Weird experience today - ox tox warning

[MikeFerrara]

Mike, I'd like to hear more about that, if possible. What do you consider the "working" part of your dive. Obviously, at depth, and I assume that could apply to someone doing salvage, hunting fish, taking photos of a wreck, whatever.

I'm considering everything other than decompression as the working portion of the dive. I don't have a problem with diving 1.4 ata PPO2 on recreational stuff but for longer/deeper stuff where the O2 exposure will be significant I like it lower.

Also, everyone has a different seizure threshold. An epileptic, for example, has a low seizure threhold and has to take meds to raise that threshold. Have there been studies that would demonstrate if one's seizure threshold plays a part in susceptibility to oxygen toxicity? A correlation, IOW.

CNS tox is hard to predict. Even the same diver can tox at different levels of exposure from day to day and dive to dive. About all they know is that above a certain PPO2 the odds of a hit go way up.

The IANTD Technical Diveing Encyclopedia references a bunch of studies.

 

[pipedope]

Also, everyone has a different seizure threshold. An epileptic, for example, has a low seizure threhold and has to take meds to raise that threshold. Have there been studies that would demonstrate if one's seizure threshold plays a part in susceptibility to oxygen toxicity? A correlation, IOW.

This is one of the problem areas.
Years ago, the Navy and commercial diving schools would test people for susceptability to O2 tox as a qualification to be a diver.

They found that the results were so variable that they were completely useless. One day a person would have low tolerance and another day they would have a very high tolerance with no corelation to anything that they could observe.

That is one more reason why the limits are lower today than they were 20 or 30 years ago. O2 tox is just too unpredictable.

 

[pipedope]

I don't have the numbers to prove it but most technical divers who tox do so because of a wrong gas switch. I bold a portion of you're statement above to point out that it might be better to put the toxing diver on your long hose rather than theirs. After all they toxed and you didn't. The gas they were breathing might not be the gas to put them on when and if they start breathing again.

I only know of one instance of a toxing diver being rescued and brought up through deco and that was AG with a student.

GUE now teaches the method which also includes purging the reg some so that there's something other than water in front of them.

Mike, you are correct.
I didn't want to type the entire mass of what is involved in a O2 tox rescue but just to make the point that it is VERY difficult to sucessfully rescue a sport diver who has a CNS episode at depth.

This also points out the importance of analyzing gas to be used and following gas switch procedures exactly.

 

[glbirch]

Mempilot, I can get in up to seven dives on most days I go out. If I can find the time, I can get out five days in a month. That would be about 35 dives right there in a single month. If I can do that for six of the 12 months, then that is 210 dives for half the year. If I can sprinkle in another 80 dives for the rest of the year, then that would explain it.

Scubaholic, what kind of profiles are you diving to get in seven in one day as an average? My computers start complaining after four or five unless I keep them shallow and/or short.

 

[Scubaholic]

Scubaholic, what kind of profiles are you diving to get in seven in one day as an average? My computers start complaining after four or five unless I keep them shallow and/or short.

This is very aggressive diving and not recommended, and I certainly don't want to encourage it here. But yes, using Nitrox, it is entirely possible to get in not only seven dives, but seven dives in which all seven are below 100fsw. I have done it using the supposedly ultra conservative Suunto Cobra. I have also made the seventh dive when everyone else on the boat using more liberal computers said "I'm done, my computer won't let me dive." Understand though, that seventh dive might be "I've got 2 minutes at 130." Also, diving this aggressivly usually mean you will flip into mandatory decompression stops a couple of times a day, but I have also done it in which all dives were no-deco.

Diving shallow dives (60 - 70fsw), I think I could dive all day. Especially if I used a high mix for safety stops.

 

[Curt Bowen]

Thanks to those of you who did not respond in a condescending manner. To those of you who responded with comments like “stop diving nitrox” and “my instructor would kill me for diving this profile,” I would advise that your beloved instructor is likely not qualified to carry my lunch bag for me on a dive trip. While there are many people that will write off an experience like this as a “Darwin Award” candidate, those that dive aggressive profiles will appreciate that I can talk about it.

Scubaholic

Oxygen toxicity is unpredictable but I don't think you should have had any type of problem with a one minute exposure to 1.6 PPO2's.

In most of our exploration dives our decompression gases are adjusted to a 1.6 PPO2 in an attempt to eliminate unwanted nitrogen and helium and also to cut our deco to an acceptable time. We are on these PPO2 for hours, but do take the required 5 minute air breaks when at our 20 foot, 100% oxygen stop. In a few of the extreme exposures we have had oxygen exposures to 1200% and not the 100% recomended by standard nitrox dive computers. We worry more about a problem we called burned lungs which can be caused by High Partial Pressures of Oxygen over long periods of time.

When I took the nitrox course back in 1991 by Billy Deans we were taught a PO2 of 1.6 for a bottom mix was OK. They changed the standards since.

In the days of deep air we normally exposed ourselves to PPO2's of 2.0 and higher for 10-15 minutes. How we lived, I am not sure?

 

[Spectre]

there are two things you can not exceed: max PO2 for
a mix AND max time at that PO2 for that mix.

thus, if you exceed 1.6 ata on a gas, even for a short time,
there is the risk (albeit unpredictable and small) that you'll
suffer CNS toxicity.

OR you could stay within 1.6 ata but exceed the max time
at that ata, and then, again, you'll risk CNS toxicity.

hope i stated this clearly

You stated it clearly; but it's wrong.

There is clock values for higher exposures then 1.6. We all know 1.6 is 45 minutes, 1.5 is 120 minutes [iirc]. If you spend 15 minutes at 1.6 exposure then go to 1.5; theoretically you have 80 minutes at 1.5 for your exposure. It doesn't go from 45 minutes to 0 minutes. There is exposure times for the higher exposures as well.

-however- you have to look at how those times were generated. It was simply an experimental datapoint where the subjects didn't show any symptoms in the time period. If you look into it some; you will note that it was all mostly random; in that someone could handle 2.0 exposure 10 times without a problem; but based on different environmental factors found themselves toxing on the 11th time.

Now add in the fact that his computer modelling had him hitting 100% exposure?! I would be spending my time looking at the complete proflie and try and figure out where the dive didn't go the way it was planned. If -you- think you shouldn't be at 100% exposure; and your computer does; then most likely there is something your computer knew about that you didn't notice...

 

[mempilot]

This is very aggressive diving and not recommended, and I certainly don't want to encourage it here. But yes, using Nitrox, it is entirely possible to get in not only seven dives, but seven dives in which all seven are below 100fsw. I have done it using the supposedly ultra conservative Suunto Cobra. I have also made the seventh dive when everyone else on the boat using more liberal computers said "I'm done, my computer won't let me dive." Understand though, that seventh dive might be "I've got 2 minutes at 130." Also, diving this aggressivly usually mean you will flip into mandatory decompression stops a couple of times a day, but I have also done it in which all dives were no-deco.

Diving shallow dives (60 - 70fsw), I think I could dive all day. Especially if I used a high mix for safety stops.

I'm not trying to be a smartass, but what kind of diving are you doing when you do seven dives in one day? Are you actually changing sites. At the bottom times your getting by allowing 7 dives, each dive must be very short(you speak of doing all seven below 100). I'm assuming your not wearing doubles, since the total run time of all seven dives could be done in just a couple of dives (I ran it on Decoplanner using NDL's). Just curious. I know that on liveaboards, we've done up to 5 dives on nitrox, but by the 5th, we getting pretty much no time, and only the first couple of dives are below 100'. Do this consecutive days and by the end, the O2 alarm is going off, so we step back to air. Could you provide an example of your 7 dive day profile?

 

[Scubaholic]

I'm not trying to be a smartass, but what kind of diving are you doing when you do seven dives in one day? Are you actually changing sites. At the bottom times your getting by allowing 7 dives, each dive must be very short(you speak of doing all seven below 100). I'm assuming your not wearing doubles, since the total run time of all seven dives could be done in just a couple of dives (I ran it on Decoplanner using NDL's). Just curious. I know that on liveaboards, we've done up to 5 dives on nitrox, but by the 5th, we getting pretty much no time, and only the first couple of dives are below 100'. Do this consecutive days and by the end, the O2 alarm is going off, so we step back to air. Could you provide an example of your 7 dive day profile?

Affirmative on the examples. I have not downloaded my dives in two years, but I'll go to Suunto.com and download the software, and then upload my dives and post an example of seven dives in a day. It may take a few days, but I'll get to it.

 

[Scubaholic]

Now add in the fact that his computer modelling had him hitting 100% exposure?! I would be spending my time looking at the complete proflie and try and figure out where the dive didn't go the way it was planned. If -you- think you shouldn't be at 100% exposure; and your computer does; then most likely there is something your computer knew about that you didn't notice...

But that doesn't explain why two different Suunto computers came up with two different results. I thought maybe I had forgot to plug in the right gas on one of them, but not the case. I'd like for Suunto to explain that to me. As it is, the Cobra is history. There are far better designs on the market that actually enable you to see the OLF data. On the Cobra, if you can see that you have all the bars, then you have better eyes than me, and I am 20/20 bilat.