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

[Cacia]

(this was also the only person not diving nitrox for the week... he didn't see any reason for that either.)

Maybe he did not think there was an appreciatable difference in the risk?

I use it when it is free, or I need to time to accomplish something at a certain depth, not to be safer as a strategy, really. I understand the theoretical advantage but it usually is not worth the money to me. And having to pick up the tanks to analyze them myself always causes moral /sleep/ethics dilemmas. On occasion, I have been fine with somebody's word on a mix...and that upsets people, so I just prefer air, usually. (if I trusted that guy that just flew me in here in that belching thing, it doesn't seem like too big of a leap)

Sometimes I do a sloping deco if I am taking pictures, and have a reason. Being aware of your profile is really what matters. No doubt it is easier to guide a group by having some organized approach, but if some old school diver does his own thing and looks okay, I am fine with that. (he thinks you are nuts for no snorkel) Some operators will require the stop or you miss the second dive. I have a friend that will not let tourists go on the second if they have less than 500 psi left on the boat. He is limiting his exposure and tells them up front, that is the deal.

 

[jeter]

If anybody doubts the value of a saftey stop have them open three coke bottles. Open one very quickly and watch the a ton of bubbles form.

Open the second one very slowly. Alot less bubbles form.

On the third, crack the top letting a little preasure escape. Wait a few minuites, then open it completely. Almost no bubbles form.

CO2 in coke can't be much different than the nitorgen in your blood. Even if we are talking micro bubbles.

A slow assent can't quite do the same job as a good solid saftey stop.

Jay

 

[Cacia]

A slow assent can't quite do the same job as a good solid saftey stop.

I don't agree with that. Has that been shown to be a statistically significant difference?
lol, because if it has then I am wrong and should put my seat belt on before starting the car.

unless you are talking .0000000999999, and then I just don't care enough to always follow the rule. (I usually do)

The problem with too many rules is that too many divers can fake understanding with themselves.

 

[ClayJar]

A slow assent can't quite do the same job as a good solid saftey stop.

I don't agree with that.

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.

 

[all4scuba05]

If anybody doubts the value of a saftey stop have them open three coke bottles. Open one very quickly and watch the a ton of bubbles form.

Open the second one very slowly. Alot less bubbles form.

On the third, crack the top letting a little preasure escape. Wait a few minuites, then open it completely. Almost no bubbles form.

CO2 in coke can't be much different than the nitorgen in your blood. Even if we are talking micro bubbles.

A slow assent can't quite do the same job as a good solid saftey stop.

Jay

So deep stops are not the best thing to do? Just do them in the shallows?

 

[Cacia]

hmm, thanks.

Well, does this matter in a significant way?

And, I do half depth stops but wonder if this diver on your table is descending too slowly before *that* depth.

I don't seee how you can make this rule, not knowing the depth from where the ascent began. You have to know where you will begin offgassing and not crowd the various tissue compartments, no?

 

[MXGratefulDiver]

If anybody doubts the value of a saftey stop have them open three coke bottles. Open one very quickly and watch the a ton of bubbles form.

Open the second one very slowly. Alot less bubbles form.

On the third, crack the top letting a little preasure escape. Wait a few minuites, then open it completely. Almost no bubbles form.

CO2 in coke can't be much different than the nitorgen in your blood. Even if we are talking micro bubbles.

A slow assent can't quite do the same job as a good solid saftey stop.

Jay

Actually, a slow ascent can do quite the same job ... or even a better one ... than a good solid safety stop. It all depends on how quickly you ascend to safety stop depth, and how quickly you ascend to the surface afterward.

The body's tissues are a lot more complex than a soda pop ... remember that your body comprises literally dozens of different tissues, each with its own offgassing rate. And you are creating a continuous pressure gradient as you ascend, as opposed to a discrete gradient when you pop the top off that soda pop bottle.

Using the soda pop as an example is useful for the novice diver ... but it's a model that quickly loses its similarities when you look at the issue more closely.

The "ideal" offgassing model would be one where you maintain a constant ... and suitably low ... offgassing rate throughout the ascent. This would require you to go progressively slower as you got closer to the surface. Since that's extremely difficult for most recreational divers to do, we use the "30 feet per minute and 3 minutes safety stop" rule of thumb. It's, at best, a simple and achievable method ... but by no means the best.

... Bob (Grateful Diver)

 

[ClayJar]

So deep stops are not the best thing to do? Just do them in the shallows?

For recreational "no-deco" divers, deep stops are a very good thing to do, at least according to that 2004 paper and other sources I've read.

Tech divers can (and I'm sure, do) run various deco scenarios to determine what procedures will be best suited to the particular dive plan they are to follow. Deep stops may actually be detrimental, considering their specific constraints. (For example, perhaps a deep stop would increase the loading in one of the medium or slow tissues that would not be an issue to recreational "no-deco" divers.)

The "ideal" offgassing model would be one where you maintain a [-]constant ... and[/-] suitably low ... offgassing rate throughout the ascent. This would generally require you to go progressively slower as you got closer to the surface.

(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

 

[Blackwood]

Deep stops may actually be detrimental, considering their specific constraints. (For example, perhaps a deep stop would increase the loading in one of the medium or slow tissues that would not be an issue to recreational "no-deco" divers.)

Loading the slow tissues for a minute or so is a small price to pay to unload fast tissues.

1 minute to a 5-minute compartment means a whole heck of a lot more than 1 minute to a 120-minute compartment. [/arbitrary example]

The "10ft-to-surface" is probably the most critical point.

I'm not sure I agree with that. In most cases I suspect that if you take a hit on a dive during which you did a 'safety' stop, it's not resulting from the last 1.3ATA of gradient. Rather, I suspect you already had bubbles too big to exhale AT your safety stop.

So deep stops are not the best thing to do? Just do them in the shallows?

As long as they are sufficiently shallow (relative to the bottom) to allow significant offgassing of the fast tissues, they are good. Starting deep stops at 195 after a dive to 200 would be rather inane. Starting deep stops at 160-150 after a dive to 200 makes sense.

The goal of a deep stop is to give the fast (type-II) tissues a chance to unload. If you bend them on the way up there is a good chance you won't be able to fix them without recompression.


And really, I agree with the sentiments that a super-slow ascent and a staged ascent is a six-to-one-half-dozen-to-the-other sort of a proposition. A minute (say 30 up, 30 stop) per 10 feet is a whole lot easier than constant 10FPM. But the overall shape of the curve will be the same.

 

[robint]

Maybe he did not think there was an appreciatable difference in the risk?

I use it when it is free, or I need to time to accomplish something at a certain depth, not to be safer as a strategy, really. I understand the theoretical advantage but it usually is not worth the money to me. And having to pick up the tanks to analyze them myself always causes moral /sleep/ethics dilemmas. On occasion, I have been fine with somebody's word on a mix...and that upsets people, so I just prefer air, usually. (if I trusted that guy that just flew me in here in that belching thing, it doesn't seem like too big of a leap)

Not what I was talking about... we were on a liveaboard in Palau doing 4-5 dives per day. The DMs fill the tanks but we stand there with them and analyze them, you don't take anyone's word for anything. No picking up tanks, they are there on the boat waiting for us.
He probably didn't do the nitrox because it cost $$$ and he was a cheap dude who whinned and complained about the dives being too deep and too difficult, too. My husband and I watched him jet from 60' deep or more right to the surface over and over. I have no clue why he didn't get bent or blow a lung. He was just plain unsafe. Several people on the boat tried to talk to him about it, discuss the "safety stop" issue..... he didn't want to hear it because he had been diving longer than we had - problem is he only does 10-15 dives per year which doesn't make him a good diver. He has learned alot of bad habits. Nitrox would have benefitted him especially because he was unfit, over 60, and doing dives that were over his skill level. Sad. I doubt he will ever go to the Pacific again, he loves Bonaire and it is much more his speed. His not using the nitrox didn't bother me half as much as the popping to the surface at the end of every dive. Sheesh!


Just my 2 cents.

robin