Nitrox and the Myth of the Longer Dive

[trigfunctions]

Maybe someone here can answer this question -
One benefit of Nitrox is that breathing gas with less nitrogen allows the body to off-gas nitrogen faster. The Nitrox gas also has less CO2 than regular air. Since the urge to breathe is determined by CO2 accumulation in the body, does breathing nitrox allow the body to off-gas CO2 faster, thus reducing the urge to breathe and lowering the breathing rate?

Does that make sense?

 

[DIR-Atlanta]

The Physiological benefit of breathing Nitrox over air ends when you go to the same No Deco time limit on nitrox as you did with air .. your body then has same nitrogen loading (just takes longer time to get there)

I m confused - I thought we were talking about divers who are hitting their air limits before they hit their nitrogen limits.

is there a shorter surface interval if you've dived all the way to the NDL .. wouldn't your nitrogen loading now be the same as it would be with air ?

The Nitrogen loading will be the same as for a dive at a shallower equivalent depth. This will often result in a shorter surface interval to get the desired bottom time on your repetitive dives.

This is pretty obvious if you run some table profiles. In particular, take a look at some repetitive profiles in the 50-80 foot range on something like 36%.

 

[Doc Intrepid]

No you're not confused. And your post was dead on.

GDI's point was well taken that "Nitrox while it may allow for longer bottom times DOES NOT translate into an actual increase in bottom times".

OTOH, your point is equally valid when discussing repetitive diving which will, over time, create situations where limitations on bottom times are not constrained by the volume of the gas in the tank.

FWIW.

 

[DA Aquamaster]

Maybe someone here can answer this question -
One benefit of Nitrox is that breathing gas with less nitrogen allows the body to off-gas nitrogen faster. The Nitrox gas also has less CO2 than regular air. Since the urge to breathe is determined by CO2 accumulation in the body, does breathing nitrox allow the body to off-gas CO2 faster, thus reducing the urge to breathe and lowering the breathing rate?

Does that make sense?

No. The CO2 is produced by you and is not in the gas to start with.

But...there is a related issue that needs to be discussed. The trigger to breathe is caused by the rise in CO2, so it is more or less irrelevant how much O2 is in the gas as in most cases you will not pause between breathes with Nitrox any longer than you would with air anyway.

However, if a diver believes that he or she can extend their bottom time by breathing slower due to more O2 in the mix they are both right and wrong and sdestined for a problem. Assuming you need a given amount of 02 per breath, the higher PPO2 allows you to get more O2 per breath, but your CO2 production remains the same. This means you will be retaining CO2 due to reduced respiration rates and will be increasing the CO2 levels in your blood, etc.

This can cause incredibily painful headaches as the CO2 is a vaso dialator but more importantly, it can increase your susceptibility to both oxygen toxicity and nitrogen narcosis.

So...artificially slowing yoru respiration rate on nitrox to extend bottom time is a bad idea as you are extending your bottom time on a higher PPO2 mix with elevated CO2 levels that increase your susceptibility to oxtox on a dive where your judgment is potentially more impaired - all in all a pretty good way to take yourself out of the gene pool.

The US Navy experimented with enriched air in the 50's and decided not to use it operationally in part because of the wide variation and unpredictability of oxygen toxicity. Part of the problem was that the test subjects had long histories of being helment divers and had developed a high CO2 tolerance in the poorly vented Mk V helmet that created a situation where they retained CO2 and increased the risk of oxtox on scuba gear using enriched air.

The current recomemended PPO2's are much lower, but I would still not push it as it is a lot simpler and safer to just buy a bigger tank if you feel the need to extrend bottom time.

 

[oxyhacker]

Actually, there are some tests and studies (none which I can lay my hands on at the moment) that suggest that nitrox WILL give a slight but significant improvement in gas consumption. That is to say, that most divers will find a tank of nitrox will last them longer than a tank of air, given similar dive profiles.

Best I can recall, the mechanisms were unclear, but the effect was measurable. There was an article in SCUBA DIVING a few years ago that mentioned it.

 

[reo]

Question .. is there a shorter surface interval if you've dived all the way to the NDL .. wouldn't your nitrogen loading now be the same as it would be with air ?

To answer the second part of your question--if you dived to your NDL, your nitrogen loading is the same regardless of the O2 percentage you used during your dive to the NDL.

Now for the first part of your question. If you are planning a repetitive dive your recommended surface interval will be shorter if you are using nitrox vs air for the repetitive dive. Your surface interval will decrease your nitrogen loading. Your repetitive dive will start to increase your nitrogen load again, but nitrox will do so at a slower rate, giving you either more time (vs air) for your repetitive dive with the same surface interval, or a shorter surface interval (vs air) for a repetitive dive with the same duration.

Ron

 

[trigfunctions]

No. The CO2 is produced by you and is not in the gas to start with.

Right, the CO2 is produced by the body, but air does have a fraction of CO2 (about 0.04%), and nitrox would have a smaller fraction. So, I'm wondering if the smaller fraction of CO2 in nitrox would allow the body to off-gas CO2 at a faster rate to reduce the CO2 accumulation enough to reduce the breathing response. It may be that the amount of CO2 in air is so low that it does not significantly impact the off-gassing - that's what I'm wondering if anyone knows. I could do the calculations myself later but I would imagine someone has already done them.

 

[jviehe]

Do some people really misunderstand longer bottom times to mean less air consumption? I havent had that experience yet with my students. If anything they will quickly learn that by diving, when they run out of air before NDL. I havent used a nitrox al80 in a long time, so time is my limiter still.

 

[DIR-Atlanta]

Do some people really misunderstand longer bottom times to mean less air consumption?

Some folks do have that misconception, but I think it is mostly people who have not taken a Nitrox class. They hear other divers talking about "increased bottom time with Nitrox", and assume (based on their limited training and experience) that Nitrox somehow lowers a diver's SAC rate. They don't really "get" the idea of Nitrogen absorption being a limiting factor, because they have only ever dove with air ("when the only tool you know how to use is a hammer, everything looks like a nail").

Most trained Nitrox divers that I have encountered do seem to understand the difference.

 

[TSandM]

I actually asked a question about inhaled CO2 (partial pressures at depth) during my OW class. None of the instructors could answer it, but the shop owner (who owns and maintains the compressor) told me that one of the filters on compressors for diving gases removes the tiny percentage of CO2 in ambient air. So CO2 in the tank is not pertinent.