Nitrox and the Myth of the Longer Dive

[Meng_Tze]

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.

CO2 is a byproduct of respiration. You exhale between 4%-and 6% (by vol.) CO2 with each breath. The variance of this percentage is in direct relation to the exercise level. The more you work, the more CO2 you produce, but basically between above mentioned levels. Our body is very sensitive to C02 levels as it triggers our breathing reflex.

At 2% by vol. you already notice breathing rate increase, at 5% it becomes uncomfortable, and at 10% you start getting confused and start going hypoxic.

Depending on depth and CO2 levels in the gas (Pg=Fg*D) your breathing rate may actually increase. PCO2 would then be {FCO2 (matabolized)+FCO2(inspired)}*D. While you exhale this in OC, it still triggers the breathing reflex. At PCO2 of about 0.15 bar, you would experience muscle rigidity, spasms and overall breathing difficulties.


This is one main reason why deep, steady breathing is important. You can not always control the level of inspired CO2 (even though it may be small), but CO2 build up WILL have an effect on your breathing. By ensuring to 'vent' as much CO2 as possible, we avoid going into hyperventilation like breathing patterns. CO2 buildup is one of the main catalysts for Oxtox symptoms. At elevated levels of O2 (EANx), this could be coming sooner given that the blood vessels (think brain as well) dilate.

CO2 buildup also could allow for more N2 to be absorbed through the increased blood flow to the cells and uptake of gases. As we know N2 is the main cause for DCS.

CO2 can help to off gas through it's breathing reflex trigger. But it is a VERY fine line and your body is sensitive to it.

 

[ianw2]

Wow! This misconception keeps coming up! It just won’t die! Let’s see why…

" …Nitrox permits longer dive times, less nitrogen absorption equals longer bottom times but it does not lengthen the dive…."

" Nitrox changes only the nitrogen/oxygen ratio of the gas one dives and yields a resulting change in the depth the body perceives it is diving based upon the nitrogen content."

"Nitrox while it may allow for longer bottom times DOES NOT translate into an actual increase in bottom times".:11:

Those comments are taken directly from previous posts in this thread!

Re-read those comments and you’ll see why divers are confused.

Take out the fluff and here’s what was written: "…Nitrox permits longer dive times but it doesn’t lengthen the dive…" and "…Nitrox allow(s) for longer bottom times but that doesn’t mean longer bottom times…":icon5:

And you wonder why divers are confused about Nitrox?

Nitrox must be some goo from a warp in the space/time continuum. It can lengthen the time a diver can stay on the bottom but it doesn’t lengthen the dive. Nitrox can lengthen the dive time but not the length of the dive. :icosm13:

Nitrox is also some wonderful gas that changes the physical properties of things around it. Nitrox somehow reduces the pressure on the human body to reduce the amount of nitrogen absorbed.:eyecrazy:

Why don’t we put out the word that Nitrox decreases the nitrogen loading during dives. Over multiple dives, this can mean being able to make longer dives due to the fact that no decompression limits are extended based on lower nitrogen levels. The nitrogen loading while using Nitrox will be less than the loading while using air on the same dive profiles, the surface interval will be shorter because there is less nitrogen to off gas during the surface interval.

I dive on Nitrox when I’m doing multiple dives over multiple days. That twentieth dive on the sixth day lasts a lot longer diving Nitrox all week than it would have diving air all week because the No Decompression Limits are longer due to the lower residual nitrogen in my body.

Ian

 

[charlesml3]

I'm the only Nitrox diver in my usual group and I get asked regularly "Should I get my Nitrox certification and start diving it?"

It's very simple:

"Are you coming up because you're low on air or because you're out of bottom time?"

If "low on air" is the response then I recommend they hold off on Nitrox. It won't hurt to use it, but it won't help either.

If the response is "out of bottom time" then we get into a discussion of how nitrox extends the NDL.

-Charles

 

[ReefHound]

At a dive shop yesterday and some divers were talking about using nitrox. While this is not a unusal discussion What often comes out of it is a statement of a diver who believes they can and are diving longer becasue they are using nitrox.

It seems to me that the real problem here is that you are making a bunch of assumptions. They didn't say that their air consumption was better. You just assumed that. It's quite possible that their dives are longer if they are being limited by NDL and not air. Without knowing their profiles, how can you say otherwise?

 

[String]

I've heard the "Im breathing more oxygen in so i use less gas" said many many times.
Its right up there with "Nitrox? Why do i want that? I never want to dive that deep."

2 very commonly heard statements, here at least.

 

[Walter]

At a dive shop yesterday and some divers were talking about using nitrox. While this is not a unusal discussion What often comes out of it is a statement of a diver who believes they can and are diving longer becasue they are using nitrox.

Well, I also believe that to be true. It's not always the case because sometimes my gas supply is the limiting factor, but quite often the NDL is the limiting factor and I can and do dive longer because I use nitrox.

 

[redacted]

If you are not getting more bottom time when you are diving nitrox, you may be able to fix that with larger tanks. I don't use nitrox often. But when I do, I enjoy both substantially longer bottom times and somewhat lower N2 loading.

 

[Nemrod]

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, not exactly. First, Nitrox, will not affect your SAC rate. Nitrox will allow for longer bottom times dependent upon the percentage mix but in order to make use of that longer bottom time (NDL for a given depth) the diver must have a larger capacity tank in many cases. Simply put, for a given SAC rate at a given depth the diver on Nitrox will consume more gas simply because he is down longer. Maybe I am not saying it well and I think that factually most people who say things like "Nitrox let's me stay down longer" fully understand they are talking about a longer NDL and not decreased gas (air, nitrox or whatever) consumption--all things being equal.

The rates of nitrogen uptake and offgasing are determined by the Laws of Partial Pressures.

I do not believe that Nitrox has less CO2 than "regular" air. No and any minor difference would be of no consequence. A divers SAC rate on Nitrox will be very similar to that on "regular" air. I certainly have noticed no difference.

N

 

[GDI]

Well, I also believe that to be true. It's not always the case because sometimes my gas supply is the limiting factor, but quite often the NDL is the limiting factor and I can and do dive longer because I use nitrox.

Yes Walter good point against the direct sense of my wording Given a consistant SAC rate and depth one could stay down longer using Nitrox because of improved SAC rate on the same volume cylinder. Provided such a SAC rate on air had the diver coming close to or exceeding the NDL. I have no problem on a Al 80 reaching 55 min at 60 ft with plenty of gas to spare. These are cases of when SAC vs NDL is the limiting issue to dive time, When a diver develops a SAC rate that allows them to exceed the NDL then nitrox would be the gas of choice.

It seems to me that the real problem here is that you are making a bunch of assumptions. They didn't say that their air consumption was better. You just assumed that. It's quite possible that their dives are longer if they are being limited by NDL and not air. Without knowing their profiles, how can you say otherwise?

I assumed nothing I questioned the divers on their experience levels and the types of dives they were doing since nitrox certified. All these divers were relatively newly certified and only certified in nitrox within their last month or two.

I apologize as my writing may not have reflected the context of the discussion in particular what was said verbatum.

The divers above were not thinking tank volume vs SAC or experience (example: as per Walter's comment and Walter having added experience). They believed that having nitrox to breath was increasing their time at depth because it magically improved their breathing rates. One of them had a "Very WELL" maintained log book showing his total of 15 dives made including cert dives. I pointed out to him that his dive times were relatively the same on nitrox as they were on air. Log Books are a great tool to help improve ones diving I think.

Breathing Nitrox does not vastly improve your SAC rate in a short period of time as if almost overnight. Better skills practice and a more relaxed diving discipline does. Your lung size doesn't change overnight. The cylinder has a fixed based volume. Yes You can change cylinders. You can change your relaxed diving posture and of course experience will come with more dives. BUT even this doesn't mean a better skill level or improved breathing rates.

 

[ReefHound]

I assumed nothing I questioned the divers on their experience levels and the types of dives they were doing since nitrox certified. All these divers were relatively newly certified and only certified in nitrox within their last month or two.

I apologize as my writing may not have reflected the context of the discussion in particular what was said verbatum.

The divers above were not thinking tank volume vs SAC or experience (example: as per Walter's comment and Walter having added experience). They believed that having nitrox to breath was increasing their time at depth because it magically improved their breathing rates.

Well you know what they say, GIGO (garbage in - garbage out). We can only discuss the incident as you related it. I'm sorry if I *assumed* that you were making assumptions as opposed to just failing to express yourself accurately. Your OP said nothing about the divers believing their SAC rate was improved but simply that dive times were increased. I hope you explained the facts to them clearer than you related the incident to us.

I'm not sure why the last two paragraphs were included - everybody here knows that nitrox doesn't affect SAC significantly if at all. But it seems you think that it's a rarity for a dive to be limited by NDL rather than psi, and that it takes skill to develop a SAC rate to do that. I think it's rather common on deep dives (over 100'), especially on repetitive dives. I doubt that divers with 15 logged dives are diving deep enough to be limited by NDL.