How can air volume shrink as you go deeper, when the tank itself doesn't shrink?

[CT-Rich]

Because you bleed it out by breathing through regulator. Take a spare tank with a pressure gauge on the tank that shows, say 3000 psig and don't breathe from it and see if that spare tank pressure would go down or not. I bet it'll stay the same at 3000 psig at depth.

Unfortunatel, the change in temperature will a have a big impact, so this experiment is hard to do.

I was watching a video about wreck of the Hornet in~1,500 m of water. One of the pictures showed an oxy-acetaline tank that was completely pancaked from the pressur.

 

[Dan]

Unfortunatel, the change in temperature will a have a big impact, so this experiment is hard to do.

I was watching a video about wreck of the Hornet in~1,500 m of water. One of the pictures showed an oxy-acetaline tank that was completely pancaked from the pressur.

We are talking about basic, open water case, air in the tank, so I don't want to complicate small thing and stay on KISS principle. Say we go down to 60 ft depth where the water temperature may drop by 10F, the effect of that temperature drop would make the air in the tank becomes 2% denser [small drop in pressure assuming ideal gas, PV = nRT, where V (volume), n (moles of air) and R (gas constant) are constant values, so the change in P (pressure) will be directly proportional with the change in T (temperature in degrees Rankine)].

 

[James79]

Thank you for mentioning that. I always thought you lungs volume got smaller as you go deeper but after reading your post and everybody else's posts, now I understand why the lung volume at any depth would be the same as on the surface.

This also is CRITICAL to understand for safety. @The Chairman and a couple others mentioned it, but it's important enough I want to spell it out as clearly as possible; at 33 feet, it takes twice as many air molecules to fill your lung volume. As you ascend, that one lungful of (twice as many molecules) air would expand to twice the volume (two lungfulls) at the surface. There are no nerves inside the lungs to feel overpressure... If you hold your breathe ascending that overpressure (as the air in your lungs expands) will force air into your bloodstream (embolism). This is the why of the rule to never hold your breath. If you keep breathing the airway stays open, and as you ascend the expansion is just exhaled. It's also why a CESA involves exhaling all the way up.... As you ascend the residual air in your lungs is expanding so there is constantly more to exhale.
As another mentioned, I applaud you for asking the question and seeking understanding... It will make you a better and safer diver!

Respectfully,

James

 

[Scott G. Bonser]

Usable volume in the tank changes with depth.
For normal dives it's not not noticeable and irrelevant..

But I 800 ft deep a certain amount is locked in the tank unusable until you ascend....

That is why if you run out of air at depth, you will get a breath or 2 on the way up.

…personal case in point. 1977, adverse conditions at 45‘. My ‘J’ valve deployed on a rebar survey stake using a state provided regulator….I didn’t notice as I was cold and fatigued from fighting the current. This was while I was volunteering w/Yorktown Shipwreck project. I ran out of air, reached to flip my ‘J’ valve it already was flipped as it was not flipped before my ‘buddy’ and I entered the water, ( no gauge on these regulators, so we had to rely on ‘J’ valve ) My SCUBA training kicked in. Choices were, Buddy breathe after swimming 220’ upcurrent or ascend and hope for the best. I made effort to safely ascend in full gear, wt belt and all. I got a full breath around 30’, another about 8-10 feet from the surface and all was well. Boyles Law ( I believe ) as it applies to SCUBA diving. Dive gear at that time was just experimenting with BCD just being developed. Horse collar BC vests w/CO2 inflator cartridges and no octopus regulator as it was just catching on as a safety feature. State of Virginia wouldn’t add gauges to their gear. After that I used my personal regulator and gauge. Next season we were using hookah rigs. best I can tell you is…It Works.

Scott G. Bonser
giantfroginthepool

 

[John C. Ratliff]

I think you're reply caused a light bulb to go off in my head. Near the beginning of the PADI Open Water online course they were talking about how air supply and breathing as you go deeper. There is this:

1. "Your air supply lasts ½ as long at 10 metres/33 feet (2 bar/ata) than at the surface (1 bar/ata)."
"Your air supply lasts 1/3 as long at 20 metres/66 feet (3 bar/ata) than at the surface (1 bar/ata)"

2. "the air gets denser as you go deeper; denser air is harder to breathe than air at normal surface pressure. The deeper you are, the more
energy you use to breathe."

So now that I know that the air molecules in the tank are unaffected, how do you explain the statements above?

As you go deeper, your lung volume goes down. Because of this, you have to breathe more faster and more frequently in order to get the same amount of air you would breathe in at the surface. Because you are breathing more frequently, your air supply doesn't last as long the deeper you go. Do I have this right? Does this explain point #1 above?

Because you're breathing faster, this requires more energy. Does this explain point #2 above?

This part of your description is not correct. The air is denser, but not so much that breathing should be shallower or faster. Lung volume stays the same, but the air is denser. It is the density of the air that changes.

SeaRat

 

[John C. Ratliff]

I under what you are saying, but what about this:

The PADI online course says, "because the air you breathe while underwater is denser than at the surface it requires more effort to breathe. And the denser the air and the faster you breathe, the more energy you use breathing."

Why does it require more effort to breathe?
Why do you breathe faster?

You have had some good answers to your question, but I’d like to point out one thing that may have some bearing on this discussion. Most scuba regulators now provide a very good “Venturi Effect,“ making breathing effort nearly effortless at depth. This has been designed into most second stages now on the market.

I think the PADI statement above applies more to rebreather diving than to traditional scuba diving using open circuit. With rebreather diving, the diver’s lungs provide the effort to inhale the gas. This is true throughout the breathing cycle. The denser the gas, the more effort it takes to move it.

But with an open circuit scuba, the second stage actually “shoots” air at the diver’s mouth, and after the effort to start the regulator functioning (breaking effort), the regulator’s Venturi effect will cause effort to decrease. When the diver starts exhaling, the Venturi effect is cancelled by the increased pressure from the diver to exhale (some regulators can actually be “tuned” to not do this, and can overpressure the diver‘s lungs if set incorrectly—the Scubapro Pilot has this possibility but has been discontinued for decades now).

So from my perspective, this PADI on-line course quote was probably taken out of context, or PADI worded it poorly.

SeaRat

 

[J-Vo]

I think PADI intended to say you consume air faster, not your breath rate is faster.

I would like to clarify something that for some reason blocked my understanding a as a "D" average high school student taking SCUBA. When we reference breathing "compressed air" it's not because the air is compressed into the tank, it's because the air we inhale is compressed by the water pressure at depth.

 

[Scott G. Bonser]

We are talking about basic, open water case, air in the tank, so I don't want to complicate small thing and stay on KISS principle. Say we go down to 60 ft depth where the water temperature may drop by 10F, the effect of that temperature drop would make the air in the tank becomes 2% denser [small drop in pressure assuming ideal gas, PV = nRT, where V (volume), n (moles of air) and R (gas constant) are constant values, so the change in P (pressure) will be directly proportional with the change in T (temperature in degrees Rankine)].

…like the tank pressure decreases as your tank cools after a ‘hot fill’…
giantfroginthepool

 

[Scott G. Bonser]

I think PADI intended to say you consume air faster, not your breath rate is faster.

I would like to clarify something that for some reason blocked my understanding a as a "D" average high school student taking SCUBA. When we reference breathing "compressed air" it's not because the air is compressed into the tank, it's because the air we inhale is compressed by the water pressure at depth.

Bingo

 

[BlueTrin]

Well this really changes things. Since the air in the tank is an air space, I thought that the air in the tank would get compressed as you go down deeper.

When the air in your tank is at 232bar, it is already 232 times compressed, but you don’t see the metal tank expanding when you fill it.

The tank is quite rigid so it can bear a certain amount of pressure.

Like others said, after the first stage, the gas you inhale will be at the ambiant pressure (which is function of your depth) but others explained better so I’ll leave it at this.