Nitrox and the Myth of the Longer Dive

[Jrock1062]

I am new and I have already gotten my Nitrox cert. IMHO because i am new to diving. i see things from different perspective. first my Nitrox class (TDI) taught me more about scuba diving, breathing, on gas, off gas, breathing rates, then my OW. They did agreat job, so for the learning aspect alone it was great, and any learning you do makes you better. second, i was saying your premis of because you aren't a good diver then you should not get the certification is like saying just because you're not a great rider and only go local you shouldn't own a big Harely. i dont see it that way. third like i have said in another thread it all about the instructor, if he/she is good, you will have more knowledge and knowledge is king. In every sport you are going to find clueless people if those people would like to think there sac rate is better because the use Nitrox and thats all they got out of the class the have bigger issues under water. IMHO I beleive everyone should have to go through the class even if they never use nitrox.

 

[Scubakevdm]

The funny thing is, is that when a diver's just starting out you sell him on nitrox instruction because it makes the dives longer. He gets a few dives under his belft and then you sell him on more nitrox instruction, because it makes the dives shorter!

 

[ianw2]

"...then you sell him on more nitrox instruction, because it makes the dives shorter!"

I never thought of it that way!

Ian

 

[DA Aquamaster]

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.

It makes sense that having a higher percentage of O2 will increase the uptake of oxygen and allow slightly lower repsiration rates - the same thing is essentially happening with someone walking in the mall with impaired lung function and an O2 tank. Without the O2 under that same workload, the person would be breathing faster to process more O2 to offset reduced O2 blood gas levels.

But it misses the point that a 12% reduction in respiration rate also means a reduction in the amount of CO2 being expelled as the amunt expelled per breathe is constant regardless of whether nitrox or air is being used. The link to the blind study you are referring to does not show the study protocols. However I suspect it was of reasonably short duration as they refer to a "course" and also as the level of effort required to maintain a 1 mph rate of speed in full scuba gear is quite high. This is problematic for 2 reasons:

1. If the duration was longer, the test subjects would have essentially been world class athletes whose results are then not typical of the average diver, and

2. If it was indeed of short duration, then the divers were able to limit the increase in CO2 levels that would have occurred over a 60 minute course and in effect would have artifically avoided the increased risk of oxygen toxicity related to elevated CO 2 levels.

If you did a sufficient number of trials at a 1.6 PPO2 with the divers exerting themselves heavily for the entire dive (something they admit is neccesary to get the full 12% improvement noted) gas consumption for some of them would be drastcially decreased as they would tox, expel the regulator and if unassisted would die.

in addition to any testing of increased oxtox potential under the same conditions they indicates would be needed to get an extra 360 psi at the end of the dive, there was, as far as I can tell, no testing of CO2 levels and no testing of potential differences in narcotic effects at higher workloads due to CO2 retention.

The article also mentions a 2 day industry workshop where a consensus was reached about several things including the consensus that "CO2 retention screening is not routinely required.". "routinely is I think a key word as that consensus is based on recreational experience where divers are not exerting themselves to any great extent and are not encountering situations where they may be retaining more CO2 than normal and nitrox cert classes are not suggesting that nitrox improves bottom time nor are they suggesting students reduce respiration rates. There is a reason for that.

So in short, I don't see any evidence that Nitrox will improve gas cosumption for the average recreational diver nor do I see any evidence to refute the original US Navy concerns regarding the increased risks of combining elevated CO2 levels with higher PPO2's.

 

[perdidochas]

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.

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.

 

[perdidochas]

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?

How does nitrox have less CO2 than air? (I understand that it would have a slight bit less due to trace amounts in the air that's displaced by O2, but I doubt that the difference between .04% CO2 in air and .02% CO2 in nitrox is physiologically significant).

 

[Rhone Man]

Reminds me of the amusing product safety warning on my (non-air integrated) dive computer: Bottom time remaining is a calculation of your no decompression limit - it is not a calculation of your remianing air. Monitor your SPG at all times. I wish they would not put warnings like that on - it interferes with the evolutionary process.

 

[SparticleBrane]

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.

Did you read past the first post?

 

[wedivebc]

I actually asked a question about inhaled CO2 (partial pressures at depth) during my OW class. None of the instructors could answer it, but the shop owner (who owns and maintains the compressor) told me that one of the filters on compressors for diving gases removes the tiny percentage of CO2 in ambient air. So CO2 in the tank is not pertinent.

The compressor filters usually contain hopcalite which converts carbon monoxide to CO2. A CO2 filter on a compressor would be huge and have to be changed frequently to collect all the CO2 in the air. It would really serve no purpose since we are quite well adapted to small amounts of CO2.

 

[The Kraken]

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