Safety stop immediately after surfacing

[MXGratefulDiver]

You've got to ask yourself one question: Do I feel lucky? Well, do ya?

The real point is ... there is no "correct" answer. It depends on a lot of things ... many of which will be unique to the individual and the dive profile they just did.

What are NDL's anyway? They're not some line in the sand which, on the one side you're safe and on the other you'll get bent. They're a look at all the factors that go into decompression ... including risk factors applicable to some individuals and less so to others ... and a "best guess" at what amount of risk you'll find acceptable.

My response to the OP would be "it depends on how fast you came up". Every time you dive, your body absorbs more nitrogen than it was designed for. Because the air you're breathing is pressurized, and because of the water pressure you're exposing yourself to, your body soaks up nitrogen like a sponge and suspends it in your blood and other body tissues. When you ascend, decreasing pressure in both the air you breathe and the ambient pressure of the water around you causes all that excess nitrogen to be released. If you come up slowly enough, your lungs can easily deal with the excess nitrogen that's coming out of solution from your blood and tissues and will expel it when you exhale. If you come up too quickly, nitrogen comes out of solution faster than your lungs can deal with it ... the nitrogen forms bubbles that expand with decreasing pressure ... and the chances of DCS will increase accordingly.

So if you came up at a sufficiently slow rate, then no ... I wouldn't go back down. Your normal breathing already took care of the excess nitrogen before it could become a problem. But if you came up quickly, then yes ... I would ... assuming I had sufficient air reserves to do it without adding another complication.

The question is ... how will you know what "safe enough" is? The answer is ... you won't. There's too many potential variables. That's why even the experts can't predict with any accuracy how much decompression is "safe". That's why there are so many conservative factors added into dive computers and dive tables.

So it gets back to the Harry Callahan quote ... Do I feel lucky?

Well, do ya ...

... Bob (Grateful Diver)

 

[captain]

Bubble pumping, a reason not to go back down.

http://www.scubaboard.com/forums/basic-scuba-discussions/308532-blow-safety-stop-redescend.html

 

[MauiScubaSteve]

Captain gives a good thread link as food for thought, but here is more to consider;

How many OW Instructors with 4 students on OW dive 2 or 3 will do 2 CESA's near the beginning of a dive and 2 CESA's at the end of the dive, and then what does the Instructor and last student do after that? Anybody heard of issues for just the student?

Also, it is likely that Suunto's penalize 2nd dive times for both fast ascents and missed non mandatory safety stops, so if you are only getting a 45-50 minute SI before a 60' max depth up to 60 minute second charter boat dive, the minimal risk of bubble pumping vs the joy of the second dive may tip in favor of going down and letting the Suunto clear.

 

[DCBC]

Jarek,

If a safety stop (or a decompression stop) is missed, it's usually required that it be completed. As a commercial diver I have often passed decompression stops to go to the surface, get into a chamber and be recompressed (similar scenario as you've described). The only variables are a shortage of in-water gas supply or injury, other than that the stop must be completed.

I'm sure that the usefulness of safety stops could be discussed, but if they are in the dive plan, they should be adhered to. Too often divers make a plan, include a safety stop, miss it and figure oh well... Dive your plan. I've found that anything that's important enough to include before the dive, should be considered to be just as important afterwords.

 

[idocsteve]

I'm just trying to reassure the OPthat if a safety stop is missed and the NDL isn't violated then survivability is possible. IMO safety stops when diving well within the NDL is like safety switches on lawnmowers.

"It's possible to survive a dive where a safety stop was not done".

Nice.

As if that statement wasn't bizarre enough, he follows up with "Safety stops while scuba diving are the equivalent of a safety switch of a lawnmower".

Whatever AfterDark is smoking, I don't want any, thanks.

 

[TSandM]

NW Grateful Diver has, as usual, written a lovely post analyzing the original question. I have little to add; safety stops are a method of slowing ascents and are not mandatory decompression stops. Especially if the preceding dive did not push no-deco limits, there is probably no benefit to go back down and try to complete the stop.

However, there is a GREAT benefit in learning how NOT to blow your safety stop! I speak as someone who had enormous trouble learning buoyancy control in the shallows, and I blew many stops before I could reliably hover at 15 feet for three minutes. But it isn't just whether you can hold the stop -- if you can't, you probably aren't controlling your ascent rate in the shallows very well, either, and that's far more important.

It is WELL worth the time to do some buoyancy work in shallow water. Try starting from 30 feet and going up ten feet and stopping for a minute, and then 10 feet more and stopping for a minute, and then drop ten feet and sit for a minute. If you do this in very shallow water, and don't do too many ups and downs, you won't hurt yourself, and you will slowly learn how to control your buoyancy in that difficult last 30 feet. (Don't do what I did, and do a night dive to practice this and do 7 or 8 poorly managed ascents -- you'll fall asleep at the wheel on the way home!)

Good buoyancy control is within anyone's capabilities, because it was, eventually, within mine. You just have to work at it.

 

[Garrobo]

Just do the three minues at 15-20 feet and your problems are solved.

 

[Bubbletrubble]

Something else to keep in mind is this: just because your dive computer "says" (or perhaps implies) that going back down is OK/correct/safer...doesn't necessarily make it the best possible course of action. Contemplating this type of scenario is a wonderful springboard for learning more about decompression theory, the algorithms used in various dive computers, the value of controlling ascent speed, and the benefit of including stops at various depths.

Many last-generation (and some current) dive computers do not recognize/allow for "deep stops." Does this mean you shouldn't do them? Do some research and find out for yourself.

There's still so much we don't understand regarding deco theory. Review some of the science that's been done on the subject in the past few decades. The online literature archive of the Rubicon Foundation is a great place to start.

 

[fnfalman]

Let me throw in another question.

Safety stops (not mandatory deco stops) are done at anywhere around 15-ft or less. So what's the difference between holding a safety stop at 5-ft or 3-ft or 2-ft? If you happen to bob up to the surface, just exhale and sink back down a few feet then hang out for three minutes. Right?

 

[Thalassamania]

Think about it this way:

Decompression stops are required and if they are missed they need to be made up somehow ... and it needs to be done rather quickly. Conventional wisdom would be to follow ommitted decompression procedures within 5 minutes if non-symtomatic and to evacuate to a chamber breathing pure oxygen, if symptomatic.

Safety stops, on the other hand, are (by defintion) not required and may be ingnored with impunity. They should not be made up, in fact returning to the water to make them up puts you in danger (however slight) from bubble pumping due to the short nature of the safety stop.