Nitrox on Air Tables - One Step Further

[Blackwood]

I know there is a lot of debate about whether diving nitrox on air tables provides an added safety margin over air. I personally believe that, regardless of whether or not we have data to 'show' it, it is likely true (since people who dive well past air NDLs on Nitrox aren't coming up bent the majority of the time, I must conclude that the nitrogen levels in one's breathing gas do in fact affect nitrogen loading).

That principle (hopping a depth column or two to the right to 'over-restrict' your NDL) in mind, I'm wondering if anyone has ever dove air on the tables for a hypoxic blend to gain a similar safety margin.

 

[Doc]

I'm wondering if anyone has ever dove air on the tables for a hypoxic blend to gain a similar safety margin.

Whazzat? Maybe, "if anyone has dove on air tables using a hypoxic blend...."

Zat what you are asking? Sure, people do it all the time. Just watch that column that takes you past the Ox Tox depth.

If that's what you were asking.

Otherwise:

 

[Blackwood]

Whazzat? Maybe, "if anyone has dove on air tables using a hypoxic blend...."

No, that's not what I meant.

Example: dive air but use an EAN18 deco schedule.

 

[ClayJar]

It would seem much more logical to just arbitrarily adjust your table numbers. Obviously, you always round up, but going to the next higher time or next deeper depth would get your more conservatism without using obscure schedules. (This is the NAUI recommendation for when you have an exceptionally cold, stressful, or tiring dive, for example.)

The reason people use the air tables while diving nitrox is because it's a very convenient way of increasing (true or perceived) safety, and it works quite well when you want that extra margin while diving with UAN21 divers. (Unenriched Air Nitrox, 21%) There's *always* an air table around... (for sufficiently small values of "always", at least).

 

[limeyx]

No, that's not what I meant.

Example: dive air but use an EAN18 deco schedule.

Well, in theory that is going the opposite way. Diving EAN on an air schedule is more conservative than air to the same schedule. This is less (you have more N2 in the mix)

People however dive hypoxic trimix all the time however, but they have varying methods of dealing with that.

 

[ClayJar]

Okay, in order of decreasing fraction of Nitrogen, you could have:

- EAN18 (82% N2) (would that be DAN18? Depleted Air Nitrox, 18%?)
- Air (79% N2)
- EAN32 (68% N2)
- EAN36 (64% N2)
- EAN40 (60% N2)
- EAN50 (50% N2)
- O2 (zippity-do-dah N2)

(...and plenty in between.) Ignoring everything but nitrogen loading for determining NDLs, going up this list is moving to more conservative. You can always use a less-O2-rich nitrox table for determining your NDLs, et al, and you can never use a richer O2 table. This only applies to nitrogen-oxygen blends, of course (with argon and the trace gases, most likely).

 

[limeyx]

No, that's not what I meant.

Example: dive air but use an EAN18 deco schedule.

Yes, I dont see why you couldn't do that as the 18 schedule would give you less time.

 

[Charlie99]

You could also accomplish your goal of adding conservatism by simply not diving to the limits of your air table. That's probably simpler and less confusing than using an EAN18 table.

As you noted, you could also add conservatism by "hopping a depth column or two to the right to 'over-restrict' your NDL", but it seems like it would be simpler and more reasonable to simply use the tables as they are designed, but with reduced NDLs of your selection. You could either go in and recalculate new NDLs using a fraction of the allowable M-values, or more simply, just move a couple spots over on the repetitive dive groups and call that your NDL.

The advantage would be that you are still tracking your best guess at N2 loading, but have lowered the limits. The other methods (bogus depth or bogus gas mix) change your "guess" rather than the limits.

 

[jonnythan]

The question that comes to my mind is: What problem are you trying to overcome by arbitrarily restricting your NDL by using an incorrect table?

If you consistently got skin bends diving air tables while breathing air, I could understand this. However, I'll bet that you don't get bent diving air on air tables. Your effort would probably be much better spent on slowing your ascents, especially the final 10-15 feet. That's where the subclinical DCS (headaches, fatigue) come from. Average times for the final 15 feet of most divers' ascents are in the 3-6 second range, giving an ascent rate of 150-300 fpm.

 

[SteveAD]

Maybe I'm missing something, but aren't most hypoxic blends trimix?

In which case your ppn2 and pphe are each lower than ppn2 on air and would therefore give longer ndls/ shorter deco schedules.

if you want to be more conservative why not just dive more conservatively?