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On Sep 21, 2:32pm, Dan Masi wrote: >>`On Sep 21, 11:04am, Marc Dufour wrote: >> Subject: Re: Why? - Pony Gauges >> ... >> > Since the first stage is 'balanced' to ambient pressure by its >> >diaphragm or piston, why, isn't the interstage pressure 8-12 bar *above >> >ambient*? >> >> Have you noticed that *BIG*, *HEAVY DUTY* spring in the first stage, >> merrily pushing *HARD* against the piston/diaphragm??? >> >> It makes sure the intermediate pressure is a good number of bars above >> the ambient pressure.... > >Right. 8-12 bar or so, as the first mail presumed. > >But we're talking about this in the context of ponies, and whether or not >they should be turned off or left on. When you turn on your pony at the >surface, you pressurize the lp hose to the intermediate pressure of, >say, 150 psia. Now turn the pony off. Presumably (no leaks), that >*absolute* pressure will remain in the hose. Which means that if you go >to a depth which causes ambient pressure to be greater than 150psia, water >can begin to enter the system. > >While I have no immediate plans to hit 300fsw, a "free-flow" in the second >stage would probably amount to a half-second's worth of bubbles to >depressurize the LP hose, and would likely go unnoticed. > >Dan Masi >(braving his first time on techdiver) I'm having trouble understanding how water would enter the system in this example. Here are some assumptions: 1) There are no leaks 2) The regulator mouth piece is not plugged (water can flood inside the regulator housing). 3) The tank valve is closed 4) The system is pressurized to 150 PSI intermediate pressure (measured at the surface). 5) Lets say we are using a balanced piston 1st stage. 6) We have a balanced second stage connected with no other hoses, or pressure gauges. As you descend ambient pressure would increase by about 1 PSI for every 2 feet, so at 300 fsw ambient pressure would equal the pressure in the LP hose and 1st stage (both compartments). So far, the only thing that should have adjusted is the piston in the first stage (it would open) as ambient pressure increased. Since the tank valve is shut off no more air would be added to the system. As you descend further ambient pressure would increase and would be greater than the IP in the hose, but water would not enter because the poppet would be pushing against the orifice at full spring (poppet spring) strength plus the difference in ambient pressure minus the IP in the hose. The demand lever would not move because the pressure inside the regulator housing is the same on the outside (it's flooded), so there is no path for the water to enter the LP hose. Am I missing something here? It seems pretty simple to me. I can see a potential problem with the diver accidently hitting the purge button which would unseat the poppet allowing water into the system. I imagine that the demand lever would be pretty stiff at this point since you have to overcome the full spring force plus the greater ambient pressure to move the poppet. I prefer to leave the air on in my pony. In an emergency I don't want to play with a valve in order to get air. One of my dive buddies had a 1st stage failure under water which caused high pressure air to flow through his regulator (it was ripped out of his mouth). If this happened to me I don't want the added stress of trying to turn my bailout air on. Happy Diving Jon
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