Curious Question - Wave action compared to ATA

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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?

2. First stage intermediate pressure is present in the hose and the part of the SS before the SS stage valve opens. It is intermediate pressure in all parts upstream of the valve when it is closed to the first stage including any parts of the SS stage housing that is upstream of the valve.
 
I'd also argue it also depends where you are in the water column. If you understand a little about wave dynamics, you'll know that the water in a wave really isn't moving left-to-right... The individual water molecules are just moving up-and-down, and they'd take you along with them.
At the surface (or just under the surface), you'd just bob up and down 33ft.
In mid-water, you'd bob up and down some fraction of 33ft.
Only if you are hanging onto the bottom (you'd probably have to be in seas that high!) your depth would change by 33ft between peak the peak and the trough of the wave.

I know I'm reading too much into the question, but isn't that what threads like this are for? :wink:
 
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?
If there is any difference, the pressure in the gas is actually lower than ambient, not higher. https://en.wikipedia.org/wiki/Bernoulli's_principle

Is that why it is rushing out so quickly bubbling all the way out like a fountain
Um, no. Kinetic energy is why it is rushing out so quickly bubbling all the way out like a fountain.

The intermediate pressure just COMPLETELY and utterly goes down from 140 psig down to 0 psig ambient instantaneously?
Yes.

First stage intermediate pressure is present in the hose and the part of the SS before the SS stage valve opens. It is intermediate pressure in all parts upstream of the valve when it is closed to the first stage including any parts of the SS stage housing that is upstream of the valve.
Correct. And when the gas passes the valve and enters the second stage, it expands and is at ambient pressure. This expansion also leads to a decrease in the gas' temperature, which is why a 2nd stage can freeze up and freeflow when you hit the purge button in cold water.
 
+1 for Storker's explanation. Well, maybe +0.90. To be picky, the pressure doesn't instantaneously drop from the IP until the gas fully expands. Consider a leaf blower. The air is compressed, then shoots out of a tube into ambient pressure air, yet still applies force to the leaves some distance away. Pressure = Force / Area . The more the gas has expanded into the ambient atmosphere, the lower the pressure difference.
Back to the 2nd stage regulator: by the time the air has expanded into your lungs there is only a slight over-pressure. To some extent, the over-pressure is related to the rate at which the gas is allowed to escape when there is demand on the 2nd stage. (Put your hand 1ft in front of a tank without a regulator. Crack the valve a little - feel a gentle breeze on your hand. Crank it wide open and hope you don't force bubbles into your skin. But in both cases the difference between the tank pressure and "ambient" is the same. )
 
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?

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.

This is fairly basic stuff. Given your tone towards me in previous posts about regulators (insulting and condescending) I would hope that you would have a better understanding of how these things work. Maybe you'll learn something.
 
+1 for Storker's explanation. Well, maybe +0.90. To be picky, the pressure doesn't instantaneously drop from the IP until the gas fully expands. Consider a leaf blower. The air is compressed, then shoots out of a tube into ambient pressure air, yet still applies force to the leaves some distance away. Pressure = Force / Area .

Unless I'm mistaken, leaf blowers do not compress air, they simply direct it with a fan. I guess you could say the air "compresses" against the fan blades. But it's not a good analogy with regulators.

Compressed air will expand to the ambient pressure whenever it has the opportunity. Sometimes there's quite a bit of force associated with this expansion, like if a tank ruptured while it was being filled. Sometimes there's a little, like air going through the 2nd stage valve. But once that air exits the valve, it is for all intents and purposes at ambient pressure within the 2nd stage. It cannot still be pressurized because there is nothing containing it.

Another issue with regards to pressure in the 2nd stage is venturi action. Air moving quickly in a linear fashion lowers pressure in it's immediate vicinity. This is how airplanes work; I'm sure you know that. So as air moves quickly through the 2nd stage body under purge, it actually lowers the pressure in the case.
 
if a 33 foot wave passes over head while on scuba - would that essentially change the ATA (adding 1ATA)

No. The surface does indeed rise 33/2=16,5 feet but then it drops back down to zero and then to minus 33/2 and then up and down and up and down... It averages to zero. Shaken, not lifted.

I cannot say more about wave pressure than
a) what is "surface"? Flat surface or wave bottom? I say flat surface.
b) at great depth the added (wave) pressure certainly averages to zero over time (one wave lengt) as there will be less/more water as compared to flat surface. Averages, but is it actually constant?
c) the instantaneous relative effect (e.g. in %) is greatest at the level of the bottom of the wave (lots of water versus no water). This is actually "at depth". The effect averages to half a wave worth of pressure of course, which is only natural as the wave bottom is 1/2 wave below flat surface.
d) Water is a viscose and nearly incompressible thing and all. It affects how pressure propagates.
e) Also remember that wave action equals to water going up and down, and at shallow depth that would also affect the diver.
f) Not my specialty. Looks complicated. Experiences, anyone?

You will need a pretty robust dive boat would you intend to dive in 33ft waves.

If you feel unsafe then replace that 100% oxygen with something more diluted.
 
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I think this discussion got off topic a bit so bringing it back a bit...if you are in 10' seas, at 60', the proportional atm change will not be enough to cause you big concerns. As you near the surface, you will move up and down to some degree with the swells, so that 10' sea will not result in an instant 1/3 atm change in pressure.
 
Right, we're getting off topic, and there's a risk of getting pedantic. So, I'll leave it, even though I could disagree with some of the explanations above. <sits on hands> :wink:
 
I asked a related question a long time ago: If you dive down to, say, 100' inside a tube (that you can just fit into), is the pressure on you the same or less than if you were at 100' in open water? The reply was that it is the same, because the only water that affects you pressure-wise is that which is exactly above you. I assume that is correct.
 
https://www.shearwater.com/products/swift/

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