expanding air

[lermontov]

im trying to get my head around the effects of the air as we go deeper -the compressed air form the tank is reduced through the first stage and then through to the second stage at ambient pressure (?) -is the reason we use more at at depth because of the ambient pressure of the surrounding water on the diaphragm thus compressing the air again?

is that correct?

 

[Steve_C]

The ambient pressure does not compress again. It controls how much the pressure is reduced. For a given volume of air, the higher the pressure, the more molecules of air are in the volume. So when you breath more highly pressurized air the more molecules you use up per breath. Our lungs and the surrounding water are at ambient pressure so (at OW depths) we do not noitice that we are breathing higher pressure air.

 

[lermontov]

ok thanks you've also answered my next question too! re molecules

 

[dumpsterDiver]

When i was thirteen and trying to understand this stuff my dad explained.. the pressure of any bubble in the water is EXACTLY equal to the pressure surrounding it.. If the outside pressure was more, the bubble would compress until equal or conversely if the pressure is higher in the bubble - then it will expand until exactly equal in pressure to the outside pressure. .

So the "bubbles" that come out of the regulator mouthpiece are the same pressure as the water pressure..(plus atmospheric pressure of course).

 

[andrei_c]

Hope not being late with a comment.
Why are we using more air at depth? Because the density of gas is direct proportional with the pressure. remember oyur OW course? A balloon of 10 liters at surface, brought at 10m depth will have a volum of only 5 liters but the density of gase will be double the one at surface. In other words, a breath at surface is equal with four breath at 30m for example, where the pressure is 4 ata and because of that the density is increased by 4.

 

[dumpsterDiver]

No it is increased by 3

 

[boulderjohn]

I think we have a second language problem. I think you are both saying the same thing.

At 4 atmospheres of pressure, the density of a gas is 4 times what it is at the surface. If you use the word "increased," though, it sounds like addition rather than multiplication.

I am going to use some ridiculously wrong numbers to make it easier to understand.

Let's say your lungs held 10 molecules of air at the surface. If you held your breath and descended to 4 atmospheres, you would still have 10 molecules, but your lungs would be 1/4 the size. If you instead breathed compressed gas on scuba, your lungs would be the same size as they were on the surface, but you would need 40 molecules (4 X 10) to fill them. Your lungs would now have 30 more molecules than they did on the surface. They would have increased by 3 times the original number.

 

[Kevrumbo]

im trying to get my head around the effects of the air as we go deeper -the compressed air form the tank is reduced through the first stage and then through to the second stage at ambient pressure (?) -is the reason we use more at at depth because of the ambient pressure of the surrounding water on the diaphragm thus compressing the air again? is that correct?

Empirically for a given tank fill at the surface and a given diver's lung tidal volume, the increasing depletion of gas supply at depth is fundamentally due to the Ideal Gas Law relating Pressure, Density and Temperature, essentially stating that Pressure is directly proportional to Gas Density; and the greater breathing gas density at that depth as delivered by the designed function of your regulator. Therefore at 30 meters (4 ATA), you will use up your air supply 4 times faster than you would breathing it at the surface at 1 ATA.

The pressure of the inhaled breath must balance the surrounding or ambient pressure to allow inflation of the lungs. It becomes virtually impossible to breathe air at normal atmospheric pressure through only a simple long snorkel tube to the surface, even at only three feet under the water.

By always providing the appropriate breathing gas at ambient pressure, demand valve regulators ensure the diver can inhale and exhale naturally and without excessive effort, regardless of depth, as and when needed. But again, because of the regulator's mechanical design & function, you must realize you will use up your air supply faster, the deeper you go.

Diving regulator - Wikipedia, the free encyclopedia

 

[TMHeimer]

When i was thirteen and trying to understand this stuff my dad explained.. the pressure of any bubble in the water is EXACTLY equal to the pressure surrounding it.. If the outside pressure was more, the bubble would compress until equal or conversely if the pressure is higher in the bubble - then it will expand until exactly equal in pressure to the outside pressure. .

So the "bubbles" that come out of the regulator mouthpiece are the same pressure as the water pressure..(plus atmospheric pressure of course).

DD,Yes, this is exactly what is explained in the Encyclo. Rec. Diving PADI as I'm sure you knew well before I did! You don't really need a whole page on Henry's (pail) Law....

 

[andrei_c]

Thanks Bouldejohn. I had in mind multiplication, but used the wrong word. Sorry if I cause a confusion with this.

Empirically for a given tank fill at the surface and a given diver's lung tidal volume, the increasing depletion of gas supply at depth is fundamentally due to the Ideal Gas Law relating Pressure, Density and Temperature, essentially stating that Pressure is directly proportional to Gas Density; and the greater breathing gas density at that depth as delivered by the designed function of your regulator. Therefore at 30 meters (4 ATA), you will use up your air supply 4 times faster than you would breathing it at the surface at 1 ATA.

I think Kevrumbo lines are very clear