You offgas faster at 5m depth than on the surface

[Tom_Ivan]

I was diving with someone the other day who had to drive to an altitude of about 800m afterwards. In order to reduce his risk he decided to extend his safety stop on the final dive from 3 minutes to 7. This was at 5m.

He reckoned that you offgas faster under slight pressure. Something to do with increased circulation and having a greater number of small bubbles instead of a small number of large bubbles.

Does anyone agree with this? Any articles or science papers that have tested this?

 

[kaylee_ann]

That makes no sense. What makes the gas go out of solution to bubbles is decreased pressure.

 

[Belzelbub]

Yeah, that’s not right. Offgassing occurs due to the differences in tissue pressure and ambient. That difference would be greater at the surface than at 5M.

 

[michael-fisch]

Switch to pure O2 at the greatest depth your training allows, then ascend very slowly and stay on O2 at the surface for a good long time.
Breathing O2 while going over a mountain pass after a heavy dive is also not a bad idea.

Michael

 

[tbone1004]

I was diving with someone the other day who had to drive to an altitude of about 800m afterwards. In order to reduce his risk he decided to extend his safety stop on the final dive from 3 minutes to 7. This was at 5m.

He reckoned that you offgas faster under slight pressure. Something to do with increased circulation and having a greater number of small bubbles instead of a small number of large bubbles.

Does anyone agree with this? Any articles or science papers that have tested this?

that doesn't track, both the circulation and the bubble size concept. The pressure differential is what causes them to come out of solution, you can still technically saturate at 5m. You want the longest surface interval possible for a reason. Get on pure O2 if you have to do that regularly and that will flush the rest of it out.
Personally though 800m isn't all that high. Equivalent of 0.91 ata so it's barely 1m if water depth.
Below is the US Navy altitude table. The letter groups refer to a level of tissue saturation. If you go down to page 497 you'll see repetitive group designations. You did not report what the dive profiles were but for example, your buddy who can go straight to 800m after diving to 30m for 25mins. You can work the tables yourself, but even after repetitive dives you're going to likely be fine to go straight to 800m

https://www.navsea.navy.mil/Portals/103/Documents/SUPSALV/Diving/US%20DIVING%20MANUAL_REV7.pdf?ver=2017-01-11-102354-393#page=492&zoom=100,70,90

 

[ScubaBadger32]

Im no brilliant guy who knows everything, but why would more pressure give you less pressure?????? that makes no sense.

 

[Lorenzoid]

I was diving with someone the other day who had to drive to an altitude of about 800m afterwards. In order to reduce his risk he decided to extend his safety stop on the final dive from 3 minutes to 7.

That part makes sense.

He reckoned that you offgas faster under slight pressure.

That doesn't make sense. It's possible that he is simply mistaken, as others have pointed out. I suppose it's also possible you misunderstood what he said.

 

[tursiops]

Any articles or science papers that have tested this?

I read something about this. I think it was in the same magazine that had an article proving the Earth is flat.

 

[Wibble]

I was diving with someone the other day who had to drive to an altitude of about 800m afterwards. In order to reduce his risk he decided to extend his safety stop on the final dive from 3 minutes to 7. This was at 5m.

He reckoned that you offgas faster under slight pressure. Something to do with increased circulation and having a greater number of small bubbles instead of a small number of large bubbles.

Does anyone agree with this? Any articles or science papers that have tested this?

What was he breathing? The way you've written this infers it's a recreational/NDL dive on air or basic nitrox. If that's the case extending the stop will have very little or no effect.

However, if breathing pure oxygen 100% it will have an effect in that it's a high pressure of Oxygen (100% at 1.5 Atmospheres -- ATA or BAR) which will "displace" the nitrogen absorbed in the diver during the dive. i.e. the body will absorb the oxygen and will eventually metabolise it, but there's a big pressure differential in the nitrogen the other way, e.g. 0% in the inhaled gas, so the nitrogen will be exhaled. This is how Accelerated Decompression works.

Thus two people doing exactly the same dive and gas profiles; but one doing a 3min deco stop then ascent and the other doing a 7 min deco stop and ascent; the one doing the longer stop should have less nitrogen loading if measured at the same time. All of this is conjecture as an additional 4 mins makes little difference if diving to NDLs -- technical divers may spend an hour or more at 6m decompressing on oxygen rich gasses.


Has a feel of a confused partial story.

 

[Lorenzoid]

The way you've written this infers it's a recreational/NDL dive on air or basic nitrox. If that's the case extending the stop will have very little or no effect.

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.