Safety stop at 15'..........always

[captain]

Although there are reports out since this one (including some that show that there is no *universal* "best" answer -- best practices for "no-deco" recreational dives may be bad ideas for tech deco dives), one bit of decent reading from 2004 is Deep Stops: Can Adding Half the Depth of A Safety Stop Build in Another Safety Margin (or the actual paper referenced in that article).

Grabbing a couple lines from a table to whet any curious appetites:

Ascent Rate|Stops|5 min Tiss. Sat.%|10 min Tiss. Sat.%|Bubble Score BSI|Total Time to Surface (mins)
3 m/min|No Stop|48|75|8.79| 8
10 m/min|6 m / 5 min|43|65|5.23| 7.5

Note that the slow ascent had greater total time to the surface, but it ended with higher saturations in the fast tissues and a higher bubble score. The paper is quite interesting.

It make sence that a slower ascent may result in continueing to on gas in the fast tissures where as getting to the safety stop faster slows down the rate of on gassing by the slow tissues and also results in a higher differential partial pressure by getting into a lower pressure area faster which would increase off gassing in the fast tissues.
I'm no deco expert but I did sleep at a Holiday Inn last night.

 

[MXGratefulDiver]

(Red edit by me, original by Bob.) Correct me if I'm mistaken, but the ideal offgassing model would not necessarily result in a constant rate of offgassing, as the limiting tissue may vary through the ascent. The concept is indeed correct, of course, but I figured you would allow a bit of pedantry regarding "ideal" models.

(I added the "generally", as I can't do the calculus right now to figure out whether the ascent rate is *necessarily* monotonically decreasing in all cases. :biggrin

OK ... I'll buy that. I was trying to speak in broad terms, but I like your choice of words better ...

... Bob (Grateful Diver)

 

[JiveMaster]

Always..always..always...AND never go straight up from your safety stop! Try to take ~ 3 min from your stop to the surface...

 

[AlaskaDiver]

I try to do 3 min stop on all dives deeper than 33'. I'll also do stops at between 40-66' based on how deep I go - recreationally speaking. A slow ascent rate is the key.

 

[Buoyant1]

Yeah, the guys I dive with all do 15 for 3 minutes...we really got into that habit last year, and even though the quarry we dive in has a good slow slope to swim up we always make it a point to do the stop.

When I was in Marathon this past November, we did a 1-2 minute stop at half our depth on deeper dives, then our 15 ft stop. Nothing to do with being into deco, just a precaution...my computer counts it down also.

I look at it this way, if that guy the OP was talking about didn't want to do them, it's his micro bubbles not yours!

 

[robint]

Yeah, the guys I dive with all do 15 for 3 minutes...we really got into that habit last year, and even though the quarry we dive in has a good slow slope to swim up we always make it a point to do the stop.

When I was in Marathon this past November, we did a 1-2 minute stop at half our depth on deeper dives, then our 15 ft stop. Nothing to do with being into deco, just a precaution...my computer counts it down also.

I look at it this way, if that guy the OP was talking about didn't want to do them, it's his micro bubbles not yours!

Yes, it was his bubbles not mine, BUT being on a liveaboard (something he says he'll never do again) it affects everyone. If we had been somewhere else with the boat far from land, the entire boat may have had to leave in order to evac him. That would have been my $$$ being wasted and I would have been really PO'ed.

For those who asked was he Nitrox certified, he said he was.
But 3 other divers showed up on the boat who weren't, they did the class the first 2 days with one of the instructors onboard and dove nitrox after day 1 week. They were all VERY happy they did and amazed at the difference that their computer read at the end of the dives. (They all had to do deco time on first day on air, never after that). All 3 of them were experienced divers who had been diving awhile and were very good on air. One guy was so good he was last back on the boat every dive, he even outlasted the dive guides a few times.... he was very very pleased with the nitrox! All our surface intervals were about 2 hours (nice since dives tended to be so deep).

As far as people diving Nitrox without getting the C-card ... I have been a few places where they offered it as long as the person had a computer that did nitrox and gave them a briefing of half hour or less.

I am glad to see such a lively discussion about this subject. It is one of my pet peeves.

robin

 

[mrlipis]

I make the cutoff at 30'. Shallower no, deeper yes.

What?.. NDL is based on depth and time. 60 minutes at 30' and I'm doing a safety stop, heck 15 minutes at 30' and I am doing a safety stop. That's like not wearing your seat belt because your only going a mile or two.

We have a safe sport because we have redundancy; buddies, octos, ponies, dual computers (I wear two), slow ascents and a safety stop I am never in a hurry to get out of the water.

Bruce

 

[robint]

What?.. NDL is based on depth and time. 60 minutes at 30' and I'm doing a safety stop, heck 15 minutes at 30' and I am doing a safety stop. That's like not wearing your seat belt because your only going a mile or two.

We have a safe sport because we have redundancy; buddies, octos, ponies, dual computers (I wear two), slow ascents and a safety stop I am never in a hurry to get out of the water.

Bruce

I usually don't do a safety stop if dive was less than 30' deep. I was taught that at less than 30' you are offgassing as fast as you are taking on the nitrogen.

So there is no need to do the safety stop necessarily on shallow dives...... sometimes I do it because there is so much to see, lots of extra air so no need to get back to surface (Bonaire is one example). There have been lots of dives where I spent over half the dive at less than 30', so as far as I am concerned, I did do a safety stop.

 

[TSandM]

I was taught that at less than 30' you are offgassing as fast as you are taking on the nitrogen.

This really isn't true, if you're talking about a dive you do with a max or average depth of 30 feet. When you descend, you are ongassing. It doesn't matter the depth, you are absorbing nitrogen in excess of what you had at the surface. What IS true is that, at such shallow depths, and within the time constraints of normal tank sizes, you are not going to absorb enough nitrogen to exceed the amount you can surface with, except in the very fastest compartments. A normal (30fpm) ascent from those depths will permit direct surfacing with very little risk.

It IS possible to execute a dive such that, at 30 feet at the end of the dive, you might be offgassing some compartments and still ongassing in the very slowest ones. Those slow compartments are rarely of significance for DCS risk in recreational profiles.

 

[Blackwood]

That's like not wearing your seat belt because your only going a mile or two.

He's applying the theory that a 1.9 atmosphere gradient isn't enough to cause clinical DCS.