Curious Question - Wave action compared to ATA

[spectrum]

1. So when you press the purge button (which is pressing on the diaphragm) the air RUSHING/GUSHING OUT of the second stage mouthpiece is at ambient pressure? Is that why it is rushing out so quickly bubbling all the way out like a fountain and isn't coming out at intermediate pressure and is subsiding as it moves away from the valve opening? The intermediate pressure just COMPLETELY and utterly goes down from 140 psig down to 0 psig ambient instantaneously? Is this what you are saying?

That IP air is at pressure when it is contained, i.e. in the hose. Once it gets past the remand valve the velocity increases with expansion and there is some locally higher pressure as it diffuses in the case but this is in no way full IP. What you represent as full IP is more like force, the combined effect of some pressure combined with velocity.

Statement 1 of post 3 is at best poorly worded.

 

[BoltSnap]

That IP air is at pressure when it is contained, i.e. in the hose. Once it gets past the remand valve the velocity increases with expansion and there is some locally higher pressure as it diffuses in the case but this is in no way full IP. What you represent as full IP is more like force, the combined effect of some pressure combined with velocity.

Statement 1 of post 3 is at best poorly worded.

I agree with you, it was worded poorly, it should have been: "When the air leaves the SS valve, it is at IP value as initial state but the this air pressure drops as air expands after it leaves the valve into the SS body."

However, it doesn't drop to ambient pressure immediately and/or instantaneously as most people above implied. This just doesn't happen.

 

[BoltSnap]

Air expands to ambient as it goes through the valve in the 2nd stage, which BTW is immediately at the end of the hose. This expansion to ambient happens more-or-less instantaneously. When you press on the purge, you are allowing a constant source of pressurized air from the 1st stage to depressurize (expand) in the 2nd stage. This "pushes" the existing air out of the 2nd stage.

You need to understand that any air in the water or in a flexible container (like a balloon or 2nd stage with flexible diaphragm) in the water will necessarily be at ambient pressure, because the water surrounding it will compress it until the two pressures equalize. This is why bubbles expand as they rise.

If there is "force," there is pressure (pressure is force per unit area).

 

[spectrum]

If there is "force," there is pressure (pressure is force per unit area).

I said that.

 

[AfterDark]

So when the 33' wave is over head the diver is 33' deeper then he was the moment before the wave, then it passes and the diver is suddenly 33' less. Sounds like a very rapid descent / ascent? I think the regulator is the least of a divers worry at that point.

 

[RyanT]

So when the 33' wave is over head the diver is 33' deeper then he was the moment before the wave, then it passes and the diver is suddenly 33' less.

Except that the water molecules in the wave are moving up and down. So the diver will move up and down with the waves, by about 50% of the wave height. The net result is a reduction in the pressure differentials as the waves move overhead. As I mentioned previously, in 10' seas, you'll experience about a 5' change in pressure near the surface; as long as your airway is open, its not a big deal. If you're diving 33' seas, you've got bigger issues.

 

[tursiops]

Except that the water molecules in the wave are moving up and down. So the diver will move up and down with the waves, by about 50% of the wave height. The net result is a reduction in the pressure differentials as the waves move overhead. As I mentioned previously, in 10' seas, you'll experience about a 5' change in pressure near the surface; as long as your airway is open, its not a big deal. If you're diving 33' seas, you've got bigger issues.

Almost. A neutrally buoyant particle moves up and down as the waves pass by an amount proportional to depth, the height of the waves, and the crest to crest wavelength of the waves. If you don't move up and down, but rather stay fixed in space, then the pressure goes up and down, again reduced by the depth you are at. The rate of decay of vertical motion or pressure is exponential with depth, so you need not be very deep to greatly mitigate the effects of a wave passing by.

 

[RyanT]

tursiops: correct, I should have qualified my statement, by saying "near the surface." Indeed, this effect becomes mitigated with increasing depth as you no longer move up and down with the wave. But of course, the proportional pressure change that you experience is also reduced with increasing depth when waves are passing over head.

 

[tursiops]

tursiops: correct, I should have qualified my statement, by saying "near the surface." Indeed, this effect becomes mitigated with increasing depth as you no longer move up and down with the wave. But of course, the proportional pressure change that you experience is also reduced with increasing depth when waves are passing over head.

Well, there are two absolute pressures...the static pressure due to the depth you are at if the surface were flat, and the dynamic pressure caused by the wave motion itself. The former, of course, increases linearly with depth. The latter decreases exponentially with depth. The proportional pressure changes from the dynamic pressures also lessen linearly with depth, just because you are normalizing the dynamic pressure with the (increasing) static pressure. But the exponential falloff with depth is certainly more important than the linear falloff.

 

[dberry]

So when the 33' wave is over head the diver is 33' deeper then he was the moment before the wave, then it passes and the diver is suddenly 33' less. Sounds like a very rapid descent / ascent? I think the regulator is the least of a divers worry at that point.

But there's not enough time to load tissues with gas, so maybe it's not really like a rapid ascent? I suppose someone might get really unlucky and inhale exactly when the pressure is max and hold breath while the pressure drops back and bust a lung, but no one should be holding their breath anyway.