What you wrote is all wrong!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?
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.