1.4 ....1.6...or ?????

[Blackwood]

...diving a higher PO2 (1.6)... just tells me you are pushing the limits unnecessarily...

It could tell me that. Or it could tell me that in your nitrox class you learned:
(1) The NOAA maximum exposure table, and
(2) a 'best mix' equation.

Those things combined point to EAN32 at 130 feet. With so many differing opinions on the maximum acceptable PO₂ for the working portion of the dive, I'd find it hard to fault someone for applying their basic nitrox training to the NOAA table.

 

[ScubaSteve]

The staff? I decide this stuff for myself based on the site (depth) information available. I would never someone else make this kind of call for me.

I agree but they are the only ones that have that information so until I get site specific information from them, I cannot make an informed decision. Poor wording on my part and thank you for calling it.

 

[TSandM]

As with so many things in diving (like the NDLs, for example) there is no hard and fast line where you can say that 1.4 is "safe" and 1.5 is "not safe". The studies that were done to look at oxygen toxicity showed that there is very wide variability between divers, and more importantly, with an individual diver on repeated exposures. The variations were really huge -- it's quite daunting to look at the figures.

So you have to look at WHY you are using Nitrox. The two benefits that are usually mentioned are that it permits increased bottom time, if you are using it to its NDL limits, or that it improves safety, if you are diving it to air limits. The price you may pay for increasing your bottom time at high ppO2s is dying -- few people have survived a seizure underwater, and those only because they were immediately adjacent to a well-trained buddy who knew PRECISELY what to do to get them to the surface.

This really is the problem with 130 foot recreational dives. Bottom time there on air is almost uselessly short, if you don't want to incur a deco obligation. Nitrox mixes that keep the ppO2s safe give you very little advantage. Really, as Uncle Pug has observed repeatedly, dives to that depth should be planned and executed as staged decompression dives, with the right gases and the right training.

At any rate, I keep my ppO2s below 1.4. There's nothing deeper than that that's worth the risk of dying for it. I read the account of the woman who died in Ginnie Springs, and her husband's valiant efforts to try to get her out of the cave while she seized. I don't ever want somebody to have to try to rescue me that way.

 

[Meng_Tze]

It could tell me that. Or it could tell me that in your nitrox class you learned:
(1) The NOAA maximum exposure table, and
(2) a 'best mix' equation.

Those things combined point to EAN32 at 130 feet. With so many differing opinions on the maximum acceptable PO₂ for the working portion of the dive, I'd find it hard to fault someone for applying their basic nitrox training to the NOAA table.

So you are telling me that as long as you follow the table and best mix equation you should be fine going to a PO2= 1.6 on the working part of the dive? Mind you, not 'faulting' anyone for doing so, just do not get why one would push their risk factors that high. Just want to clarify this one point.

 

[Blackwood]

So you are telling me that as long as you follow the table and best mix equation you should be fine going to a PO2= 1.6 on the working part of the dive? Mind you, not 'faulting' anyone for doing so, just do not get why one would push their risk factors that high. Just want to clarify this one point.

I'm saying that there are various standards, various classes, and various teachers who teach various things. They may simply have been informed differently than you and I about the risks.

Like I said, I wouldn't do it. 1.2-ish is AOK. I wouldn't dive to 130 on anything less than 25/25 either, but I've clearly had different training than they.

All I'm saying is that looking at the broad strokes of nitrox training, their choice is justified and possibly even encouraged (by that 'best mix equation').

 

[battles2a5]

The toxing incident was not at Ginnie, it was at "The Crack" and it was a deeper trimix dive. There is still a lot of speculation as to why the diver toxed at a "low" PO2. She had participated in a chamber study and stated that she felt like she was sensitive to elevated PO2's so that may have been the cause, although she seemed to have plenty of experience at this level. Some think that the O2-tox threshold is lower with trimix. Some also speculate that is was allergy medicine. During my last trip to the allergist, I had the doc get on the phone to see if any of my meds were "suspect". DAN confirmed that some rumors were out there, but no drugs have been proven to increase your risk of O2 tox. So back to my previous point (and that of several others), you can't predict it so the best you can do is plan at a level that is both useful (in terms of extending NDLs, etc) and safe according to your level of risk tolerance.

One point I would like to bring up to those of you who routinely conduct the working portion of the dive at 1.6; what do you do in terms of contingency? Blackwood mentioned dropping down to grab dropped gear or a buddy, etc. At 1.4 you have that contingency. Dropping to 1.6 for a few seconds is not a big deal. But what if you are at 1.6 and have to drop to 1.8? 1.9?

 

[battles2a5]

I'm saying that there are various standards, various classes, and various teachers who teach various things. They may simply have been informed differently than you and I about the risks.

Like I said, I wouldn't do it. 1.2-ish is AOK. I wouldn't dive to 130 on anything less than 25/25 either, but I've clearly had different training than they.

All I'm saying is that looking at the broad strokes of nitrox training, their choice is justified and possibly even encouraged (by that 'best mix equation').

All the nitrox training I have received (basic nitrox, advanced nitrox, trimix from three different instructors) has given guidance to plan for 1.4 for the working portion of your dive, 1.6 for contingency or deco. You use the "best mix" calcs to work out your bottom mix at 1.4, not 1.6. So while they can easily calculate a best mix at 1.6, I doubt they were trained to do so.

 

[Damselfish]

This really is the problem with 130 foot recreational dives. Bottom time there on air is almost uselessly short, if you don't want to incur a deco obligation. Nitrox mixes that keep the ppO2s safe give you very little advantage.

while the table time for a square profile to 130 may be too short to be very useful, a short visit to that range to see something specific, combined with a multilevel profile, is pretty common some places. And not so limiting of your overall dive time whether on air or better yet on some appropriate Nitrox mix.

 

[John_B]

I agree but they are the only ones that have that information so until I get site specific information from them, I cannot make an informed decision. Poor wording on my part and thank you for calling it.

No, I clearly misunderstood what you were saying. My bad.

 

[Kern]

xxxx Sorry, seem to have responded to something that doesn't exist. Must be going mad.