how does second stage reduces Intermediate Pressure

[phgachoud]

Am doing my padi IDC and had a question (dont remember where...) about second stage reducing the IP to the ambiant pressure the diver needs... am really not understanding how that can be. From my understanding, correct me if I'm wrong.

• IP is around 8-9Bar + ambiant pressure 0m=8-9Bar + 1Bar | 10m=8-9Bar + 2Bar | ...
• Second stage open its valve on demand, meaning the more you suck the more it opens its valve pushing on the lever. So the maximum air pressure it can deliver is given by the surface of the biggest hole from the IP down to the mouthpiece right? it doesnt seem to me there are holes smaller into a hose down to a second stage making sure this pressure is never > 1Bar on ambiant pressure. So, one question would be, can a second stage deliver more than 1Bar on ambiant pressure or not, and if so why and how is it regulated so that you dont receive 9 bars on the mouth and if not how is this 1 bar limited??
• If I suck as hard as I can, how much pressure can I take from my second stage?
• Are we talking about demand valves because you'll regulate your pressure with the force you suck limiting the size of the hole between the pupet and the inlet cone?

Lets take for the below examples an IP of 8Bar (knowing many second stages support up to 12-14 bars even if really not recommended)

Depth (in m)	Absolute IP (Intermediate Pressure from First stage) in Bar	Second stage pressure minimum on mouthpiece in Bar	Second stage pressure maximum on mouthpiece in Bar
0	8+1=9	1	?
10	8+2=10	2	?
20	8+3=11	3	?
30	8+4=12	4	?

On the attached picture, the maximum pressure is probably defined by widest space for the air to flow (in green above) is that what defines the maximum pressure a second stage can deliver and its proportional to the first stage pressure? if so are we talking about always a maximum of my above minimum column or more? Sorry might be redundant and hope you got my question and point...

 

[Bob DBF]

A second stage is designed to close at ambient pressure. When you breathe in, the pressure drops below ambient and the reg flows until the pressure reaches ambient, at which point the valve closes. Ambient pressure depends on depth.

If a second free flows due to a high IP, the pressure in the second can be higher than ambient, but this is a failure, not normal operation.

 

[davehicks]

The first stage delivers an IP that is "constant" relative to the ambient pressure. As you go deeper the IP gets higher to compensate for the ambient. The 2nd stage uses a set of springs and levers to keep the valves balanced closed on a knifes edge. A very slight inhalation creates a low-pressure space in the 2nd stage body that lets gas leak out of the valve to inhale, at a pressure that is slightly in excess of ambient. Since the pressure is at ambient your lungs are able to inflate against the pressure of the water around you.

 

[phgachoud]

The first stage delivers an IP that is "constant" relative to the ambient pressure. As you go deeper the IP gets higher to compensate for the ambient. The 2nd stage uses a set of springs and levers to keep the valves balanced closed on a knifes edge. A very slight inhalation creates a low-pressure space in the 2nd stage body that lets gas leak out of the valve to inhale, at a pressure that is slightly in excess of ambient. Since the pressure is at ambient your lungs are able to inflate against the pressure of the water around you.

Thx, I am pretty aware of the functioning, am looking about details on what is the physic principle that garantees this slightly excess of ambiant you are talking about. Is it the space between the knife edge and the puppet (rubber piece that closes the inlet)?

 

[davehicks]

Thx, I am pretty aware of the functioning, am looking about details on what is the physic principle that garantees this slightly excess of ambiant you are talking about. Is it the space between the knife edge and the puppet (rubber piece that closes the inlet)?

I can't help you there. I am sure there is some Bernoulli equation in play, but I don't know the physics. The 1st stage IP is going to stay fixed at around 135psi above ambient ATA. The 2nd stage valve is balanced lightly closed and gas will flow out a greater than ambient allowing you to breath. Now if you forced open the 2nd stage valve then the gas flow would significantly exceed ambient and you would have problems.

I've seen a second stage regulator become detached underwater due to a failed swivel. You get a torrent to air that will drain the cylinder in minutes.

 

[rsingler]

I think you're over-thinking this a little bit.
The second stage valve is just a spring-loaded trap door. Behind the door is 130 psi over ambient, at any depth (thanks to the first stage).

Your octopus is full of water, and the pressure of the water inside is "ambient".
Your primary second stage, with your mouth on it, if you hadn't taken a breath all the way to the bottom, would be at "surface ambient" and your tongue would get sucked into the hole.
But of course, you have taken a breath, and every time you did, the IP gave you as much air as you wanted. You didn't feel 130psi, because you barely crack the second stage valve. But that extra gas at 130 psi over ambient helps keep your mouth and chest from collapsing at depth. In other words, every breath keeps your chest and cheeks "at ambient". You just don't notice it, because you breathe during descent, during which time the  absolute IP is increasing.

The exhaust valve controls overpressure in the second stage. Any pressure above ambient simply bleeds out the exhaust valve. Any pressure below ambient disappears with your next breath.

 

[rsingler]

one question would be, can a second stage deliver more than 1Bar on ambiant pressure or not, and if so why and how is it regulated so that you dont receive 9 bars on the mouth and if not how is this 1 bar limited??

Then here, you don't get a full bar above ambient because the second stage valve spring closes the valve before pressure rises above "zero" (ambient). You have to suck -1.2" WC to open the valve, and unless you freeflow, it'll close before it rises to zero. Anything above zero (ambient) during a freeflow blows out the exhaust. What @Bob DBF said...

 

[Akimbo]

This might be helpful to some readers:

How Diving Regulators Work

We all use them, but how many divers understand how they work? Diving regulators are not only impressive bits of technical kit, but are also likely to be one of the first things new divers buy when they start diving. They have undergone many changes since the first demand regulator in 1864...

www.obsequio.ltd

 

[halocline]

Your primary second stage, with your mouth on it, if you hadn't taken a breath all the way to the bottom, would be at "surface ambient" and your tongue would get sucked into the hole.

I don't think so. The air inside the 2nd stage case, when subjected to increasing pressure by the water on the outside of the diaphragm, would compress, which collapses the diaphragm, which pushes the lever, which opens the valve, which restores ambient pressure to the inside of the 2nd stage case.

Basically the flexible diaphragm ensures that the pressure inside the 2nd stage case is always equal to or very close to ambient, never dropping below. As you correctly pointed out, the exhaust valve ensures that the pressure inside the case never exceeds ambient.

The only difference is the difference in depth between the exhaust valve and the diaphragm, which you're very familiar with as case fault geometry. This is why most 2nd stages need to be set around 1" or greater of cracking pressure, to account for the exhaust valve being 1" shallower in normal diving position.

To the OP; we're talking about very slight differences in pressure, nowhere near 1 bar. For your question, the simple answer is that the air pressure inside the 2nd stage case, whether at the mouthpiece flange, diaphragm center, or exhaust valve location, is always very close to ambient pressure. You could force the issue by blocking the mouthpiece opening with your tongue and then pushing the purge to fully open the valve, which would result in a violent force of air entering the 2nd stage, but it immediately exits the exhaust valve. How far above ambient the actual pressure on your tongue would be is minimal, and really irrelevant.

 

[rsingler]

Of course you are right, @halocline , as you always are. But absent valve opening, it was a nice visual to make a point.
"Diver dies with crushed chest and blue tongue as scuba equipment fails. "
I can see the headline now...