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

[Angelo Farina]

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?

What you wrote is all wrong!
Didn't you read what I posted here above?
Let's go step by step through what you wrote:
1) air consumption is due by the product of your breathing volume (usually around 3 liters) by the number of complete breathing cycles per minute (usually 6÷7). The resulting vented volume is around 20 liters/minute. This does not change significantly with depth.
What changes is the ambient pressure. The regulator reduces the high pressure inside the tank to the ambient pressure. So, at surface the pressure is 1 bar, and your air consumption is roughly 20 normal-liters per minute (one normal-liter is a liter of gas at 1 bar and 0 °C).
At 10 m the pressure is 2 bar, hence the air density is doubled, and your consumption becomes 20 normal-liters per minute. And so on, the deeper you are, the higher the ambient pressure, the larger will be gas consumption due to its larger density.

2) as density is proportional to pressure, of course when you dive deep you are venting the standard 20 liters/minute, but they weight much more. At surface (1 bar) the air density is 1.2 g/liter, so you are venting 24 g/minute. At 10m (2 bar) the air density is 2.4 g/liter so you are venting 48 g/minute. However density has no effect on the amount of work required for venting. What matters is air's VISCOSITY, not density.
As also viscosity increases with pressure, the work required for venting your standard 20 liters/minute also increases with depth. So experienced divers usually REDUCE the vented volume per minute when diving deep, keeping the same vented volume (3 liters) but slowing down the rate of breathing (4÷5 cycles per minutes instead of 6÷7).
This reduces the amount of work required for breathing and helps also reducing the air consumption.

3) when you go deep with a scuba system, your lung volume does not change, as you are breathing normally, venting the same 3 liters at each respiratory cycle.
Novice divers usually keep the very same breathing rate (6÷7 cycles/minute) as at shallow depth. As already explained, experienced divers slow down the rate, but the lung volume remains the same.
The lung volume decreases only for free divers, as they are holding their breath, and cannot inhale new air for restoring the lung's volume.
If a free diver meets a scuba diver, and the latter gives him a full breath of air from his regulator, the free diver will inhale those 3 liters of air, restoring his lung volume to the original volume at surface: but now he faces a severe risk! If the free diver, now with full lungs, ascends breath-holding, the gas inside his chest will expand significantly, rupturing his alveoli and causing a bad air embolism.

4) some VERY inexperienced and crap divers effectively can start to "shallow breath", due to panic and/or over-exertion.
When the vented volume reduces to less than 1 liter, you are basically breathing in and out the same air, as the trachea has a "dead volume" and you are not getting fresh air in your lungs. This makes the concentration of CO2 in your blood to increase, which triggers the "fast breathing" reflex.
It is a vicious circle, you start venting very fast and very shallow, without any efficient gas exchange in your lungs. After a minute of such horrible sensation of not getting enough air, usually the diver abruptly ascends to the surface, risking the same air embolism as the free diver in the previous example.
So it is recommended to novice divers to breath with a good vented volume (at least 3 liters) and keeping a slow rate (never more than 7 cycles per minute - the slower, the better).
If you feel the need of breathing faster, it means that CO2 is building up: you need to stop, calm down, breath profoundly, until this need of breathing fades away.

5) regarding the larger air consumption at depth, this is not due to faster breathing, but to breathing the same volume per minute as at the surface, but of a more compressed and dense gas.

 

[BackAfter30]

L13 said that the gas in your tank is unaffected. If that's the case, then how can the pressure in the tank change with depth?

Read that again. @CT-Rich said that the RELATIVE pressure changes with depth. Remember where you learned that there are two ways to measure pressure - Absolute Pressure, and Gauge Pressure? Absolute pressure is the pressure of the gas relative to total vacuum - think out in space; gauge pressure is the pressure that you measure against ambient pressure - 1 bar, 14.7 psi at sea level, etc. The relative pressure that CT-Rich referred to is the generic case of gauge pressure. The pressure of your gas in your tank is useful only relative to your local ambient pressure. The deeper you are, the higher pressure you experience and the higher the pressure the air in you tank has to oppose to get through your regulator system and into your lungs. This is the basis of the reason that if you OOA at depth you want to keep trying to breath as you ascend since you will be facing lower pressure; the gas that couldn't exit the tank at greater depth now is opposed by less ambient pressure and can make its way to your lungs.

 

[Angelo Farina]

Read that again. @CT-Rich said that the RELATIVE pressure changes with depth. Remember where you learned that there are two ways to measure pressure - Absolute Pressure, and Gauge Pressure? Absolute pressure is the pressure of the gas relative to total vacuum - think out in space; gauge pressure is the pressure that you measure against ambient pressure - 1 bar, 14.7 psi at sea level, etc. The relative pressure that CT-Rich referred to is the generic case of gauge pressure. The pressure of your gas in your tank is useful only relative to your local ambient pressure. The deeper you are, the higher pressure you experience and the higher the pressure the air in you tank has to oppose to get through your regulator system and into your lungs. This is the basis of the reason that if you OOA at depth you want to keep trying to breath as you ascend since you will be facing lower pressure; the gas that couldn't exit the tank at greater depth now is opposed by less ambient pressure and can make its way to your lungs.

Correct. The absolute pressure of air inside the tank does not change with depth.
The SPG measures the relative pressure, so if you descend quickly you will see the pressure shown by the SPG to decrease slightly (1 bar every 10 meters).
But in reality the pressure inside the tank is not changing...
It is the ambient pressure which is increasing with depth.

 

[Duke Dive Medicine]

In the picture below, at 50 meters the gas volume is only 17% of what it was at the surface. At that depth, surely the tank itself isn't 17% of what it was at the surface. Although the tank is exposed to much greater pressure at 50 meters than at the surface, how does the air in the tank actually get compressed into such a small volume? The tank is sealed with no openings so it's not like water can get in there to compress the air that is in the tank.

Here's another question. If you were to visualize the arrangement of the air particles when the air is compressed at any given depth, what would that arrangement of air particles look like? Would all of the air be concentrated at the bottom of the tank (due to gravity), concentrated at the top of the tank, spread out evenly throughout the tank, something else?


View attachment 794741

A very common question with some excellent responses so far. My take, FWIW:

1. The tank is for all practical purposes rigid and incompressible, so the ambient water pressure on the tank will not reduce its volume while you're diving.
2. The higher the tank pressure (the pressure reflected on your SPG), the more gas molecules are packed into the tank.
3. Diving equipment is designed to deliver breathing gas at a pressure that will allow you to inflate your lungs against the surrounding water pressure, whatever that pressure is. The first stage senses the surrounding pressure and delivers gas to the second stage at a set pressure above that ambient pressure.
4. Your lungs are only compressed when you descend while holding your breath. Because of the action of your regulator (#3), your lungs during normal respiration while diving contain the same gas volume as they do on the surface. There are just more gas molecules packed into that lung volume. The deeper you go, the higher the pressure will be in your lungs, therefore the more gas molecules your lungs contain.
This is why your tank is used up more quickly the deeper you go - deeper = more gas molecules in your lungs = using the available gas molecules in your tank more quickly.

As you progress in your diving, you'll learn to calculate approximately how long your tank will last at any given depth given your own personal air consumption rate.

Best regards,
DDM

 

[RXTdiver]

Here is an example:

Say you have a tank with a volume of 1/2 cubic foot (cuft) at surface pressure. If we compress air into it to a typical operating pressure of ~200 atmospheres of pressure, it would now have the molecules of 100cuft of surface air in it (in the US, this tank would be labeled as having a size of 100cuft). On the surface it would last a typical person a couple hours.

Now we go down to 33 feet deep. the pressure of the water is is about 2x what it was on the surface. When you take a breath, it takes twice as many molecules to fill your lungs as it did on the surface, so the tank lasts half as long, and the air you are breathing is twice as dense.

Now go to 100ft, the pressure of the water is about 4x what it was on the surface, and it takes 4x as many molecules of air to fill your lungs, so the tank lasts 1/4 as long.

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?

 

[JoeTPhilly]

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?

Think of it like a kid with cotton candy; when all puffy one can take little bites and it lasts long and is easy to eat. Press it into a ball one might be able to eat the whole thing in a few bites but might choke on it because it is so compressed and hard to swallow.

 

[RobPNW]

That graphic is for a non-rigid bubble of gas, such as in your BCD, lungs, or dry suit.

Plus the tiny bubbles in a wetsuit.

 

[agilis]

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?

1. Denser thicker air
2. Exertion activity

 

[RobPNW]

So, what would happen if you went on a deep dive after a couple of your lunch burritos had time to ferment at depth and then you ascend?

 

[Angelo Farina]

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?

I already explained this. It is not the higher density which causes an increased work of breathing, it is increased ViSCOSITY.
This increases the friction losses in the fluid flowing through your airways.
The friction losses are proportional to viscosity AND to air speed inside the duct (trachea).
So you spend more energy if you breath a gas with higher viscosity (improperly called "density" by Padi) , or if you breath faster than normal (which you should never do).
As an high viscosity "brakes" the air flow, going deep you should breath more slowly, not more fast.
When a diver start breathing fast, it is a big problem.
Fast and short breathing is an involuntary reflex triggered by too much CO2 in your blood. This can be caused by over-exertion (most novice divers cannot swim efficiently, due to bad kicking technique, using their hands for paddling, breathing improperly, etc.).
It can also be caused by panic or strong emotions.
When you breath fast and short, CO2 will skyrocket as you are not properly changing the air in your lungs.
So the problem becomes worse and worse, until you cannot breath anymore and sprint towards the surface, risking an embolism.
One should never breath fast when diving...