Nitrox extends NDL but does it have other benefits?

[archiebald]

My daughter and I are planning on doing an Enriched Air course while diving this summer. I would like to increase my understanding a little before taking the course.

I know that using nitrox;

1. results in longer NDL times, OR,
2. reduces surface time intervals, AND,
3. can cause oxygen toxicity at certain percentage / depth combinations.

(1 and 2 being related to the lower take-up of nitrogen)

(EDIT - added the "OR" and "AND" statements to clarify my meaning).

In addition, it was also my initial assumption that with the same diver, same sized tank and same level of exertion, a tank of nitrox would last longer that an tank of straight air. The logic being that the higher percentage of oxygen flowing into your bloodstream will automatically reduce your respiration rate for a given set of conditions.

However, I have read that your respiration rate actually has nothing to do with oxygen (except near to death). Instead it is related to the percentage of CO2 in your blood so the usage of nitrox will make no difference to respiration rates and your SAC will remain the same.

Despite being a fairly simple website, this page 4 of a 6 page article mentions both of the above items playing a role in respiration rates. HowStuffWorks "How Your Lungs Work" This particular page suggests that both oxygen and CO2 play a role.

So, First Question....
Theory is fine, but in actual diving conditions would any of our experienced members like to offer any practical opinions and /or results on SAC rates with and without nitrox?

And, Second Question....
I have read several topics on various sites that disagree more or less 50:50 on whether the use of nitrox reduces the evening fatigue that you get from a day full of diving. I know it is a subjective question but can anyone offer their own experiences here?

 

[Tigerman]

1. Nitrox has no effect on SAC in anyone I know, nor myself.
2. There is no PROOF of less fatigue or other benefits of nitrox over air. Some people claim to feel a difference, but theres no evidence to support it. There is however a theory involving subclinical DCS

 

[victorzamora]

Your SAC rate does have to do with your CO2 levels AND your O2 levels....you're right about that. However, CO2 is typically the limiting factor. An AL80 will last the same amount of time whether it's on air or on nitrox.

As for your first two points (shorter SI and longer NDLs).....I haven't really planned a repetetive dive on Nitrox and then Air to compare the difference, but I'm pretty sure that these two are fairly exclusive. If you want to do the same two dives on air and nitrox, you WILL have a shorter SI.....OR you can stay down longer on nitrox. The reason your surface interval is what it is is because of the amount of nitrogen buildup in your system. If you stay down longer on nitrox, you'll eventually end up absorbing the same amount of nitrogen.....that amount of nitrogen is what gives us NDLs. So, if you dive to NDLs on air and on nitrox....you'll have the same nitrogen loading (theoretically). Does that make sense?

 

[rhwestfall]

1. Enriched air will do nothing in regard to consumption rates. A tank of "A" will last the same as a tank of "B" given the same profile.

2. The bodies reaction to micro-bubbles forming due to returning to equilibrium is to treat them as an "infection". The bodies defense systems hence go into "high gear", and are considered to be the cause of fatigue. A completely controlled ascent is the key in bubble control (it is believed micro-bubbles form with all diving, but the strategy is to control them to the greatest extent possible - prior it was believed that you could prevent them from occurring). If you really want to expand your knowledge, "Deco For Divers" by Mark Powell, or Steve Lewis' "The Six Skills" & "Staying Alive" present the greatest bit of technical information in a non-clinical manner. It will take you far beyond the Enriched Air classroom text.

I have no anecdotal evidence for you as there are numerous factors that can play a role in post dive feelings, but the theory/science/speculation seem plausible to take it into consideration.

 

[archiebald]

As for your first two points (shorter SI and longer NDLs).....I haven't really planned a repetetive dive on Nitrox and then Air to compare the difference, but I'm pretty sure that these two are fairly exclusive. If you want to do the same two dives on air and nitrox, you WILL have a shorter SI.....OR you can stay down longer on nitrox. The reason your surface interval is what it is is because of the amount of nitrogen buildup in your system. If you stay down longer on nitrox, you'll eventually end up absorbing the same amount of nitrogen.....that amount of nitrogen is what gives us NDLs. So, if you dive to NDLs on air and on nitrox....you'll have the same nitrogen loading (theoretically). Does that make sense?

Okay, I understand - points 1 and 2 should possibly include an "OR" statement between them

 

[Wookie]

Here is some anecdotal evidence for you about DCS. I've been running a liveaboard since 1998. Same liveaboard. During that time, we see about 500 customers per year, who complete in average 7500 dives. From 1998 to 2006, we charged an extra $10 per day for nitrox, some used it, some didn't. In 2006 we raised the price for the dive trip to include nitrox, so most everyone used it. From 1998 to 2006 I saw 4-8 cases of DCS per year, which meant we had to bring the trip home early. Since 2006 we've seen zero cases of the DCS, and one case of Immersion Pulmonary Edema, and suspect a second case, but both of those divers were on rebreathers.

 

[archiebald]

2. The bodies reaction to micro-bubbles forming due to returning to equilibrium is to treat them as an "infection". The bodies defense systems hence go into "high gear", and are considered to be the cause of fatigue. A completely controlled ascent is the key in bubble control (it is believed micro-bubbles form with all diving, but the strategy is to control them to the greatest extent possible - prior it was believed that you could prevent them from occurring). If you really want to expand your knowledge, "Deco For Divers" by Mark Powell, or Steve Lewis' "The Six Skills" & "Staying Alive" present the greatest bit of technical information in a non-clinical manner. It will take you far beyond the Enriched Air classroom text.

I have no anecdotal evidence for you as there are numerous factors that can play a role in post dive feelings, but the theory/science/speculation seem plausible to take it into consideration.

Thanks, that makes sense, and thanks for the references.

 

[Nirvana]

Hello, archiebald.

The benefits from nitrox come from the fact that it is mixture containing less nitrogen than air. The less inert gas being inspired, the longer you can stay at a certain depth before the total amount of inert gas dissolved in your body becomes too great to allow a direct ascent to the surface, that is, before you reach your NDL.

In actual use, it can serve to allow for longer dives - since, as explained, it increases the NDL - or to provide a greater level of conservatism, if the diver breathes nitrox while respecting air NDLs.

The downside of nitrox, though, is the greater risk of oxygen toxicity. Oxygen, when breathed at increased partial pressures (determined by mix and depth), can cause some negative effects, the most serious and life-threatening of which is a convulsion. Therefore, when using nitrox, a diver must be more aware of his oxygen exposure and limit his maximum depth in accordance to his mix.

As to consumption, as you noted, it is driven by how much carbon dioxide needs to be eliminated. The more one exercises (and produces more CO2), the harder he has to breath. Nitrox will not increase the duration of a tank.

Finally, personally I have not felt any different when breathing nitrox.

I hope I've been of assistance.

 

[Tigerman]

What mixes do you usually dive Nirvana that has MORE AIR than NITROGEN?

 

[victorzamora]

From 1998 to 2006, we charged an extra $10 per day for nitrox, some used it, some didn't. In 2006 we raised the price for the dive trip to include nitrox, so most everyone used it. From 1998 to 2006 I saw 4-8 cases of DCS per year, which meant we had to bring the trip home early. Since 2006 we've seen zero cases of the DCS.

I wonder.....what was your rate of divers wearing computers from 98-06 compared to 06-now. I wonder if that was a more direct causal link.

Also, to address something in the OP I obviously missed: Feeling better after a nitrox dive.

I've never felt better after a nitrox dive vs an air dive, but I believe that to be due to diving for longer instead of adding conservancy to my diving. However, the best I've ever felt after a dive was after decoing out on pure O2. It was a dive I had done several times in the past and always exited VERY drained. Since I've started O2 at the end, I always feel a little bit better. I've never experienced this with nitrox, though.