100 % O2 breathing at safety stop

[iainwilliams]

Hello

This is two-part question.

If I breath 100% oxygen for say, 10 minutes during a safety stop (within acceptable partial pressures) after a dive using a standard 21/78 air mix, there is a very low probablity of oxygen toxicity. However, if I am doing a number of dives during a 24 hour period, do I have to worry about the cumulative affects of O2?? If so,

Do I add up the totals of the time spent breathing oxygen at the safety stops and ensure that they do not exceed 45 minutes (assuming the safety stop was at a partial pressure of 1.6).

Would I do the same if I was using NITROX??

Thank you for your time and advice.............Iain

 

[Spectre]

iainwilliams once bubbled...
if I am doing a number of dives during a 24 hour period, do I have to worry about the cumulative affects of O2??

Yes... of course.

Do I add up the totals of the time spent breathing oxygen at the safety stops and ensure that they do not exceed 45 minutes (assuming the safety stop was at a partial pressure of 1.6).

Well, it's 45 minutes per dive, 150 minutes over a 24 hour period [@ 1.6]. However the clock starts @ your dive, not your safety stop... so you have to take into account the O2 exposure you got from your full dive.

For example, if you spent 50 minutes @ 1.4 during your dive, then you only have 30 minutes @ 1.6 remaining on your clock.

If you do that same dive twice [50@1.4 + 30@1.6], you are left with [(50 + 50)/180 = .55 * 120 = 67] [67 + 30 + 30 = 127]. 23 minutes of exposure remaining @ 1.6, or 27 minutes of exposire remaining @ 1.4.

This is the same for whatever O2 percent you dive, but O2 tracking becomes more important with Nitrox diving, as your bottom time spins the O2 clock faster the higher the O2.

 

[iainwilliams]

Thank you Jeff. I believe I understand what you are saying.

What I am trying to do is figure out (in a rough way) whether I will be SAFE using oxygen on safety stops to increase nitrogen out-gassing using a 21/78 mix when diving to average SCUBA depths (ie: no deeper than about 40 metres), taking into account that I do 2 dives per day.

I understand that NITROX will GREATLY increase the risk and probablilty of oxygen toxicity, however, for the diving I am doing, it is unnecessary to use 100% oxygen on a safety stop when using NITROX - Just use the NITROX!

Cheers......................Iain

 

[Spectre]

iainwilliams once bubbled...

What I am trying to do is figure out (in a rough way) whether I will be SAFE using oxygen on safety stops to increase nitrogen out-gassing using a 21/78 mix when diving to average SCUBA depths (ie: no deeper than about 40 metres), taking into account that I do 2 dives per day.

Realistically you should be fine, but running the numbers on the O2 exposure limits clock is a good practice to get yourself into in any rate... Of course I'm making a lot of assumptions regarding your dive profile [assuming NDL on air + 3-5 minutes safety stop @ 15], so always run the calculations for the dive profile you are planning.

Likewise make sure you get adequate training...I've not taken the course yet, but I believe the practices we are discussing are covered in advanced Nitrox courses....

 

[iainwilliams]

Yes Jeff you are quite right re: training. Your assumptions are also correct re: NDL on air with SS @ 15 feet. Thanks for your help. Iain

 

[BIGJC]

Personally I would ALWAYS account for my O2 exposure no matter how slight.. survival rates on underwater tox hit are VERY LOW!

O2 on your hang is a good idea though!

get some dive software and run some scenarios.... the exposure should not be that great... after 6 dives on air with 100% O2 on your safety stops your only at 30% exposure.. no PROBLEM!

:boom:

 

[fins wake]

... I'd be less worried about the cumulative effects of oxygen and more worried about the risk of a bad gas switch (wrong gas, wrong depth) if untrained ...

I've not taken the course yet, but I believe the practices we are discussing are covered in advanced Nitrox courses....

Yes, you are trained for up to 100% O2 on a TDI Advanced Nitrox course. Only 50% on an IANTD Advanced Nitrox course (the 100% comes with the IANTD Technical Nitrox course).

Of course, as you're a PADI OWSI, you might appreciate the excellent PADI/DSAT TecDeep course. ApprenticeTec allows for gas switches up to 60%, TecDeep up to 100% O2. And you really, really get to work on NO TOX gas switch drills, believe me ... very useful. But all three organisations run good courses provided you get a good instructor. I've had experience of two of them.

Of course, you'll be able to track the CNS clock after these courses, as well.

Doing O2 switches without training is not a good idea. :bonk:

 

[Pez de Diablo]

this into an agency comparison. The point is, get the apropriate training.

Here is a good thread on the Decostop about the DESAT course. DESAT discussion

I enjoyed the course and would recommend it. However I have not taken the equivalent course from another agency so I cannot compare them.

Like anyone telling you something is good, how do they know what *good* is. Ask, what is their definition of good.

 

[TheDarknessLord]

the best way to know the %CNS and OTU unit, is using a computer. I have two computers: Uwatec Air-X and Delta VR3. The second one is much better because you can use 10 gases on it...

cheers!

 

[Braunbehrens]

Take a look at what the wkpp is doing (www.wkpp.org).

If you look at their profiles, you'll see that they far exceed the recommended O2 allowance. How do they do it?

The answer lies in taking frequent low PO2 breaks.

The current PO2 limits are based on very thin research...

There is another thread on this, but what the wkpp guys are doing is to keep the bottom PO2 very low, and then taking frequent breaks during high pO2 sessions.

If you keep the bottom PO2''s low you have the added advantage of not having to track PO2's.

I would NOT do any of this without taking a class. Blowing it with high PO2's will kill you faster than you can say "Bob's your uncle".