How Much Reliance on Dive Shop Planning?

[dmaziuk]

At 35 feet, you are a hair beyond 2 ATA, and according to Haldane's calculations, that would indeed be pretty much a no limit dive. Those calculations have since been made more conservative, but if you went to a 30 feet dive, you would indeed be pretty close to a no limit dive.

I think they revised the original 10 metres to 7.8 as in 78% N2 in the air. And the safety stop depth is half that.

From watching my computer (cressi leonardo), if I'm coming straight(-ish) up it'll show the SS icon as I get close to 5 metres and start counting down the 3 minutes. However, if I'm coming up along the bottom and spend enough time above ~8 metres, e.g. shore dives on Bonaire, SS icon never comes up at all. And it seems to be crediting those "extended safety stops" towards longer NDLs on subsequent dives, although that's hard to tell when you're doing shallow dives anyway... But with more rectangular profiles it does seem to be showing NDLs (as opposed to "99" for "no limit yet") more often.

 

[JohnN]

This is all VERY confusing and VERY hard to understand.

Sorry, what part is confusing? The greater differential in N2 partial pressure (between your tissues and gas in your lungs) , the faster you'll outgas. That's just physics. .

 

[halocline]

"One thing that's useful is that studies have shown extended shallow stops to be very effective in reducing N2 load in recreational diving. "
Can you share some URLs?

It's in a book, the PADI encyclopedia of recreational diving. I don't know if there's an online version.

 

[halocline]

The best way to reduce the N2 load is to get the highest possible gradient between the N2 levels in the body and the N2 levels in the air, provided that enough N2 has already left the body to make that pressure safe.

Sure, you're going to off-gas quicker on the surface than at 15 feet. But the point is in situations where a recreational dive might push the limits of NDL, an extended stop provides some additional off-gassing prior to surfacing. It has definitely reduced post dive fatigue for me in fairly aggressive and/or repetitive recreational dives.

 

[TrimixToo]

Get the computer. They're cheap enough now that there's no reason NOT to have one.

What she said. Use the eRDP for pre-dive planning and then dive the computer.

John

 

[boulderjohn]

Sorry, what part is confusing? The greater differential in N2 partial pressure (between your tissues and gas in your lungs) , the faster you'll outgas. That's just physics. .

I was just talking about deco theory in general. Having taught it to DM candidates and tech divers, I have seen their eyes glaze over.

 

[boulderjohn]

Sure, you're going to off-gas quicker on the surface than at 15 feet. But the point is in situations where a recreational dive might push the limits of NDL, an extended stop provides some additional off-gassing prior to surfacing. It has definitely reduced post dive fatigue for me in fairly aggressive and/or repetitive recreational dives.

That is very much inline with what I said. I will extend my decompression stop/safety stop for that reason if I feel it MIGHT BE to my advantage to do so.

 

[MichaelMc]

Ok, a question on the current tangent, having read well but not taken deco procedures. Say I've done a dive on air and accumulated some tissue N2 loading. I come up to 15' to off gas some safely before surfacing, as safety stop or deco stop, without violating my M values. My tissues are a safe amount above my breathing gasses ppN2, the only gas around, so I am safely off gassing. Still on air. Now I start breathing 100% O2. My tissues are now way above my gasses pp N2, and to some extent far above my lungs ppN2. Why is this not suddenly very bad?? As in exceeding my safe M values? Because the gas that matters is the gas in the micro bubbles? which is still N2 rich? And if we are bubbling directly into the lungs, that is actually what we want?

 

[boulderjohn]

Ok, a question on the current tangent, having read well but not taken deco procedures. Say I've done a dive on air and accumulated some tissue N2 loading. I come up to 15' to off gas some safely before surfacing, as safety stop or deco stop, without violating my M values. My tissues are a safe amount above my breathing gasses ppN2, the only gas around, so I am safely off gassing. Still on air. Now I start breathing 100% O2. My tissues are now way above my gasses pp N2, and to some extent far above my lungs ppN2. Why is this not suddenly very bad?? As in exceeding my safe M values? Because the gas that matters is the gas in the micro bubbles? which is still N2 rich? And if we are bubbling directly into the lungs, that is actually what we want?

The gradient you are worried about is the gradient between tissue pressure and ambient pressure, not the tissue pressure and the contents of the breathing gas.

 

[MichaelMc]

The gradient you are worried about is the gradient between tissue pressure and ambient pressure, not the tissue pressure and the contents of the breathing gas

Thanks. Duh. I guess I see that. I kept thinking of Henry's law which talks about another gas being around.