The Truth About Nitrox

[Rick Murchison]

The Navy used 30' for pure oxygen and a 2.0 PO2 for years without incident (or at least without any *known* incident). After a confirmed O2 convulsive hit they and NOAA started revising that downward until reaching 1.6, where, to my knowledge there has never, ever been a convulsive hit unless there has been considerable exposure to over 1.6 immediately prior... at 1.4 I don't believe there is any case of a convulsive O2 hit under any circumstances (epilepsy excepted).
FWIW, in my opinion 1.6 is a perfectly fine limit so long as it's never exceeded (and within the single/daily dose limits), and 1.4 is so conservative as to border the paranoid.
Rick

 

[turnerjd]

Originally posted by Rick Murchison
The Navy used 30' for pure oxygen and a 2.0 PO2 for years without incident (or at least without any *known* incident). After a confirmed O2 convulsive hit they and NOAA started revising that downward until reaching 1.6, where, to my knowledge there has never, ever been a convulsive hit unless there has been considerable exposure to over 1.6 immediately prior... at 1.4 I don't believe there is any case of a convulsive O2 hit under any circumstances (epilepsy excepted).
FWIW, in my opinion 1.6 is a perfectly fine limit so long as it's never exceeded (and within the single/daily dose limits), and 1.4 is so conservative as to border the paranoid.
Rick

Rick

- it really took one hit in many years use of a ppO2 of 2.0 for the limmit to be lowered to 1.6??

Wow!

But how did they choose the 1.6? was it just a best guess? - this probably explains the french 1.9 - it is probably the max they thought acceptable after one hit at 2.0 with many trouble free years at that value.

Jon T

 

[TexasMike]

ID: Point taken, and that makes pefect (and safe!) sense. My instructor didn't qualify the statement like you said. So I guess if I take something other than AL80's to Cozumel, I had better take the time to ensure it's done correctly (AND offer them a larger tip!).

--TM

 

[devjr]

I wouldn't worry about this level for light duty. When I was in the chamber, they cycled me at 3.0 atm off and on, mostly on.

However, I was in the chamber in the first place for only a slight infraction of the tables. When I was younger, would not have been a problem. You see, the rules are for the lowest common denominator. Darwin? Heh! What'l you guys come up with next?

 

[Rick Murchison]

Originally posted by turnerjd

- it really took one hit in many years use of a ppO2 of 2.0 for the limmit to be lowered to 1.6??

Wow!
Jon T [/B]

The operative word here is "known." There had been lost divers before - but after the known convulsive hit the research began in earnest to explore exposures previously thought safe - and the 1.6 limit eventually established. I think the agencies are taking the "well, the Navy only uses prime physical specimens in their sample so we'd better be more conservative" approach. The assumption that "good shape" equates to resistance to OXTOX is a pretty giant leap, though, and to my knowledge not supported with real science.
Anyway, if anyone knows of a single incidence of a hit that has occured at a PO2 of 1.6 or below where the victim had *not* been recently exposed to greater than 1.6 or otherwise already exceeded the single or daily dosage (CNS clock) I'd sure like to know about it.... I don't believe there's ever been one documented.
Rick

 

[John Reinertson]

Originally this data was derived from incidents of seizures occurring in UDT training with oxygen rebreathers, plus data from chamber seizures.
The first Navy limit of 2.0 was considered safe enough for use with rebreathers which had straps on the regulator to keep it in the mouth or full face masks, and is still used with the understanding that risk of seizure is somewhat increased..
The survivability of an oxygen induced seizure with a full face mask and straps holding it on is much better than with conventional SCUBA, and some Navy missions assume increased risk in exchange for decreased likelihood of being observed..
The 1.4 to 1.6 limit was derived by trying to estimate at what level would there be no appreciable increase in seizure risk compared to air.
Dr Frank Butler of the Navy EDU ran through the math and data in some detail in a diving medicine course, and it was convincing to me, but I don't have a solid enough command of the statistics and math to be able to present his explanation in a more rigorous manner. Drfred@scubamed.com may be able to enlighten you more precisely or give a better reference. I sat in an audience of Diving Docs and they all felt it was a reasonable limit, so I sure abide by it.

The risk is relative, however and is time dependent. If I dropped a $1,000 camera at a depth giving me a PP of 1.6 and I had to go to 1.9 for a few secs to retrieve it, I would probably take the risk. Others might choose not to..
We all have to know the risks of our decisions on each dive and dive accordingly.

Dive safe and seizure-free,
John Reinertson

 

[captdave]

Just a note to remind you that the increased Oxygen feeds your tissues as well as minimizes the nitrogen absorbtion. This produces a lessening of the psyshiological work on your entire system. You do not get as tired. I dive all my dives with nitrox for this very reason. Especially the shallow ones. I find I can dive more and party better. I don't crap out at 10PM..This is why the term geezer gas was coined as the older divers notice the difference..Plus there is other differences to consider. What type of Oxygen is used in the blend as well as what type of air. Is it hospital grade or aviator grade oxygen..Makes a difference in dehydration. is the air e grade or oxygen compatable. Makes a difference...food for thought.

 

[Liquid]

Greg-
With EAN32 you have exactly 1.6 PPO2 at 40 meters. Diving for up to 45 minuets, for a single exposre, with 1.6 PPO2 is considered within the most conservative range.

 

[Lost Yooper]

I would agree that the 45min rule holds water under ideal conditions with no stressors. However, I suspect that time limit might substantially decrease if things were to go sour on a dive -- Murphy's Law. The risk of seizure at 100'+ sure doesn't seem worth it to me -- especially when it's so easily avoidable. I kind of think that being a bit conservative when the risk of seizure and drowning are possible make a little sense.

Even deco at 20' on 100% O2, it's recommended to take a back gas break (5mins) every 25 mins or so.

Take care.

Mike

 

[Liquid]

Lostyoopers:

I belive that the 1.6/45 is for EVERY diver in EVERY condition, not for "pull" conditions only. That's why it's in the basic table. Same as the NDL tables, that area lso suposed to be like that (though, someone that belongs to risk groups should still be conservative).