Where does that extra breath or two come from?

[Lorenzoid]

Partially from ... your lungs.

I don't think that was the question here. As I understand it, in some OW courses the instructor mentions this (apparently near-mythic) "extra breath" that might, under some circumstances, be delivered in response to the diver inhaling during ascent. I vaguely recall hearing it myself.

The air in a diver's lungs expands on ascent, for sure. But I don't think that addresses the "extra breath" question.

 

[scagrotto]

I was having a discussion with my instructor trainer and I thought it would be from the tank, but he insisted that is was not from the tank, but from the expansion of the air in the hose and second stage.

"Instructor trainer"? Just to be clear, you mean it wasn't "just" an instructor who told you that. It was somebody who trains instructors? That gives me enormous confidence in the system.

 

[EricTheDood]

Likewise, if you suck your tank empty and you have zero PSI at 130 feet, when they find your body later that day on the surface, your tank is still gonna have zero PSI in it. Your lungs don't work the way the solid steel rigid tank does because your body is not a rigid capsule.

This is accurate except you can never actually suck your tank empty. Your regulators won't allow it. Even at the surface you can't get below 1ATM. In contrast, if you rigged a hose directly to the tank, you could probably suck out a small amount of air. Your lungs/diaphragm won't take it down anywhere near vacuum, but you'll get something.

A 40m/130ft, ambient pressure is 5ATM, or about 73 psi. Your SPG is zeroed at 1ATM, so it'll actually read 4ATM, or about 59psi. So "empty" at this 130ft is about 59psi. Take that tank up to the surface, and it'll still be at 59psi, but you'll be able to breath it down to "zero" (1ATM). That's where those "extra breaths" are coming from.

 

[Subcooled]

as you get shallower you may get an extra breath (...)
Where does that extra breath come from? (...) Anyone have a definitive answer?

A pressure difference means that something wants to move (from the area of higher pressure to the area of lower pressure). If there is more pressure inside the cylinder than there is outside, then air wants to get out of the cylinder -> You can breathe. If the pressures are equal, you cannot breathe, because nothing is coming from the cylinder. The pressure inside the very rigid tank is not affected by external pressure. It is only lowered by breathing (if any).

For a given amount of air (atoms/molecules/grams/pounds) in constant temperature, pressure times volume is constant. You can suck a plastic bottle empty (it collapses, volume drops) but you can't do that with a steel bottle (your lungs cannot make it to collapse). You would need pretty strong lungs to bend metal and make the cylinder smaller...

When you ascend, the pressure inside the tank remains constant. A fixed amount of air (in weight or molecule count or whatever measure you prefer) in a fixed volume (V) at constant temperature (T) means a fixed pressure (P). P times V equals constant times T is a valid model. P V=nR T. Ideal gas law.

When you ascend, the pressure inside the rigid cylinder remains constant, but the external pressure drops. Hence, air wants to flow out and you can breathe. When you breathe, the amount of air inside the tank (the "nR" part in the equation) reduces, and so does the pressure. After a breath or two you cannot breathe again. But, if you ascend and the external pressure drops again... you can breathe once more.

 

[archer1960]

Thank you gents! I was having a discussion with my instructor trainer and I thought it would be from the tank, but he insisted that is was not from the tank, but from the expansion of the air in the hose and second stage.

For a "definitive" answer I'm going to take my 18 cu ft pony bottle out, take it down to about 100, breath it dry and then shut off the valve. Then I'll surface and see if I get anything just out of the hose/regulator. The I'll open the valve and see what I get. Probably not much with that puny pony tank, but it will be an interesting experiment.

Your instructor is incorrect; the gas comes from the tank as the ambient pressure drops below the pressure in the tank.

 

[archer1960]

Thank you gents! I was having a discussion with my instructor trainer and I thought it would be from the tank, but he insisted that is was not from the tank, but from the expansion of the air in the hose and second stage.

For a "definitive" answer I'm going to take my 18 cu ft pony bottle out, take it down to about 100, breath it dry and then shut off the valve. Then I'll surface and see if I get anything just out of the hose/regulator. The I'll open the valve and see what I get. Probably not much with that puny pony tank, but it will be an interesting experiment.

That would be a good way of demonstrating it, and you WILL get some air out when you get back to the surface. Going from 100 ft back to the surface, you might get 2 or 3 full breaths out of it.

 

[toby43]

Anything below 10m you can forget it, stay close to your buddy.

 

[Subcooled]

When a given amount of air, e.g. 100 grams, leaves the cylinder, and its pressure drops to 1/100 (like 200 ATA -> 2 ATA), then its volume increases to 100x. If the pressure drops to 1/50 (e.g. 100 ATA -> 2 ATA) then the volume increases to 50x. The weight of the consumed gas is constant of course (one gram in cylinder is one gram in lungs), but in smaller pressure it takes more space, in higher pressure it gets compressed. This is a very rough model, but adequate for now.

If the pressure inside the cylinder is 2 ATA and outside it is 1.8 ATA then the pressure drop is only 0.2 ATA, 10%, and not much expansion takes place. The volume of what little air comes out is hardly sufficient to fill your lungs. The longer the ascent the bigger the chance of getting that extra breath or two. Those breaths are more easily available near the surface (where relative pressure drops quickly),

An ascent from 10m/30ft to surface would be easiest (external pressure drops to half). An ascent from 30m/100ft to 10m (again pressure drops to 1/2) would give you the same amount of breaths, but there is twice the distance to swim. It gets even worse at greater depths. Don't rely on those breaths at 70m

 

[Bob DBF]

One problem with extra breaths is the mythology. Back in the day we had unbalanced regs that would get hard to breathe as you went deeper and also as the tank pessure decreased. As you surfaced you had the effect in the tank that EricTheDood explained, the fact that the regs woûld give air if breathed harder, as well as they breathed easier shallower. Back when I started diving, before SPG's, breathing hard was how you knew you were OOA and then you could ascend at 60'/min breathing more or less normally on the "extra breaths", and still have air at the surface.

The problem is that new regs are not as forgiving.


Bob

 

[LariatAdvance]

This is accurate except you can never actually suck your tank empty. Your regulators won't allow it.

Well, I didn't mean literally. I realize you're not going to get your tank PSI to a negative ATM by sucking on the reg. I meant breathing until there's nothing left to breathe, and I've done that. Of course it was shallow 30 feet and not at 110 feet. I've cracked a tank after "sucking a tank dry" (so to speak) to blow the water off the reg cover and nothings come out, so I assume there's at least 14.7 PSI still inside the tank.