Another (legit) nitrox ?

[jon m]

hey all-
i have a question i haven't seen asked about nitrox .
i have seen some folks recommend diving nitrox using air tables(or comp) for an added degree of safety. i thought this was a good idea, but we are going to Rotan soon. so here is the ?
if doing multiple, repetative dives (say 4-5 dives a day for a week) to 50-90' is there a danger of too much o2 buildup? just a thought... thanks!

 

[JahJahwarrior]

Have you taken a nitrox course? Shouldn't that be covered???

the first worry is with OxTox--you have to make sure that your gas won't kill you at depth. I won't explain how to calculate MOD because that should be covered in your nitrox class, and I don't want to be teaching you how to do something if you don't really know what you are doing with it. (and if you want to know how to do it badly enough, you can find the equation on the internet) Basically, you need to figure out what PPo2 you are happy with, and then analyze the gas to figure max depth and make sure you don't exceed that.

It turns out I was wrong, there is a concern with residual o2.
http://www.iantd.com/articles/95-2gurr.html
http://www.scubaboard.com/forums/ask-dr-decompression/30393-cns-clock.html

(for what it's worth, I haven't taken a nitrox class either )

 

[phunk]

Yes, there is a concern for overall o2 exposure with repetitive enriched air dives. Take a nitrox class before you use it, it covers this.

 

[DA Aquamaster]

I am assuming you are asking in part to see if there is a benefit to getting a nitrox certification before the next dive trip. (it's nicer to assume that than to give the harsher sounding and very unhelpful "it's covered in the course" response.)

With a maximum PPO2 of 1.4 you have to dive an awful lot to come anywhere close to meeting the exposure limit in a 24 hour period. And realistically, the average PPO2 on most rec dives is less than the max of 1.2. So in effect, it should not be an issue.

For planning purposes you can use the exposure chart arranged by max PPO2 to get an idea of how many minutes per single dive and per 24 hour period you dive. If memory serves, that will be in the 150 minute range for a single dive and 180 minutes per 24 hour period. That is however also very conservative as it has no provision for "offgassing" between dives which does in fact occur. So in effect with an hour or two between dives with two or even three 60 minute dives per day you should not have any issues.

O2 exposure limits begin to come into play more with technical diving where you may have longer bottom times with a PPO2 in the 1.2 to 1.4 range followed by long deco times with a PPO2 in the 1.4 to 1.6 range where the single dive limit may be pushed and where repetitive dives may potentially exceed the 24 hour limits.

 

[SteveAD]

We don't refer to it as residual o2, but rather as OTU (oxygen toxicity units). Per the DSAT (PADI) nitrox tables divers should not exceed 150 minutes at a ppo2 of 1.4.

Assuming the OPs schedule has a max depth of 90ft and a max fo2 of 36% it is unlikely (but theoretically possible) to exceed recommended oxygen exposure while staying within no deco limits.

 

[mempilot]

OK. The original post asked about the safety of repetive Nitrox dives after using air tables or computer.

So. To actually answer the question.

If you are going to use air tables or an air computer while diving mixes with a FO2 greater than .21, then you will want to track your OTU's and CNS% seperate from your computer, as an air computer will not do so, and a nitrox computer set to air wouldn't give you accurate information for these two items, but will give you a nice buffer on N2 loading. You will need an exposure/dose table or the simple equation to do so. You will learn how to do it in the course if you haven't taken it yet.

Diving Nitrox on air tables, whether it be paper or computer, is a great way to increase your safety on any particular dive or set of dives. The limit for OTU's is quite high, and I doubt you will even come close to it diving recreationally. The limits for CNS% might come into effect late in the week if you are pushing high PPO2's on all your dives. The best thing to do in my opinion is throw in a couple of air dives now and then for any shallow dives you do rather than crank the FO2 up on them. This will give you a nice decay of CNS%, but keep you in the water.

Dive safe,

Eric

 

[The Kraken]

Yes, there is a maximum O2 level within a 24 hr period of diving.

the K

 

[knotical]

jon,

You don’t mention how long you anticipate the dives to be.

According to DSAT, if you did five dives on 36% EANx to 90 feet and stayed there for 30 minutes each, you would reach the daily oxygen exposure limit.

On the one hand, this scenario seems unlikely. You’ll likely spend much of your time at shallower depths and/or sometimes dive 32% or even 21%.

On the other hand,

little is presently known about physiological effects of multiple dives over multiple days.

Personally, I'd be comfortable diving as you describe, but taking an enriched air course does seem prudent.

 

[MBH]

My main concern with this thread is that shoud one decide to dive 32% while using a computer set to air or 21%, sure the computer will give you a very conservative bottom time, but....that same computer will give you a MOD that will be WAY to deep for the 32% that is being breathed. Forget that little tidbit of information and you might have a very bad day.

 

[DGriffin]

I just took the Nitrox course last night. It was TDI's version.

Great class, one of the better coursebooks I have seen, and I would say that I am highly confident of my EAN knowledge.

I suggest taking the usually one day course, rather thanseekng advice from the internet or other dive buddies. It's well worth it.