air consumption versus depth

I'm new to scuba diving. One of the first teachings that went against my intuition and physics background was the increased air consumption as depth, and breathed air pressure, increase. There is a lot of material out there that relates this phenomenon to the fact that a breathful of air at 2 atm - for instance - will in fact contain 2 breathfuls of surface air. At that rate, the tank will deplete twice as fast. What I don't get is that in that same breathful of 2 atm air, there is also twice as much oxygen. And if there is more oxygen, we should be able to take fewer breaths. After all, the amount of O2 in the tank is the same at any depth. Why is that same amount of O2 not allowing me to produce the same work at 2 atm as it would at 1? One final note: If I go to a Colorado ski resort, where the atmospheric pressure is far less than 1 atm, I will take many more breaths of the same volume as I would at sea level for the same work output. Why isn't the reverse also true when I go diving at depth? Are we so limited in the rate of oxygen we can process? Are we breathing out - and wasting - more oxygen at depth?

The human breathing reflex isn't triggered by low levels of oxygen, its triggered by high levels of carbon dioxide.

cogitoergo once bubbled...
Are we so limited in the rate of oxygen we can process? Are we breathing out - and wasting - more oxygen at depth?

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Yes....CO2 levels "drives" the breathing mechanism. (This is also part of the Skip - Breathing/ CO2 build up scenario)

What your instructor apparently forgot to tell you is that the amount of oxygen you're breathing isn't what controls your desire to take a breath. That is controlled by the amount of carbon dioxide being built up in your tissues as your blood circulates oxygen to your tissues and removes CO2 from your tissues. In other words, your demand for another breath is controlled by the waste building up in your blood, not the amount of oxygen being delivered to it. What determines CO2 buildup is how hard your body is working and your metabolism rate ... not how much oxygen you're breathing.

... Bob (Grateful Diver)

When you breathe in you will fill you lungs, not half fill them at 10m or 1/4 fill them at 30m.

What is the limiting factor is your rate of oxygen uptake. Even though there may be twice the amount of oxygen, your uptake probably doesn't change that much.

wlo93 once bubbled...
What is the limiting factor is your rate of oxygen uptake. Even though there may be twice the amount of oxygen, your uptake probably doesn't change that much.

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Your body gets all the oxygen it needs by just taking a portion of the available O2. Even at sea level the air you breathe out has only 2 or 3% less than the 21% O2 that you breathed in.

As another poster has already pointed out, you breathe in response to CO2. You generate CO2 in your body when you metabolize the O2. You get rid of the CO2 by passing it into the air that you are breathing in and out.

cogitoergo, that is actually a very good question. I wish the answer was simple. It is true what most replied that the breathing trigger is Co2 not O2. But there are other factors. Much has to do with physilogy, physics, "general gas law" to be specific.
Just as an example, many divers with reasonable consumption can easily spend 30 minutes bottom time at 4ata on a single 80 cuft tank and still have plenty left. It is unlikely that the same diver under the same stress load could swim for 2 hours just under the surface and have enough air from a single 80. Interesting concept.
With todays computeres it can be easily proven. Check air consumption at 4ata, 3ata, 2 ata's etc., under the same conditions. Sure, the deeper a diver is consumption will increase, but not exactly in proportion to the ata's. Interesting test.

The best way to reduce air consumption is not so much to strive to use less oxygen (as you only use maybe 20% of the oxygen in each breath even at sea level anyway) but to produce less CO2.

So economy of motion and working an being very calm will go a long way toward generating less CO2. Less CO2 to get rid of means you can tolerate a slower inhale, a longer pause after the inhale and a slower exhale without building up an excessive amount of CO2 all of which translates to more minutes per tank.

Deeper than normal exhalation also helps clear the C02 a little more efficiently than shallower breathing. And if you are working hard during a dive and are a bit winded - just bite the bullet take those deep and rather frequent breathes and get over it. You just prolong things and aggravate the problem by trying to be stingy on air at that point.

A tolerance for higher than average CO2 levels is something that also develops over time but it can cause problems of it's own if you take it to far. Much of the variability and unpredictability found by the US Navy in their early research into the use of enriched air mixtures and the related PO2 limits was due to their test subjects who were primarily helmet divers who all had developed fairly high tolerances to elevated CO2 levels.

As it turns out higher levels of CO2 can make a diver more susceptible to an O2 hit. This makes me a bit nervous as some of the divers on our local dive rescue team are 1) air hogs, and 2) think they get more mileage per tank on nitrox, which to me, indicates they are potentially building up CO2 with a breathing gas with an elevated PO2 - a potentially bad combination.

cogitoergo once bubbled...
What I don't get is that in that same breathful of 2 atm air, there is also twice as much oxygen. And if there is more oxygen, we should be able to take fewer breaths. After all, the amount of O2 in the tank is the same at any depth.

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These two sentences contradict themselves....one saying there is twice as much oxygen (which is wrong) and the amount of O2 is the same at any depth (which is partially true)

On the sruface...let's say we have 10 cu. feet of air...knowing that we have approx. 21 percent O2 in the air....let's just round (to make math easy) and say of that 10cu. ft of air, we have 2 cu.ft of oxygen.

According to physics...we have an ideal gas law that states as pressure increases, volume decreases at a rate of 1 to 1.....

So at the surface we have 1 atm...at 33 ft we have 2atm...so the volume in the tank is now half, of what we originally had...5cu. ft. So, the amount of oxygen we have is still the same percentage, but not the same volume or amount...the actual volume is 1 cu. ft.

Since the capacity of our lungs is a fixed amount that does not change with pressure we can still take in the same volume or amount of air...so we are taking in the same amount of air/oxygen with each breath no matter where we are, but we are taking it from a different fixed "amount" or volume of air at depth....thus the tank is depleted faster at depth.