How does Nitrox work?

[halocline]

It appears that the way nitrox prevents DCS is by sparking such a lengthy debate that participants no longer have time to dive, thus eliminating any DCS risk.

Nitrox is not proven safer....only clinical statistical studies can do that and there simply is not the data to support it, to my knowledge. It is assumed that diving with nitrox lowers N2 loading and this results in a greater margin of safety from DCS, and that's a very intuitive and fair assumption. But it is still an assumption.

It sure is nice on 4-5 dive days, though. I don't know if I'd want to do a flower gardens trip, with 5 dives to 80 ft+/day, on air.

 

[DaleC]

Nitrox is not proven safer....only clinical statistical studies can do that and there simply is not the data to support it, to my knowledge. It is assumed that lower N2 loading would result in a greater margin of safety from DCS, and that's a very intuitive and fair assumption. But it is still an assumption.

Not really, it's basic decompression physics.

Of course reduced N loading presents a lower DCS risk. If one doesn't believe that then the whole premise of decompression diving goes out the window. One can also say that gravity is just an assumption but I doubt most people would jump off a cliff to disprove it.

The difference between EAN tables and air tables as far as extended BT's and shorter SI's is based on a decompression model that has its basis in N loading and its effect on a divers physiology. The end result being a probable risk of DCS if the NDL for a given depth is violated and the diver surfaces without a decompression stop. When one uses a nitrox mix the risk decreases (proportional to the mix).

Using my example from a couple of posts ago the tables (PADI) state 20 minutes BT for air at 100ft. and 30 minutes BT for Nitrox I (32%). If what you said above is true, what accounts for the difference in BT's between the two gas mixes? Why can't the air diver stay down longer? What accounts for those different values? The answers all have to do with N loading.

As a test, that same dive, my buddy and I were using the same computer (Suunto Gekkos). He set his for the nitrox mix we were diving and I set mine to air. (we wanted to see what would happen when we put the computer into deco). According to the Suunto decompression model my buddy had a voluntary 3 minute SS and I incurred a manditory 6 minute deco stop. Why would I incur that if nitrox did not decrease the risk of DCS?

Not neccisarily trying to be argumentative. Just wanting to expand on the threads theme of how nitrox works.

 

[halocline]

Sorry, what I meant to say is that "it's assumed that diving with nitrox provides a margin of safety from DCS..." I went back and edited my post.

We all know that nitrox contains less nitrogen and consequently we absorb less when breathing it under pressure. What we don't know clinically is if this results in lower incidence of DCS in recreational diving.

 

[DaleC]

Sorry, what I meant to say is that "it's assumed that diving with nitrox provides a margin of safety from DCS..." I went back and edited my post.

We all know that nitrox contains less nitrogen and consequently we absorb less when breathing it under pressure. What we don't know clinically is if this results in lower incidence of DCS in recreational diving.

I understand what you are saying if it is the arguement that Nitrox doesn't decrease the risk of DCS in recreational diving because the risk is already so low. Making it a theoretical advantage but not practically measurable. For a single dive (or repetitive dives with adequate SI's) well within the NDL I agree. If you don't want to push your bottom time or reduce your SI's nitrox doesn't offer much of a demonstable advantage.

I think it does reduce the potentiality of DCS when one pushes the limits though. The variations in the tables tell us that. I also think it reduces the potentiality in those suseptable to DCS and in the overall incidence of subclinical DCS.

I think sub clinical DCS is a fascinating topic and more worthwhile studying that full onset DCS. Many divers won't experience the latter (hopefully) but probably experience the former without even realising it.

 

[boulderjohn]

I understand what you are saying if it is the arguement that Nitrox doesn't decrease the risk of DCS in recreational diving because the risk is already so low. Making it a theoretical advantage but not practically measurable.

I agree. Part of the problem is how we deal with and talk about statistics. I am going to make up some numbers here to show what I mean--please do not mistake these for actual statistics.

Let's say that 99.8% of all dives on air that are within commonly accepted recreational limits result in no DCS. Let's say that 99.9% of all dives on nitrox (but within commonly accepted recreational limits for air) result in no DCS. We could talk about these statistics in two ways, depending upon the starting point of our comparison:

• Diving with nitrox on air limits results in only a 0.1% improvement in safety.
• Diving with nitrox on air limits cuts the DCS risk in half.

 

[MXGratefulDiver]

Statistics don't really tell you much, because you first have to consider WHY people are choosing one over the other. Most people dive nitrox because they want more bottom time. And in this respect, offthewall's initial comment, before all the silliness, had some validity ... if you don't have enough available gas to extend your bottom time you won't get a whole lot of benefit from nitrox. On the other hand, if you're pushing NDL rather than gas reserves on your dives, you probably will.

But if you're using nitrox to stay down longer, then you're not going to be ascending with any less nitrogen in your body than you would if you'd simply used air and stayed down less time. So in that respect, extra safety isn't a benefit.

There are multiple variables involved ... bottom time, surface interval, and dive profile being the main ones ... and in order to determine whether or not nitrox will benefit you, they all need to be considered. At a high level, statistics will not give you that information ...

... Bob (Grateful Diver)

 

[DaleC]

I agree. Part of the problem is how we deal with and talk about statistics. I am going to make up some numbers here to show what I mean--please do not mistake these for actual statistics.

Let's say that 99.8% of all dives on air that are within commonly accepted recreational limits result in no DCS. Let's say that 99.9% of all dives on nitrox (but within commonly accepted recreational limits for air) result in no DCS. We could talk about these statistics in two ways, depending upon the starting point of our comparison:

• Diving with nitrox on air limits results in only a 0.1% improvement in safety.
• Diving with nitrox on air limits cuts the DCS risk in half.

Who says the numbers don't lie
When I was posting my example above I couldn't decide if, by using Nitrox I, I increased my BT by 50% (20min vs 30min) or if it accounted for a 33% increase in BT

 

[Puffer Fish]

Who says the numbers don't lie
When I was posting my example above I couldn't decide if, by using Nitrox I, I increased my BT by 50% (20min vs 30min) or if it accounted for a 33% increase in BT

Well, that one is easy.. going from air to nitrox increases your bottom time by 50%... going from nitrox to air decreases your bottom time by 33%...

 

[halocline]

Statistics don't really tell you much, because you first have to consider WHY people are choosing one over the other.

This is a good point, and is one of the reasons I feel that claims of nitrox giving increased safety is problematic. If someone is diving nitrox under the mistaken assumption that just using it provides greater safety, or even with the assumption that using it on an "air profile" provides greater safety, that person might be less vigilant about other dive behavior that affects safety, like dehydration, ascent rates, going to altitude after diving, etc...

It's very easy for people to take a claim of increased safety for granted. And, the dive industry LOVES to sell stuff on claims of safety. Isn't your life worth it? One of the classic dive shop lines.

 

[boulderjohn]

It's very easy for people to take a claim of increased safety for granted. And, the dive industry LOVES to sell stuff on claims of safety. Isn't your life worth it? One of the classic dive shop lines.

I am an instructor for standard nitrox, and I myself have advanced nitrox certification from another agency. Both state clearly that nitrox does not improve safety, and that is not the reason for using it. I have never heard anyone "sell" it to anyone on that basis. Maybe I have been sheltered.