Air Consumption while on Nitrox - A theory

[nigelH]

I'm going to disagree with the whole premise of the first post.

O2 inhaled has nothing to do with O2 consumed. We make O2 available to the tissues that need it but they still only use what they want. Increasing the ppO2 will not change the O2 consumption nor the CO2 production rate.

'Desire to breathe' depends on blood acidity which depends primarily on CO2 levels. However you need to move N litres of gas through your lungs to get a 'flush' so more CO2 to get rid of means more litres of gas (at what ever is ambient pressure) to keep the same amount of CO2 travelling outwards for the same level of breathing comfort.

Hence changing the mix you breathe won't affect your gas consumption for a given depth one iota. The simple law of surface consumption multiplied by ambient pressure is as good as you are going to get. You can skip breathe and push up your CO2 levels and save some flushing gas at the expense of the classic CO2 headache or rebreathe it and let the scrubber take the strain.

The big thing that drops O2 consumption and hence CO2 production is relaxing. If I'm nice and warm and dangling on a string waiting an hour for a computer to announce the deco is finished the SPG on the O2, only a 3L tank, just stops moving. One day I'll get so relaxed I'll fall asleep, lose my hold of the reel and drift away to be remembered only as an awful warning against the evils of solo deco diving.

[Rydive]

Everybody knows that we only consume a percentage of the O2 we breath on air. So, why would we think that breathing a richer mix would increase the consumption of 02. If our tissues required more 02 there is plenty availabe in the air we breath.

02 cunsumed, not breathed = co2 produced

[Jimbo200SX]

That was the question as to whether the amount of O2 able to be absorbed was an absolute, or a percentage of the O2 available, therefore, if a higher percentage of O2 is available then a higher percentage of the total gas could be absorbed.

It seems that this is an absolute, rather than a percentage, hence irrespective of any relationship (Or lack of) between O2 absorbtion and CO2 production, nitrox would not affect air consumption as we are still absorbing the same amount of O2. I think.....

[Scuby]

Originally Posted by Jimbo200SX

It seems that this is an absolute, rather than a percentage, hence irrespective of any relationship (Or lack of) between O2 absorbtion and CO2 production, nitrox would not affect air consumption as we are still absorbing the same amount of O2. I think.....

Aye.

Of course there could be (and probably are) very small effects in gas consumption on nitrox vs air, maybe because you feel more relaxed on nitrox knowing you have a nice safety margin, or because you usually dive shallower on nitrox (less kit, less narcosis, less stress, warmer, etc), or anything like that. No easy way of reliably testing any of those though (at least not able to prove the causality) so stick with the assumption of "gas consumption doesn't change" and you'll be fine

David

[Mal Bridgeman]

I am sure that when Nitrox was first taught to the "mass market" this was one of the stories put about to increase its attractiveness.

I have been unable to find proof of it though .... I think it's another of the those diving falsehoods, like you must do your deepest dive first, it won't hurt when you take your mask off or 40m in the Red Sea is "different" to 40m in the UK..... but that does not stop people believing them

Mal

[Okeanos]

my best SAC rates have been on nitrox dives!

[Tunicates]

The amount of o2 you can draw from the inspired air is dependant on your red blood-cell count. The molecule responsible for o2 uptake is haemoglobin - found only in red blood-cells - and each molecule has only so many sites to bind o2 onto. Its more about how good a carriage system you have rather than how many things you have available to carry.

Cyclists and other athletes going for extreme records often train at altitude so as to boost the numbers of red blood cells in their body. Presumably one would uptake more o2 in the inspired air than normal if one had spent 2 weeks previously up a mountain.

[Foggy]

Originally Posted by Tunicates are Mad

The amount of o2 you can draw from the inspired air is dependant on your red blood-cell count. The molecule responsible for o2 uptake is haemoglobin - found only in red blood-cells - and each molecule has only so many sites to bind o2 onto. Its more about how good a carriage system you have rather than how many things you have available to carry.

Cyclists and other athletes going for extreme records often train at altitude so as to boost the numbers of red blood cells in their body. Presumably one would uptake more o2 in the inspired air than normal if one had spent 2 weeks previously up a mountain.

Correct.

But as stated above the CO2 is produced by respiration. It is metabolic, and depends on work rate.

It is the CO2 level that triggers the breathing response (unless you are a heavy smoker).

Now Hyperventillation (think of lots of breathing out as overdumping CO2) will reduce CO2, BUT uses more gas (breaths in for every out).

I have never noticed a difference in SAC on Nitrox.

Now relaxation - meditation / Yoga - have been shown to reduce SAC. Articles in magazines and just look at the freedivers crowd.
Some people relax more when less concerned about nitrogen (deco) comitments, and this may be the cause of anecdotal evidence for reduced SAC.

Cheers, Paul

[nigelH]

Originally Posted by Tunicates are Mad

Presumably one would uptake more o2 in the inspired air than normal if one had spent 2 weeks previously up a mountain.

No.

You can only take up more if you have more 'empty' Hg molecules. Your body only uses so much so you only have that number 'empty'. Having more full ones just going round and round does nothing beneficial.

Having more carriers is good if you are going to exceed the 'normal' flow rate but diving is hardly energetic. We hit lactic acid build up long before we get muscle oxygen starvation. That one only features in extreme endurance sports and then only at the very top levels.

[A. Berk]

Originally Posted by Okeanos

my best SAC rates have been on nitrox dives!

Which may be due to no other reason than the reduced narcotic presence of N2 causing more relaxation/less anxiety.

kerBo