nitrox MOD/TOD

[TSandM]

You guys are arguing in a circle.

ATA * %O2 = ppO2

Three variables. One gets fixed when you pick up the tank -- it has 31.4% O2 in it and that's not going to change unless you remix it.

One gets fixed when you make a decision about what the tank is used for, and what ppO2 YOU want to be exposed to while doing that. 1.4 for the working part of the dive, 1.6 for deco, or some other combination that you have learned or chosen.

Fix two variables, the third is determined by the math. ATA converts directly to depth.

YOU ask for the mix; YOU decide the ppO2, and the math decides the depth to which you can use that mix. It's that simple. And everybody who goes down breathing compressed gas should know how deep they want to take that gas, and how deep they are willing to take that gas in a "contingency". And be aware of what the outcome of those decisions can be.

 

[H2Andy]

yes, of course

ah well ... time to move on anyway

 

[ianr33]

you're talking about pulmonary oxtox

we were discussing Central Nervous System (CNS) oxtox, which can be triggered immediately if a certain PPO2 treshhold is exceeded

(i.e. there are two types of oxtox)

That is not correct. It depends on time and PO2. Just because you have MOD 132 on a tank of 32% does not mean that you can breathe that mixture all day at that depth (45 minutes is the normal recommended maximum)

Even very high PO2 levels (say 3.0) will most likely not trigger an "immediate" tox (although you might have only a minute or two) Then again you MIGHT be just fine.

I am no expert but pulmonary oxygen toxicity produces lung problems,not the seizures that CNS oxygen toxicity does

 

[grazie42]

I don´t really see the label as that important...MOD/TOD/MIX

I just put mix in big numbers and my initials on the tank.
If it´s a single, I won´t be able to read it anyway (its behind me), same goes for dubbles.

If it´s a decobottle, all the label is for is to make sure I don´t use the wrong bottle. The depth at which I´ll breath it will be written in my decoplan. I have multiple other, redundant systems, to make sure none of those to things happen (wrong gas/wrong depth)...I´d think that would be true of any trained diver form any agency...

YMMV...

 

[Charlie99]

you're talking about pulmonary oxtox

we were discussing Central Nervous System (CNS) oxtox, which can be triggered immediately if a certain PPO2 treshhold is exceeded

(i.e. there are two types of oxtox)

Both CNS and pulmonary/whole body effects of elevated oxygen levels are time dependent.

For example the CENTRAL NERVOUS SYSTEM oxtox NOAA limit is 45 minutes per dive at 1.6ata ppO2, and 150 minutes per dive at 1.4ata ppO2 (24 hour limits are 150minutes @1.6ppO2, and 180 minutes at 1.4ata ppO2.). These time limits have nothting at all to do with pulmonary oxtox.

You are a good example of how prevalent misunderstanding is in this area.

 

[BarryNL]

Even very high PO2 levels (say 3.0) will most likely not trigger an "immediate" tox (although you might have only a minute or two) Then again you MIGHT be just fine.

IIRC, 3.0 is used in bends treatment in 10 minutes doses. Generally this does not cause seizures. Of course, this is under medical supervision in a recompression chamber so seizures are not a dangerous problem.

 

[H2Andy]

You are a good example of how prevalent misunderstanding is in this area.

lordy lord lord .... am i speaking a foreign language today or something?

i am well aware of the exposure issue (time-wise) with CNS tox. that is dependent
on the PPO2, which will determine how long you can stay at that PPO2.
however, exposure is NOT the dominant factor in CNS ox tox, PPO2 is.
it determines time available.

another way to say it is that when it comes to pulmonary oxtox, time is the primary factor (i.e. after 12 hours or so of constant exposure to .06 PPO2 gas), whereas in CNS tox, PPO2 is the primary factor (as that will determine time available) (this, btw, has been my point all along ... that PO2 is what controls MOD)

however, that is NOT what we were talking about. we were talking about MOD, in other words, the maximum operating depth for a particular gas being dependent on what PPO2 you chose to live by. in that case, the immediate concern is exceeding a "safe" PP02 which could trigger an immediate (yes, immediate) CNS hit if the PPO2 is high enough (i don't know how long it would take, say, at 2.0 PPO2, but i imagine it's in the order of minutes).

obviously exposure time is not an issue WHEN DISCUSSING EXCEEDING A "SAFE"
PPO2 LEVEL since the ox tox onset could happen catastrohpically quickly (say, for example, erroneously diving a mixture to a 2.5 PPO2)


(please note my use of the word "could" before going ballistic)

 

[cerich]

lordy lord lord .... am i speaking a foreign language today or something?

could trigger an immediate (yes, immediate) CNS hit
if the PPO2 is high enough (i don't know how long it would take, say, at 2.0
PPO2, but i imagine it's in the order of minutes).


this ain't rocket science

could you direct me to some literature where a immediate CNS hit is discussed? It may present like that to the lay man but it is my understanding (and experience in chamber) that there are always symptoms prior to seizure. They may not be easy to connect but they are there. Could you back your assertions?

The best part of this thread is folks trying to find or give definative answers to something that is so variable that that answer does not exist.

 

[H2Andy]

could you direct me to some literature where a immediate CNS hit is discussed?

sure:

"The onset and severity of symptoms do not follow any particular pattern and may vary in anindividual diver from day to day. Of particular note is that there may not be less serious symptoms to serve as a warning before a full convulsion is precipitated. According to oxygen physiologists Stephen Thom and James Clark, "Minor symptoms did not always precede the onset of convulsions, and even when a preconvulsive aura did occur, it was often followed so quickly by seizures that it had little practical value."

http://www.scubadiving.com/index2.php?option=content&do_pdf=1&id=3427

also:

Central nervous system (CNS) toxicity, aka acute oxygen toxicity or the Paul Bert Effect (who published his research in 1878), manifests itself as convulsions, often with very little in the way of warning signs.

http://www.gasdiving.co.uk/pages/misc/Nitrox.htm

 

[cerich]

sure:

"The onset and severity of symptoms do not follow any particular pattern and may vary in anindividual diver from day to day. Of particular note is that there may not be less serious symptoms to serve as a warning before a full convulsion is precipitated. According to oxygen physiologists Stephen Thom and James Clark, "Minor symptoms did not always precede the onset of convulsions, and even when a preconvulsive aura did occur, it was often followed so quickly by seizures that it had little practical value."

http://www.scubadiving.com/index2.php?option=content&do_pdf=1&id=3427

So from that I read that it can be super quick, but not free of all symptoms (major and/or minor)

Thanks,

Chris