Safety stop

[Darian Dunn]

From pg169 Multilevel and Computer diving section of the PADI Adventures in Diving manual, “Safety stop have been tested to a limited degree, and show significant benefit in reducing the probability of decompression sickness.”

Why?

Yep, I have to admit I don’t really understand decompression theory.
:stupid:

 

[Rick Murchison]

The short answer is that a safety stop vastly reduces bubbling in your blood.
Just do it!
Rick

 

[Drew Sailbum]

Anything you don't understand is a good topic for questions.

Decompression theory is, first and foremost :idea: theory. One of the regulars around here is one of the docs who was involved in some of the tests, so he can go into far more detail than I can.

The work of J.S. Haldane was the basis for a model of decompression. His basic idea is that, when you breathe air at higher pressure than atmospheric pressure, some of the nitrogen gas dissolves into your tissues.

When you then decrease the pressure of the gas you are breathing(like when you ascend in scuba), the nitrogen which has dissolved in your tissues starts to be dissolve back into the blood , and is then carried to the lungs where it is exhaled.

How fast the gas dissolves out of the tissue is determined largely by how much the pressure has decreased. The less the pressure decreases, the slower the built-up nitrogen is released. The more the pressure is decreased, the faster the nitrogen is released.

According to this Haldaen model of decompression theory, DCS ("the bends") results when pressure is released "too fast" and your body can't eliminate the nitrogen fast enough. The theory is that if your body can't eliminate the nitrogen fast enough, the nitrogen (and assorted other dissolved gases) start to form little bubbles, which continue to grow in size. In theory, if these bubbles get too big they somehow interfere with nerve signals producing the pain, and other neurological symptoms of DCS. (A certain amount of small bubbles is usually tolerated without incident. These are referred to as silent bubbles.


Making a safety stop at 15ft means that your breathing gas will be a little less than 1.5atm pressure. You are shallow enough than you are eliminating nitrogen at a reasonable rate, but (in theory) not so shallow that the nitrogen will be released "too fast."

Finally, remember that safety stops are done because no dive table is absolute. Occasionally someone who has stayed within the limits of the dive tables and followed all the rules gets a DCS hit. These are what people refer to as undeserved hits. Safety stops should reduce the chance of one of these undeserved hits.

 

[Darian Dunn]

Thanks that is a perfect explanation.
I always knew how to follow the rules, but I never understood the why behind them.

Thanks

 

[Lost Yooper]

What's the single biggest mistake most (a lot of) divers do following a safety stops? They shoot to the surface. You see this a lot. Do youself a favor and make that final ascent take 3mins or so. Don't be in a hurry getting to the surface.



Mike

 

[TexasMike]

Yooper's correct....take the last bit reeeeaaaaalllllll s-l-o-w.

Besides, if you're following the dive profiles of many dive boats where they require you to surface with a minimum of so much PSI in your tank, at 20 feet, you got plenty of gas to hang out at the stop for a long time. Plus plenty of stuff will be swimming by at the shallower depths (including the whale shark I saw on my first Flower Gardens trip.)

 

[TRUETEXAN]

A safety stop is really just a precautionary decompression stop. I wish the different recreational certification agencies was just call it what it is and maybe people would take it more seriously.:mean:

 

[The Chairman]

First off, the reason we REALLY need safety stops is that we are too impatient. Our ascents are way, way too quick for our bodies. SOOOOO, to effectively slow you down (especially just before you make that last ascent) they stop you at 15 ft. Say you were at 66 ft and you came up at 30/min... it would take you only TWO minutes to get to 15 ft. THATS QUICK... now factor in that 3-5 minute safety stop, and your looking at an average of around 11 ft/min or less. Hmnnn... you could even ELIMINATE them thar safety stops IF you could ascend that slowly. Don't bet on it... do your safety stop and then take another two minutes... (5 minutes total). Your head and body will be glad you did!!!

 

[Dr Deco]

Dear readers:

Here is another of the somewhat long posts on a generalized decompression topic. Much of this has been covered before, and some of it bears repeating.

The concept of the safety stop is both derived from practice and theory. In the Haldane model, the safety stop allows the dissolved nitrogen in tissue to be reduced before the final ascent to surface. The change is small in this concept/model, however.

In another view, where micronuclei are present (Harvey, first described two-phase systems about 1943), the dynamics of gas transfer are different. Ideas presented by Drs. Yount and associates and by Weinke describe dissolved gas diffusing out of a tissue. Since diffusion by a gas molecule is random, the molecule can find its way into either a growing gas bubble (where it is essentially trapped) or into the blood capillary where it is spirited away to the lungs.

--- Obviously if there were only capillaries and no microbubbles, the diffusing dissolved nitrogen molecules would have only one pathway available to them.
--- Conversely, if there were millions of micronuclei, the chances are that dissolved nitrogen would end up in one of these microbubbles.

Reality lies in between, and the scuba diver wishes to foster the first process rather than the second.

The way this is done is to make slow ascents to keep microbubbles small in the first place. This keeps in their internal pressure high because of the compression of surface tension (the Laplace pressure). Slow ascents assure that tissue micronuclei remain below the Laplace cutoff. Slow ascents have been shown to reduce the number of Doppler bubbles found on the surface in the diver {Carturan D, et al. Ascent rate and circulating venous bubbles in recreational diving. Int. J. Sports Med. 2000 Oct; 21(7):459 - 462}.

As a demonstration, one can see this when the top is slowly unscrewed from a bottle of carbonated beverage versus rapidly removing it. The first case produces few bubbles in the liquid and the second produces many. The effect is NOT produced by “decompressing” the beverage in the sense that the dissolved gas is released and exits the liquid in one method and not in the other.

In a similar fashion, safety stops allow dissolved gas to move into the capillary system rather than being caught in growing tissue microbubbles. Since everything done by a diver on the way to the surface is decompression, a safety stop is just another form of it.

This is rather straightforward, but it is not necessarily simple to envision.

Dr Deco. :doctor:

 

[MNScuba]

Is decompression sickness something every diver will more than likely experience to some degree? It sounds like there's really no sure way to avoid it, just ways to minimize risk. Like driving a car on ice... you can be as careful as humanly possible, but you still might slide off the road.

Right?