You offgas faster at 5m depth than on the surface

[DiveProKoko]

All of my local dives are "at altitude" due to where I live. I dive regularly at about 4200 feet (1280 meters). I sometimes dive in mountain valleys nearby which are in the 6000 foot range (1800 meters), which requires driving over mountain passes back into my home valley. Some of these passes are in the 8000 to 10000 foot range (2400 to 3000 meters). The only remedy is long surface intervals before driving, so we go have dinner, hangout, etc. before making those drives. Longer safety stops are contradictory for that problem. That time could be better spent on the surface actually off gassing.

 

[tbone1004]

All of my local dives are "at altitude" due to where I live. I dive regularly at about 4200 feet (1280 meters). I sometimes dive in mountain valleys nearby which are in the 6000 foot range (1800 meters), which requires driving over mountain passes back into my home valley. Some of these passes are in the 8000 to 10000 foot range (2400 to 3000 meters). The only remedy is long surface intervals before driving, so we go have dinner, hangout, etc. before making those drives. Longer safety stops are contradictory for that problem. That time could be better spent on the surface actually off gassing.

How long are you talking and what kind of dive profiles are you doing? Check out Table 9-6 on page 9-62 of the manual I posted. You're going from 0.8ata to 0.7ata so may be worth looking at depending on how inconvenient that wait is.

 

[DiveProKoko]

How long are you talking and what kind of dive profiles are you doing? Check out Table 9-6 on page 9-62 of the manual I posted. You're going from 0.8ata to 0.7ata so may be worth looking at depending on how inconvenient that wait is.

The dive profiles vary, of course. But thanks for the tables. We've done the math, but the tables are nice to have! I realize the types of intervals we are adhering to are "overkill" for altitude diving and driving, but they are never an inconvenience. We know that we're gonna hang out for a bit, doing something (dinner, cookout, etc.) afterward anyway. I have driven almost immediately after a couple of shallow open water dives at altitude with students when I needed to get home. The time to disassemble, rinse, pack up and debrief is usually plenty of time. Thanks again for the info.

 

[Akimbo]

There "might" be impact for some tissues under some dive profiles but not all, unless you are on pure Oxygen. A few minutes difference at a safety stop breathing the deep mix will be inconsequential unless you are very close to getting bent anyway.

 

[Johnoly]

Off gassing is a VERY complex subject with small bubble, large bubbles, hidden bubbles, wannabe bubbles, hard bubbles, soft bubbles, and so many other things it will make your head hurt after walking out the of the yearly D.A.N. seminars at DEMA.

So I just want to address "Why 3 minutes? " . In a normal person at about 70 beats per minute it takes an average of 60 seconds for a single blood cell to make a full circuit of your body and that includes thru your lungs where most bubbles are "processed" {another long subject }. By staying 3 minutes that's ~3 trips thru the lungs and ??HOPEFULLY?? any bubbles will be processed by the lungs. So is 7 minutes better ? Diminishing returns & even more discussions that will make your head hurt.

 

[boulderjohn]

I'm not so sure the effect of extending a safety stop on air or nitrox will have as little effect as you imply. I believe there have been studies (DAN?) that attempted to compare the effects of various safety stop lengths on bubble formation. If I recall, longer stops were found to reduce bubble formation but with diminishing returns, so the recommendation continues to be for a 3-minute safety stop.

I am unaware of these studies. If you can find them, I would be interested.

Longer stops on recreational dives should have a benefit in making it safer for you to ascend to the surface and that lesser ambient pressure. However, once you have reached the point that it is safe to go to the surface, if you are hoping for the fastest off-gassing, the surface is where you want to be.

I am assuming that a recreational diver will not have access to oxygen during the dive. Breathing oxygen during a stop will be better than breathing air at the surface.

 

[boulderjohn]

Just to add to my last post....

During your safety stop, your fastest tissues will off-gas to the pressure at that depth. Once they reach that pressure, it does them no good to stay there any longer. During that stop, the slower tissues will be on-gassing, but they will be on-gassing to a level that should be safe to surface and will go no farther. It is of no benefit for them to stay any longer and continue on-gassing.

The benefit of a longer safety stop is for cases when because of the depth of the dive and the rate of ascent, it may take the fastest tissues longer to reach a safe level. You will have no way of knowing if this is true in your case. It will do not harm to do the extra few minutes to be sure, but it may have no additional benefit.

 

[Lorenzoid]

Here's a SB thread where a 3-minute stop versus a 5-minute stop was discussed: Safety Stop - 3 min or 5 min?

 

[broncobowsher]

On air, they are doing it wrong. Longer surface before altitude is correct, longer in the water is not.
With a strong O2, as in the PPN2 is less than at the surface, they got it right.

The tissue bar graph on a Shearwater can teach a lot. slow tissues, fast tissues.
On a long dive with my rebreather, a long deco on near pure O2, some fast tissues are actually on-gassing when I surface

But for the recreational diver on air, they got it wrong.

 

[Subcooled]

He reckoned that you offgas faster under slight pressure.

Offgassing has to do with the concentration of nitrogen in you body (and tissues and perfusion and it's complicated) and the ambient pressure. Shallower is faster. I said faster, not better.

Edit: OK, Akimbo reminded us of the fact that maximum oxygen pressure is what you want, hence depth. My bad.

7 minutes at the safety stop? Why not fifteen? Surely, there is a lot to see at that depth too, and it IS easier on your body.

OK, I get it, it might (???) be more efficient to offgas from solution to gas instead of from bubbles to gas, but this somehow sounds a bit academic. We need a hyperbaric medicine researcher to guide us here.

Surface deco with oxygen might be a good choice. DAN has studied that, at least.

Something to do with increased circulation

And how would that be related to increased ambient pressure????