Nitrox and the Myth of the Longer Dive

[perdidochas]

Did you read past the first post?

yes, after I posted. I post as I read. I don't read through a thread, and then post.

 

[perdidochas]

perdidochas . . .

Let's simplify the equation, shall we?

Let us suppose that you have a SAC rate of .75 (that's 3/4 of a cubic foot of gas at the surface).

Let us also suppose that you have a tank with a true volume of 80 cubic feet.

Let us also suppose that you are making a dive to 33 feet.

At 33 feet your SAC rate will still be .75, but you are CONSUMING 1.5 cubic feet of gas per minute.

Therefore, if you absolutely drain the tank dry at 33 feet, you would have spent 53.333 ... minutes at the bottom.

Now let us suppose that the tank on this dive was filled with regular air.

NOW, we fill the tank with EAN40.

You make the same dive to 33 feet with a SAC rate of .75.
You will still be consuming 1.5 cubic feet of gas per minute. Thus the 80 cubic foot tank will give you 53.333 . . . minutes of dive time if you suck it dry.

Nitrox does not change the VOLUME of gas you take on each breath, all other things being equal.

the K

I never claimed that, in fact, I recognized that I would use more gas on nitrox due to increased bottom time. On most dives, I am not limited by gas, but by NDL. Since that is the case, nitrox extends my bottom time (and I can use most of the gas I carry). Diving in the 80 ft range wearing an AL-80, I typically come back with 1500 PSI on air (after a 25 minute or so dive, not including the safety stop as part of the time). On nitrox, I would come up with less excess gas, but have had a longer bottom time. I said nothing about my SAC getting better with nitrox. I simply said that with nitrox I had higher bottom time, not that I had a better SAC. I should have added that the reason for the longer bottom time is the increased NDL. Also, why use 33' as the example. It's silly to use EAN40 in an AL80 to dive 33'. (I do agree that your example is correct, although, my SAC is closer to .4)

Let me quote my original statement:

I don't see how that statement is wrong. Diving nitrox, I do have better bottom times. I do use more gas, but with a fairly low SAC, I am still able to have longer bottom times.

 

[The Kraken]

Sorry, I guess I just didn't understand your position given the way it's stated.

the K

 

[mrwa]

I haven't heard anyone comment yet on the theory that nitrox can decrease the level of impaired cognition (narcosis) that divers can experience at deeper depths. It seems that if it is true, then it would be a possible advantage of nitrox. Any thoughts?

 

[perdidochas]

I haven't heard anyone comment yet on the theory that nitrox can decrease the level of impaired cognition (narcosis) that divers can experience at deeper depths. It seems that if it is true, then it would be a possible advantage of nitrox. Any thoughts?

This was a big topic of discussion on this thread:

http://www.scubaboard.com/forums/near-misses-lessons-learned/229746-first-time-getting-narced-yipes.html

 

[perdidochas]

I haven't heard anyone comment yet on the theory that nitrox can decrease the level of impaired cognition (narcosis) that divers can experience at deeper depths. It seems that if it is true, then it would be a possible advantage of nitrox. Any thoughts?

To paraphrase the argument in the thread linked in my previous response: Probably not too much, as oxygen has narcotic effects as well.

 

[String]

To paraphrase the argument in the thread linked in my previous response: Probably not too much, as oxygen has narcotic effects as well.

There are 2 real schools on this:

(i) Oxygen is as narcotic as nitrogen so no difference
(ii) Oxygen is MORE narcotic than nitrogen so makes it also.

Add that to the fact that at the depths narcosis becomes an issue for most (ie >30m) the mix is generally weaker so even if there was any benefit it was a lot less than it could be due to the mix. 40m you're on 28% for example which really isnt a lot different to air in terms of N2 content etc.

The way i see nitrox is its a way of extending no stop times or reducing the time needed for deco stops. Ive yet to see ANY other benefit what so ever from breathing it.

 

[Web Monkey]

I haven't heard anyone comment yet on the theory that nitrox can decrease the level of impaired cognition (narcosis) that divers can experience at deeper depths. It seems that if it is true, then it would be a possible advantage of nitrox. Any thoughts?

It's possible, but not likely to be a significant advantage, since at depths where narcosis is typically a problem, you can't use a lot of O2 due to hitting the PPO2 max.

For example, the optimal Nitrox mix for a 120' dive, with a PO2 of 1.4 is about 30%. Plain air is already 21%, so you're only getting an extra 9% more O2 (and 9% less N).

It might help, but I wouldn't count on anything noticeable.

Terry

 

[mrwa]

And as far as whether nitrox can improve a diver's air consumption, it sounds like there is very little scientific evidence and a lot of anecdotal reports. The only way to really know is to perform a randomized controlled trial with proper statistical power. If we trust the results of the study cited on this post, then it appears that SAC improvement may increase with the level of exertion. But the relevance of the effect may be limited by the fact that diving doesn't tend to require high levels of exertion.