes

 

[OrcasC205]

But that's what I described in my statement .. the "difference" would be lower as the spring doesn't need t hold back as much pressure ..Am I missing something here ? It's all just for debate ..You set a reg to factory specs and either it's a good breather or not .. I always just tried to get the cracking pressure as low as I could ...

Remember, the spring force is a force holding the poppet closed. IP works against that force. If the IP is too high, the poppet will open and the regulator will free-flow. Lowering the IP to just the same force as the spring is producing will make the regulator stop free-flowing and will have the least cracking pressure -- the difference between the spring pressure and the IP pressure is minimized at this point. Lowering the IP anywhere lower than that will just increase the cracking pressure.

 

[James79]

But that's what I described in my statement .. the "difference" would be lower as the spring doesn't need t hold back as much pressure ..Am I missing something here ?

On a simple downstream reg with adjustable orifice like a Scubapro 108HP or Aqualung ABS octo, you have 2 opposing forces. Spring holding the poppet shut against the orifice, and IP against the poppet trying to push it open. To use made up numbers (for easy math examples), we'll say the opening of the orifice is 1/100th of a square inch. If the IP is 135 PSI, then it is exerting 1/100th of that trying to open the poppet, or 1.35 lb force. So the spring has to exert more than that to hold it closed... we adjust that by moving the orifice in or out, increasing or decreasing the compression of the spring. If the orifice is run in to put the spring at 1.4 lb force, it takes .05 lb force to open the valve. If you drop IP to 125 (1.25 lb force on the poppet) and back the orifice out until the spring is exerting 1.3 lb force, it is still a .05 lb force to open the valve. Same cracking pressure at a lower IP is then achieved (or reverse the example to increase IP while maintaining cracking pressure the same).
On an even simpler downstream second with a fixed orifice (like my beloved Conshelf seconds), the spring pressure is fixed. You can set lever height, but the only real cracking pressure adjustment is to adjust the IP.... Higher IP results in more force countering the fixed spring force resulting in lower IP, or vice versa.
On more complex regs (think SP 109)you have an adjuster that changes the spring pressure, leaving the orifice as primarily for setting lever height (set as in the first example with the adjuster all the way out and orifice set to the edge of freeflow, then you can use the adjustment knob to "stiffen" the breathing as needed).
And most complex (for the standard downstream examples) would be balanced adjustable designs (such as the G250). Same as a the 109, only there is a balance chamber exposing an equal area on the back of the poppet to IP.... resulting in nearly no IP effect against the spring. Orifice sets lever height, and a very soft spring holds the valve closed, with the adjuster to increase spring pressure as desired.

Respectfully,

James