Nitrox and the Myth of the Longer Dive

[DIR-Atlanta]

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.

Thanks for the clarification. It certainly seems as if they didn't really understand what was going on.

One situation that I have occasionally run across involves newly certified divers who believe that Nitrox decreases their SAC rate, when in fact it is due to increased comfort on their dives as a result of increased experience as a diver. Most newer divers go through their gas pretty quickly due to excitement, inefficiency of movement, or lack of proper breathing control. Sometimes the point where they start diving Nitrox is the same point where their SAC rate starts to drop naturally as a result of experience, so they believe the Nitrox has something to do with it.

That happens rarely, but I have seen it happen.

 

[MauiScubaSteve]

I realize the search function does not result in entertaining hyperbole, but just for giggles I did a search in Basic Scuba Discussions for Nitrox Myths. Low and behold, we just had a very similar discussion (Diving using Nitrox?) started Oct 22 and last posted on Oct 27. This was the first result of said search.

I was actually looking for my last post in that thread (#90, Oct 27) after I found a web page called The Truth About Nitrox. This is the only blind study results to my knowledge that adresses the longer dives with Nitrox question with hard data.

Gas Consumption

Do breathing rates improve when a diver uses nitrox rather than air? According to our tests, the answer is a pretty clear "YES". On average, a diver breathing a nitrox mix will use gas more efficiently than a similar diver who breathes regular air.

In-water tests were conducted between 33 and 99 feet, comparing air (21 percent oxygen) and EAN 36 (36 percent oxygen). Tests were done "in the blind"-all tanks were labeled nitrox, but the oxygen percentage was not marked. The tanks were carefully analyzed, but only the dive supervisor knew which contained air and which contained EAN 36. The tanks were issued in random order to the divers, who swam at a steady rate (one mph), or sat at rest on the bottom. All tests were carried out in the ocean with divers in full scuba gear.

The at-rest tests (done at 99 feet) revealed no difference between the basic breathing rates of divers on air and divers on nitrox. However, in the controlled course runs, swimming at one mph, nitrox divers averaged 2 psi per minute better gas consumption than air divers. This result was based on 54 trials.
If this nitrox advantage held true over an entire dive of 60 minutes at 66 feet, there would be 360 psi more gas available to the nitrox user, a 12 percent improvement when using a standard aluminum 80.

Please fell free to impugn this data as you feel neccessary to maintain your hyperbole, but if you actually use the link to go to the web page and actually read the entire report you just might learn something.

 

[Nemrod]

I realize the search function does not result in entertaining hyperbole, but just for giggles I did a search in Basic Scuba Discussions for Nitrox Myths. Low and behold, we just had a very similar discussion (Diving using Nitrox?) started Oct 22 and last posted on Oct 27. This was the first result of said search.

I was actually looking for my last post in that thread (#90, Oct 27) after I found a web page called The Truth About Nitrox. This is the only blind study results to my knowledge that adresses the longer dives with Nitrox question with hard data.

Please fell free to impugn this data as you feel neccessary to maintain your hyperbole, but if you actually use the link to go to the web page and actually read the entire report you just might learn something.

Well, I will stick with what I said. Nitrox will have no real affect upon SAC. The inactive divers showed no difference and the active--working divers--under a sustained load showed a small but apparently measureable improvement of 2 psi per minute. Since most dives do not require SUSTAINED heavy or even moderate exertion then it is reasonable to extrapolate from the data that at best the average diver would see a 1 psi improvement or less. One psi (was this at 100 feet--I forget) at 100 feet for 30 minutes is 30 psi saved or at the surface about 90 psi. This is NOT a signifciant difference. It is reasonable to expect that the harder one works the more a high O2 concentration would benifit them--kind of like doping.

Here is a question, is the O2 uptake the same per breath (same volume breath) when at rest as it as say during a 10 K run? The O2 requirement goes up and thus the cardivascular system responds with increased breathing rate and pulse but due to the law of partial pressures and fixed surface area of lung alveoli and capilllary tissue--again---would not the O2 uptake be the same per breath and thus the bodies requirement to breath more rapidly and deeply when working hard. If it could magically increase O2 uptake some other way--what way would that be? How--O2 doping (red blood cell boosting via transfusions) would accomplished this as does better physical condtioning and exercise and perhaps some drugs? High altitude training?

The small increase in O2 content of nitrox has a small and essentailly negligable effect upon SAC--I think this is the result of the study that was linked to and that I read. If we were performing olympic underwater competitions I would imagine Nitrox would be illegal. In the 100 meter swim or even the mile run the difference between winning and losing might be miliseconds---I just don't think 1 psi to be significant to a diver.

N

edited because I went and put my glasses on and realized my big fingers were pushing all sorts of keys

 

[Scubakevdm]

Nitrox can also result in shorter bottom times. It all depends on what you're doing.

 

[SharkDZ]

The warm waters off Kona improved my SAC rate, which led to longer bottom times.
Oh, wait! I was using EANx32.

 

[DIR-Atlanta]

Well, I will stick with what I said. Nitrox will have no real affect upon SAC. The inactive divers showed no difference and the active--working divers--under a sustained load showed a small but apparently measureable improvement of 2 psi per minute. Since most dives do not require SUSTAINED heavy or even moderarte exertion then it is reasonable to extrapolate from the data that at best the average diver would see a 1 psi improvement or less.

While the results of the study are interesting, I have to agree with Nemrod. If I did the math right, then swimming at 1 mph translates to a 400 yd swim in under 14 minutes, which is a pretty good clip, without SCUBA gear. That's not something that most recreational divers could (or would even want to) sustain for 60 minutes. I haven't read the report, but my "hyperbolized" conclusion based on halemano's summary is that an average recreational diver on an average recreational dive would see a negligible decrease in their SAC by switching from Air to Nitrox, the referenced study notwithstanding.

I think it's an interesting topic for further inquiry, though.

 

[Nemrod]

I think it is interesting also and thank Halmaneo for the links.

N

 

[matjo1]

What about the placebo effect guys? In medical drug research trails, the sugar pill works as well as the real thing 20 to 40% of the time! If you asked one of those crazy free divers how they can stay 8 min or more underwater on one breath, the answer will likely be relaxation techniques, visualization, mind over matter etc. If a newbie thinks that nitrox will "make him feel better after the dive" and "extend his bottom time" don't you think he will be more relaxed, slow his breathing down and actually make his tank last longer! I know, I know... we want to explain, demystify, educate, but in the process of enlightenment, aren't we robbing them of what little edge they have over more experienced divers? The discussion about if it is better to tell them the truth or keep them in the dark could go on for days, but if it means more bottom time for us because our new buddy isn't "low on air AGAIN" do we really care?
Just a thought...

 

[MauiScubaSteve]

At the very least, more study is needed before an exact yes or no answer is given, if ever!

 

[Jrock1062]

I understand what your getting at about the bottom time and all but what i am not getting is ,are you saying that new divers should not get nitrox certifcations ?if so, why, because it doesnt help them right away thats like saying dont buy a a B/P wing set up because right now you are not going to be diving doubles or you should not get an advanced OW because your not not going down100 ft. I think that the more you learn,the better you are if there are still some people who think nitrox = better sac rate ok so be it.there are many people that do many things and never fully understand what they are doing