100% oxygen, safety stop depth & off gassing

[iainwilliams]

You bet Brian - cheers.

I usually use a form of modified Pyle stop.

Half the deepest depth divide by two is my first stop for 2-3 minutes – then half again until I reach 5 m. At 5 m I spend 5 minutes then proceed to 3m where I spend an additional 2-3 minutes (more time if I’m doing a repetitive dive or doing several dives over several days). I then surface (obviously). I go onto 100% O2 at 5 m and maintain this until on the boat.

Yes – probably overkill – but definitely safer………….Iain

 

[keezdiver]

when scrubbing with high mix oxygen...in this case 100%...you're not accumulating ANY added nitrogen. therefore isnt it most beneficial to do your SS at a depth closest to the "safety limit" of the mix....1.6pp?

as stated, other than slowing your final assent, the 3m stop is not doing much good.

when you have a computer that gives you credit for scrubbing....its AMAZING to see how fast the nitrogen load drops when on pure 02

 

[TSandM]

Yes – probably overkill – but definitely safer………….Iain

I'll take issue with this. You're trading an extremely small risk of DCS (for recreational dives, carried out with reasonable technique) for a small but very real risk of a seizure. Most DCS is survivable; in fact, although I can't offer any data to support it, I'd be willing to make the statement that ALL DCS which occurs in recreational diving carried out with good technique is survivable. On the other hand, seizing underwater can easily be fatal, especially if one does not have a well-trained and very attentive buddy present when it occurs.

This is not a safety strategy that I would adopt or recommend.

 

[DepartureDiver]

when scrubbing with high mix oxygen...in this case 100%...you're not accumulating ANY added nitrogen. therefore isnt it most beneficial to do your SS at a depth closest to the "safety limit" of the mix....1.6pp?

Yes. Staying under pressure minimizes any "bubbling" and increases the driving force for gas elimination. Keeping this simple (and not exact), by definition the pressure inside a bubble is equal to the external pressure (since that is what determines when a bubble stops growing) and due to the inside and outside pressure being equal, the driving force for gas elimination is lost. So if bubbles are minimized by staying under more pressure such as at 20', then the driving force for gas elimination is more efficient and faster.

 

[DepartureDiver]

... On the other hand, seizing underwater can easily be fatal, especially if one does not have a well-trained and very attentive buddy present when it occurs.

A valid point, but tech divers routinely perform safe deco on O2 at 20' and if anyone is using pure O2 they are hopefully adequately trained prior to using it. Also if a diver is simply performing a safety stop on a no-stop dive the exposure is very small.

 

[DA Aquamaster]

You bet Brian - cheers.

I usually use a form of modified Pyle stop.

Half the deepest depth divide by two is my first stop for 2-3 minutes – then half again until I reach 5 m. At 5 m I spend 5 minutes then proceed to 3m where I spend an additional 2-3 minutes (more time if I’m doing a repetitive dive or doing several dives over several days). I then surface (obviously). I go onto 100% O2 at 5 m and maintain this until on the boat.

Yes – probably overkill – but definitely safer………….Iain

There are trade offs either way. Personally if you are not getting subclinical DCS signs, such as flu like symptoms the day after diving,tc, then the "normal" ascent profile/deco you are doing is probably doing the job effectively. Adding more is just trading one potential risk for another, so you need to balance the costs and benefits.

Personally, with your deep stop approach, it may make more sense to use 50% from 70 feet up. A couple minutes at 70 feet for your deep stop, and then a slow ascent to a 15-20 ft safetey stop with perhaps brief stops every 10 feet to stop the ascent and ensure a very slow overall ascent rate, followed by a slow (maybe 10 ft per minute) ascent to the surface.

Very long deco dives using helium based mixes in a spring with no wave action may warrant a very slow ascent from the 10 or 20 ft stop to the surface but for recreatioanl or most deco dives using nitrox it is probably overkill and in 5 ft swells or waves a slow ascent shaloower than 10 feet is probably pointless.

So in effect, what makes the most sense also depends on the situation and the local conditions.
e

 

[JahJahwarrior]

I'd be concerned about the fact that you are carrying a bottle of pure o2 with yourself throughout the dive...do you have the MOD marked very clearly and plainly on this bottle? Is there anything preventing you from leaving it somewhere above it's MOD, so there is no way you can accidentally breathe from the bottle at depths where the PPO2 would be so great as to virtually garuantee oxtox?

 

[TSandM]

Technical divers routinely do stops on hot mixes or 100%. But they do it because they have a deco obligation, and shortening that deco obligation is worth some risk, and increasing the efficiency of offloading nitrogen is definitely worth some risk.

I'm just pointing out that, on a recreational dive, you have a minimal requirement for decompression, which is neither thermally stressful (or it shouldn't be) nor difficult to carry out. Even if you omit ALL decompression on a recreational dive, as long as your ascent is controlled, the likelihood is greater than 95% that you will be asymptomatic at the end of the dive. Breathing 100% O2 at 1.6 ppO2 has a small but real risk of seizures. We've lost at least two divers this year at 1.4, and another in Southern California seized but survived. If you're going to do a safety stop anyway, you aren't "shortening your deco", and the need to optimize off-gassing is minimal, if it exists at all.

Everything we do in diving makes us weigh risk. I don't think you would find very many people who are educated and trained in the use of breathing gases other than air, who would find the risk-benefit ratio of 100% O2 on recreational dives to pencil out.

 

[spacemanspiff1974]

I'd be concerned about the fact that you are carrying a bottle of pure o2 with yourself throughout the dive...do you have the MOD marked very clearly and plainly on this bottle? Is there anything preventing you from leaving it somewhere above it's MOD, so there is no way you can accidentally breathe from the bottle at depths where the PPO2 would be so great as to virtually garuantee oxtox?

Gas confusion results more from carrying multiple deco gasses. If you are slinging a single deco bottle, you know whats in it and when it can be used...or should if you checked the O2% before jumping in.

 

[dab]

Technical divers routinely do stops on hot mixes or 100%. But they do it because they have a deco obligation, and shortening that deco obligation is worth some risk, and increasing the efficiency of offloading nitrogen is definitely worth some risk.

I'm just pointing out that, on a recreational dive, you have a minimal requirement for decompression, which is neither thermally stressful (or it shouldn't be) nor difficult to carry out. Even if you omit ALL decompression on a recreational dive, as long as your ascent is controlled, the likelihood is greater than 95% that you will be asymptomatic at the end of the dive. Breathing 100% O2 at 1.6 ppO2 has a small but real risk of seizures. We've lost at least two divers this year at 1.4, and another in Southern California seized but survived. If you're going to do a safety stop anyway, you aren't "shortening your deco", and the need to optimize off-gassing is minimal, if it exists at all.

Everything we do in diving makes us weigh risk. I don't think you would find very many people who are educated and trained in the use of breathing gases other than air, who would find the risk-benefit ratio of 100% O2 on recreational dives to pencil out.

Well stated. That said, as long as the training and full understanding is there, and your buddy has the appropriate training in how to handle a tox situation, then it becomes a personal choice. For recreational diving, it's hard for me to see the benefits of carrying pure 02 when compared to the headaches and risks of dealing with it both above and below the water (and costs). My personal feeling is that one would be better off carrying a pony containing additional backgas which would have a more widespread use should it be needed.