Nitrox use question

I got Nitrox certified in the fall and used it for the first time on my trip to Coz in Feb. I've read a couple of really good threads on SB about the uses, benefits, and risks of Nitrox, but there's still one concept I don't quite get. Can anyone explain (not too technical!) why gas/air consumption doesn't decrease when using Nitrox? In other words, if I a diver uses air and then repeats exactly the same dive on Nitrox, why wouldn't the tank last longer? Common sense suggests that if the air you breathe contains more oxygen, then shouldn't your respiratory rate decrease to compensate?

The short answer is, it doesn't work that way. If your body only breathed enough air to get the oxygen it needs (the concept that you are suggesting) then it wouldn't breath anything like the air it does. (You don't use anywhere near all the oxygen in the air that you breath. That is why rebreathers work.) Moreover, if that were the case, when you go down to 33 feet you get double the oxygen you get on the surface with each breath you take. Does your respiratory rate decrease at 33 feet? How about 66 feet? If your theory worked, you would hardly need to breath at all at 99 feet since you would be at 4 atmospheres and have 4 times the oxygen available to you than you have on the surface.

Note that you need more gas at depth to maintain your body structure (i.e. to counteract the effects of the increased pressure at depth). The amount of oxygen it contains is (relatively) irrelevant to that need. The increased oxygen in Nitrox has little to do with your need for oxygen.The amount of oxygen is increased to reduce the amount of nitrogen in your gas supply because the nitrogen has (potential) negative effects on your body (DCS).

Does this help?

<TED>

The issue is that your breathing cycle is controlled by your production of carbon dioxide rather than the availability of oxygen. When the CO2 in your blood reaches a certain level your nervous system creates the urge to breathe.

What this means is that your rate of breathing will stay almost constant whether you have 21% or 100% oxygen in your breathing mixture because, all other things being equal, you are still producing the same amount of CO2.

Only factors which affect your rate of production of CO2 (such as exercise, dehydration, tension) will affect your breathing rate and hence your gas consumption.

Edit: a good way to check this, although by no means prove it (a sort of "idiot check" if you will) is think about the point that Ted raised. If, as you suggest, a higher concentration of oxygen would lower your gas consumption rate why is it that your gas consumption goes up, rather than down, as you dive deeper?

Because it takes X amount of gas to fill your lungs with each breath.

That volume does not change with the composition of the gas being breathed.

the K

The point that Kraken made is also an important piece of the puzzle. I glossed that over in my short explanation because when Winterpeg wrote this:

Winterpeg:

Common sense suggests that if the air you breathe contains more oxygen, then shouldn't your respiratory rate decrease to compensate?

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led me to believe that he/she already understood that and was wondering whether a decrease in respiration rate could compensate for the greater gas density being breathed. I mean, if an increase in O2% could result in a lower respiratory rate (which of curse it doesn't), then the increased density of O2 at depth should logically result in an even bigger decrease in respiratory rate. By demonstrating the fallacy in the latter (using everyone's personal experience of greater gas consumption at depth), I was hoping to provide more supporting evidence that the former was not true either.

I admit that it is slightly misleading/tricksy. It was using X to "illustrate" Y when X and Y weren't directly related. It was a "hand waving" explanation: something that isn't technically correct but gets the point across. I left it in just to provoke thought rather than provide a technical explanation.

4sak3n:

The issue is that your breathing cycle is controlled by your production of carbon dioxide rather than the availability of oxygen. When the CO2 in your blood reaches a certain level your nervous system creates the urge to breathe.

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Duh! I knew that! Did anyone look at my profile? I'm an RN
I guess I put 2 and 2 together and came up with 5.
Thanks everyone, all the answers make sense now that I think about it.

Please dont worry - I once taught a Dr his Nitrox course and he asked me exactly the same question !

In a normal healthy scuba diver, hemoglobin is usually 99% saturated with oxygen on room air. Even breathing 100% oxygen adds very little additional oxygen - you'll get that last 1% of hemoglobin saturated and you'll add a trivial amount of extra oxygen dissolved in the aqueous plasma. This trivial increase in oxygenation of the blood does little to reduce respiratory drive.

Nitrox is about reducing your nitrogen load, not decreasing your gas consumption.

On the other hand - take someone with a hemoglobin of 7mg/dL who has pulmonary edema and an oxygen saturation of 85%. Give them 100% oxygen and the increase in oxygenation of their blood will be significant enough to reduce their respiratory drive, i.e., they'll stop panting and relax a bit.

All excellent responses. By the way, be prepared for a bit of "sticker shock" for Nitrox in Cozumel. It's now up to $10 per TANK extra.

There is only one place in Coz that fills Nitrox so they've jacked the price. I used to dive Nitrox on every dive in Coz. With the prices where they are, I'm doing air on the first (deep) dive and Nitrox on the second.

-Charles

charlesml3:

With the prices where they are, I'm doing air on the first (deep) dive and Nitrox on the second.

-Charles

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That's what we did, but being newly certified in Nitrox, I wondered when we got back if we spent our $ wisely or not. That's why I looked up the threads on Nitrox use. From what I've read, it looks like we did, and will probably do the same again when we go back next year I just couldn't figure out why the Nitrox use didn't affect the gas consumption at all. Now I know.