Diving nitrox on an air computer

I was told that it is safer to dive using nitrox on an air computer rather than a nitrox computer. Is that true. If so it that always the case?

When you're diving Nitrox on an air computer, the computer doesn't take into account the lowered nitrogen levels. So when you're at 60 feet on Nitrox, tables might tell you that you're at an equivalent depth of say 45 feet, and a computer would calculate on- and off-gassing accordingly. IOW, the computer calculates for the full 2.4 atm of N2 pressure instead of 2 atm of N2.

I believe diving Nitrox on a correctly set Nitrox computer is about as safe as diving air on an air computer. Of course, it certainly isn't unheard of for divers to dive Nitrox to air NDL for extra safety or the better after-dive feeling Nitrox apparently gives you.

Air computers also don't track 02 exposure - which you will have to do yourself. This is of particular concern if you are doing multiple dives over a 24 hour period.

Jack R once bubbled...
I was told that it is safer to dive using nitrox on an air computer rather than a nitrox computer.

Click to expand...

As far as decompression goes, yes... the NDL limits will always be equal or higher for nitrox.

However there are two important things you will loose on the air computer. The first is the floor alarm... you have to be careful that you don't exceed the MOD. You shouldn't anyway, but remember that your computer isn't going to tell you.

The other thing you loose is OxTox CNS limit tracking, which shouldn't be too much of an issue, but is possible if your using high mixes [e.g. deco gases] and/or lots of time underwater in a day...

Both of which are easily solved by using your head, however for O2 CNS time limits, something people tend to forget 'cause they rarely come close to them...

...You must keep make sure you are within the PPO2 limits.

If you are making sure your PPO2 is on the safe side, than diving Nitrox on an air computer will give you better safety margins.

Ari

I respectfully disagree. I think that the statement is based on the fact that the no decompression limits (NDL) for air (EAN-21) are lower than that of higher percentages of O2. That does not make things safer, what would make things safer is less nitrogen loading, based on less time or depth. By setting your computer wrong, or using the wrong computer, you may be able to trick yourself into less bottom time, kind of like setting your watch 15 minutes fast to get to meetings on time.

But there is no free lunch. As mentioned you lose MOD and OxTox CNS limit tracking. Additionally should Murphy become involved, the dive plan go out the window, and you exceed the NDL time for air, all of the Deco stop information will be erroneous. This could result in a situation where you are faced with a decision that you must make with inaccurate information.

Mike

It's not safer. Its "safer" in the same way that renumbering your car speedometer would be if when you were going 60 it read that you were going 70. While this might help keep you from getting a speeding ticket, you can also get tickets for going too slow. Not to mention what it does to trip planning....

I think its a bad idea. A diver should have all the available info and it MUST be the correct info.

Tom

As long as you stay within the mod for a mix, what is "safer"is to use nitrox for "air" bottom times. But I would rather renumber the face of my depth gauge and watch. Or to really add a measure of safety we could not go diving and say we did.
Sorry I guess I'm in a sarcastic mood.

I agree with Mike (I think). I have never understood why a large number of my club mates insist on diving with Nitrox using air tables to give themselve "an added safety margin". Regardless of MOD and the CNS clock (whatever that is) it is certainly not as simple as it at first seems.

A few weeks ago I ended up diving with our club chairperson (female) to 29 metres. She was using 30% Nitrox but always uses an air computer. I was using 36% Nitrox with an appropriately set-up Nitrox computer (Alladin Nitrox Pro).

Planned bottom time was 35 minutes but we missed this by one minute (for reasons that were quite unavoidable). In consequence my buddy clocked up decompression penalties of 10 minutes compared with my own requirement of only one minute.

Either we divers trust the deco tables (employed by the computer manufacturers in this case) or we do not. They all have considerable conservatism when used appropriately. My pp O2 at the target depth was 1.4 bar and within the generally accepted margins of safety, so I was obliged to spend an 9 extra minutes hanging around with my buddy in the shallows quite unnecessarily.

I suppose it was good practice in bouyancy control but my buddy's concern was all to evident.

In my opinion sometimes one can take conservatism too far.

Dear Jack R:

Nitrox and Air Tables

In theory, the use of nitrox (more oxygen and less nitrogen in the mix) will result in a lower “dose” of nitrogen entering the tissues. In practice, the incidence of DCS is very low in recreational diving. It is in fact so low, that other factors are probably quite important. Among these, we have discussed musculoskeletal work, Valsalva-like maneuvers, large on-gassing and/or low off gassing rates, etc. The "safety edge" is probably more wishful thinking.

No “Bends/No Bends” Limit

Please recall that there is not any “Bends/No Bends” limit in physical reality. Divers have been taught to think in these terms but it is not really correct. DCS risk increases in a steady fashion with increased nitrogen “dose.” It is not an all-or-nothing response.

A light switch exhibits and all-or-none characteristic. It is either “on” or it is “off.” There is not any mid position. The accelerator of an automobile is a graded response. The more the pedal is depressed, the more fuel is delivered (“dose”) to the cylinders. Even the novice student driver knows that there is not any speedometer setting that is “mishap free” and one that is “assured accident.”

This is not to be confused with a speeding ticket where you can get your butt nailed for a couple of miles over.:egrin:

Dr Deco :doctor: