Where does that extra breath or two come from?

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CaptainHornblower

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I just don't log dives
in an emergency swimming ascent, as you get shallower you may get an extra breath, which is one reason for keeping your reg in your mouth.

Where does that extra breath come from? expansion of the air in the hose; it doesn't seem like there would be enough air in the hose to amount to a partial breath? expansion of the air in the tank; it's a rigid container, so I've been told the air in the tank would not provide that extra breath.

Anyone have a definitive answer?
 
I want to say it's because as the ambient pressure decreases as you ascend - at lower ambient pressures, the first stage might deliver the last remaining 'gasps' of air from the tank. It's a bit like the reverse that you can see if you descend with a valve only partially open - can breath easily near the surface but as you descend the first stage can't deliver the air.

It's not a definitive answer, and I'm hardly an extensively experienced diver, but I am curious to hear about the mechanics of what is actually happening - it's not easily googled :)
 
The extra breath has to come from the tank, where else could it be?
The Intermediate Pressure of most 1st stage is set at about 9 bars. So if the content of the tank is below that, no gas will be able to deliver through your 1st stage. As you ascent the ambient pressure drops, gas expands(like shooting a smb from depth) inside the tank and that should provide an extra breath or two.
My tec instructor had convinced me to do that at 40m with a twin set. Sucked the content dry and then ascent slowly. He was holding a 2nd stage in front of my face just in case. Lo and behold, there was breathing gas in my supposed empty tank as I ascent.
 
Its differential pressure. at 100 ft you cant empty your tank because ambient pressure is IE 50 psi so the tank can not bo lower than that. when you get to 20 ft the ambient is about 10 psi and the 50 in the tank can now move from the tank to the diver. there is 40psi difference between the tank and the mouth piece.
 
You can only breath from a scuba tank if the gas pressure sufficiently exceeds ambient pressure to allow regulator function (IP).

As you ascend, ambient pressure reduces. This can allow the regulator to function again - permitting (in some circumstances) some inhalation from the tank.

To a lesser degree, there is also some tiny effect from Boyle's Law in the regulator hose. There is also the issue that the pressure of gas in the hose must equal/exceed ambient pressure - otherwise you won't have the lung strength to inhale it.

At no point can you create (inhale!) a vacuum in your scuba system. There's always some gas there (sum total of cylinder regulator and hoses).

With conventional scuba equipment, the volume of air in the hose is quite inconsequential. With a longer (2m) hose, the greater volume of air contained can provide a partial inhalation.

As you ascent the ambient pressure drops, gas expands(like shooting a smb from depth) inside the tank and that should provide an extra breath or two.

Back to school for you....

Diving Physics #101 - Boyle's Law and Sealed, Inflexible Containers...

Must be a typo, right?
 
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You get the extra breaths because the ambient pressure around you decreases which allows you to inhale the remaining gas in the tank.. Your lungs must be capable of overcoming the surrounding ambient pressure, this decreases as you ascend which then allows you to inhale any remaining gas in the tank.

Several have however, given you incorrect information on regulator operation. When the pressure in the tank drops to the IP pressure of the regulator (around 135 psi/ 9 bar) plus ambient pressure, the reg does NOT stop allowing gas to flow through it, you can always get any remaining gas in the tank as long as you have the lung power to pull it out. Once the internal pressure of the tank reaches the regs IP plus ambient pressure (roughly 15 psi/ 1BAR per 33ft/ 1M) the HP valve of the reg opens fully and remains open until tank pressure is once again above IP + ambient. This can occur as you ascend but since you only get around 15 psi/1 bar for each 33 ft you ascend it's not much. As you breath the gas past the point where the reg is fully open (below IP plus ambient) , inhalation gets harder and harder with each breath but all of the remaining gas in the tank is accessible. This is because the IP is no longer being regulated but is now simply the amount of pressure in the tank above the surrounding ambient pressure. There are a number of flow restrictions in a regulator and without the assistance of the higher pressure, your lungs must supply additional energy to get the gas out.....it becomes harder to breath. It is also incorrect to say you can not "create a vacuum in your scuba system" it can in fact be done and can give you slightly more gas if you do. Breathing a tank down to slightly below 0psi (or pulling a slight vacuum) is possible. You can't pull much of a vacuum, just the amount your lungs can create over ambient pressure and certainly not a full vacuum but a slight one is possible. It's a small amount of gas and not something you should count on but "can't " is an incorrect statement...
 
the air comes from your tank and the extra breath is permitted by decreasing ambient water pressure as you ascent.

the air inside your tank is under pressure and is always trying to push its way out (similar to water constantly trying to push its way into your camera housing). things flow from high pressure regions to lower pressure regions.

as you breath down your tank, the pressure of the air decreases. at some point the pressure inside your tank / regulator will equal the ambient water pressure. no more air flow. your tank appears to be "empty". it is not. it has simply equalized with the water pressure around you. the deeper you are the more air remaining in the "empty" tank.

as you ascend, the ambient water pressure decreases while the pressure in your tank stays the same. so air can once again flow out of the tank.
 
. . . expansion of the air in the tank; it's a rigid container, so I've been told the air in the tank would not provide that extra breath.

Although "it's a rigid container" you're forgetting that it has a regulator delivering the gas from it. It's a rigid container, so the number of molecules of gas doesn't change. However, the regulator delivers those molecules in relation to whatever the ambient pressure is. At great depth, it delivers a lot of molecules with every breath. Running out of gas at depth means there are no longer enough molecules in the tank for the regulator to deliver the next such breathful. At shallower depths, it delivers fewer molecules with every breath. Those last few molecules are in there, waiting to become deliverable with yet another breath as depth decreases on ascent.
 
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.
 
from a laymans viewpoint read my response. for technical details of how the first and second stage screw up the theory, read hermans post.

Then find a different instructor...
 

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