Why 15 ft for a safety stop?

[String]

A few things from above posts.

Firstly, there was an incident here last year where someone got bent in a swimming pool. 2m. The year before an instructor got bent in 4m of water. Both of these were training dives and shallow for nearly all of it (as opposed to the end of a deco dive) so although rare bends are not impossible even in very shallow water.

As for the safety stop - it was introduced in part to get people to actually stop, get a chance to control their buoyancy and ensure a slow ascent in that critical last 10m or so of water. Without trying to hit that stop people did end up rocketing/losing it. It provides a braking mechanism of sorts.

Plus all the reports ive read have detected far few so called "silent bubbles" on doppler from divers doing even short safety stops vs none at all and a direct ascent.

On a standard no stop dive my stop depth depends purely on the dive with no real science. My tables say 6m. My computer is happy anywhere from 3m to 6m so in reality i usually aim for 5.5m under the bag.
Unless im abroad on a reef and then its usually "whatever depth that nice ledge or bit of ground to look at is at".


However, common sense should prevail - i dived with a girl a few years ago who failed on that count. Dive max depth 12m BUT the last 15 mins of it were in 4m of water. The "6m stop" was so ingrained to her she tried to signal to me to DESCEND to 6m to complete the stop before surfacing totally missing the point that we'd been shallower for some 15 minutes already.

 

[rawls]

I have a 1967, got it new as a present, a little worn now.

I knew it...Well...There is your retirement income

Then there's Wienke

He speaks of the problem of both dissolved gas (Haldane) and free phase bubbles (bubble decompression modeling)

Haldane brings the diver as close as possible to the surface for elimination of dissolved phase, whereas in bubble decompression models the diver is maintained at depth "to both crush bubbles and squeeze gas out by diffusion across the bubble film surface."
This leads to competion between transfer pathways that exist between free phase and dissolved phase gas for elimination. It would be simple to think, do a deep stop, slow ascent and do a 15 minute stop, which each arguably has its advantages, but still there are the problems of tissue tensions and ambient pressure having wide variations, as well as ascent rate factors that all have an effect on bubble growth and are not close to being easily quantifiable.
The recommendation for a 2-4 minute safety stop between 10-20 fsw has several benefits. They restrict bubble growth and, in addition, at this depth range the slower compartments...which control shallower dives...are allowed insignificant buildup of dissolved gas. In other words...The benefit of reduced bubble growth far outweighs any insignificant buildup in the slow compartments. The fast compartments are eliminating gas at fast rates at this depth. And, as has been already stated, the 15 ft stop is easier than the 10 ft stop which has considerations of surge and other other surface conditions. Finally..."Slower ascent rates afford additional advantages but safety stops in the 2-4 minute range are easier and more efficient." Thus the main basis for the greater importance of the safety stop over other methods

 

[Thalassamania]

A few things from above posts.

Firstly, there was an incident here last year where someone got bent in a swimming pool. 2m. The year before an instructor got bent in 4m of water. Both of these were training dives and shallow for nearly all of it (as opposed to the end of a deco dive) so although rare bends are not impossible even in very shallow water.

I believe the incidents that you reference were determined to be other than DCS.

As for the safety stop - it was introduced in part to get people to actually stop, get a chance to control their buoyancy and ensure a slow ascent in that critical last 10m or so of water. Without trying to hit that stop people did end up rocketing/losing it. It provides a braking mechanism of sorts.

That was a well recognized side effect. In reality safety stops began with the observation that three minutes at ten feet (now five at ten) would cover you for an error of one step too deep or one step too long. This was observed by Harry Averill who developed the circular format tables for NAUI that are now used by NOAA and others. The "Three at Ten" guideline became one of those things that was spread at the ITCs and had other "advantages" slapped on it such as slowing incompetents down and providing for additional safety offgassing. In addition, in keeping with Navy instructions of the times those non-required stops needed to be considered as part of you bottom time, which lead to the NAUI tables that cut back the Navy No-D limits by five minutes.

Plus all the reports ive read have detected far few so called "silent bubbles" on doppler from divers doing even short safety stops vs none at all and a direct ascent.

That is perhaps the best reason to continue the practice, but had no role in the practice's origin.

On a standard no stop dive my stop depth depends purely on the dive with no real science. My tables say 6m. My computer is happy anywhere from 3m to 6m so in reality i usually aim for 5.5m under the bag.
Unless im abroad on a reef and then its usually "whatever depth that nice ledge or bit of ground to look at is at".

However, common sense should prevail - i dived with a girl a few years ago who failed on that count. Dive max depth 12m BUT the last 15 mins of it were in 4m of water. The "6m stop" was so ingrained to her she tried to signal to me to DESCEND to 6m to complete the stop before surfacing totally missing the point that we'd been shallower for some 15 minutes already.

The problem with common sense is that it is so damned uncommon.

 

[String]

I believe the incidents that you reference were determined to be other than DCS.

The reverse - both the incidents were found to be DCS by the DDRC. Both happened in the UK so the incident report etc is publicly available.

 

[MikeFerrara]

SEE BELOW - "Excerpt from a DAN Divers Alert Network Article"

DAN News
Deep Stops: Can Adding Half the Depth of A Safety Stop Build in Another Safety Margin?
Last Updated: 12/28/2004 10:45:48 AM

By Peter B. Bennett, Ph.D., D.Sc., Alessandro Marroni, M.D., Frans J. Cronjé, M.D., International DAN

Although decompression tables have been significantly modified over the last 20 years, with many now giving much shorter times at depth than the original U.S. Navy tables, the incidence of decompression illness (DCI) has changed very little. Even the recent introduction of dive computers has not made a significant impact on dive injuries.

Decompression illness incidence remains consistent with the distribution for sex, age and training among divers, regardless of the computers or tables they use. The problem, as previously elaborated in Alert Diver1 appears to be a too-short time of ascent; this is the only parameter that has changed very little over the last 40 years and, accordingly, appears to possibly be the real controller of the incidence of DCI.
Ascent Rates: A Quick History
Historical guidelines as to rates of ascent are pertinent. In the 19th century, for example, the French physiologist Paul Bert in 1878 quoted rates of 3 feet per minute and the English physiologist John Scott Haldane in 1907 recommended ascent rates between 5 and 30 feet (1.5 and 9 meters) per minute. From 1920-1957, rates of 25 feet (7.5 meters) per minute were recommended.

Then in 1958, during the production of the U.S. Navy Diving Manual, the rate of ascent to be proposed came under review. Cdr. Francis Douglas Fane of the U.S. Navy West Coast Underwater Demolition Team wanted rates for his frogmen of 100 feet (30 meters) per minute or faster. The hardhat divers, on the other hand, considered this impractical for the heavily suited divers who were used to coming up a line at 10 feet (3 meters) per minute. Thus, a compromise was reached at 60 feet (18 meters) per minute, which was also a convenient 1 foot per second.

So from 1957 until 1993 the U.S. Navy tables have consistently advocated an ascent rate of 60 feet per minute, based on this purely empirical decision, with many recreational diving tables and even early computers following suit. In recent years this has been slowed to 30 feet per minute with a recommended safety stop for three to five minutes at 15-20 feet (4.5-6 meters).

NOTE: The most important things from this excerpt
1. The incidence of decompression has changed little (this would indicate that ascent rates and safety stops have little to do with your risk of DCI) in the last 20 years. Other factors (physical such as obesity, genetic issues etc..) are likely the more problematic factor. Yes, ppeople can get DCS in 10, 20 even 30 feet of water... but this just means they were pre-disposed to it regardless of depth, ascent rate and safety stops.
2. The conclusion of a "recommended" safety stop surely bears what I said... and that is that it is recommended - not required.

Happy Diving.

Well don't forget this piece of the excerpt

The problem, as previously elaborated in Alert Diver1 appears to be a too-short time of ascent; this is the only parameter that has changed very little over the last 40 years and, accordingly, appears to possibly be the real controller of the incidence of DCI.

I'm not sure I agree with your conclusions regarding the effect of ascent rates. One of the most widely used tables (DSAT) is still based on a 60 fpm ascent though many recommend a slower rate. In theory, anything within the limits of the model being used "should" be pretty "safe" statistically. However, what is recommended is not what divers necessarily do. Are divers really ascending slower and getting bent as often? I think that the effect of ascent rate RECOMMENDATIONS on DCS rates gets lost in the noise especially since so many divers still aren't very good at controling their ascent rate.

It might be dangerous (or at least misleading) to draw conclusions based on DCS rates without considering the actual behavior and conditions of the specific divers who are getting the DCS.

Personally I'm less concerned with the DCS rate within the population than I am with whether or not I get bent.

 

[offthewall1]

Well don't forget this piece of the excerpt


I'm not sure I agree with your conclusions regarding the effect of ascent rates. One of the most widely used tables (DSAT) is still based on a 60 fpm ascent though many recommend a slower rate. In theory, anything within the limits of the model being used "should" be pretty "safe" statistically. However, what is recommended is not what divers necessarily do. Are divers really ascending slower and getting bent as often? I think that the effect of ascent rate RECOMMENDATIONS on DCS rates gets lost in the noise especially since so many divers still aren't very good at controling their ascent rate.

It might be dangerous (or at least misleading) to draw conclusions based on DCS rates without considering the actual behavior and conditions of the specific divers who are getting the DCS.

Personally I'm less concerned with the DCS rate within the population than I am with whether or not I get bent.

Any research can be confusing as can any conclusion... because even though "scientific," all research has so many variables the conclusions made are just that - conclusions. They do not and can not possibly apply to all who dive. In medicine, clinical trials studies (often double blind) are done in an effort to get as true and accurate an account of the impact of a drug as possible. In diving, most research is at best theoretical. The impact of increased atmospheric pressure, while at extremes can be determined to effect virtually any human, it's impact under routine (non-extreme) conditions is at best unpredictable.

In other words, two people can dive the exact same profile. One will get bent, one will not. For that matter, the same person can dive the same exact profile one week and be fine... and the next week get bent. The contributing factors are ever changing in the human body.

I did not mean to imply that a safety stop was never needed. There are many situations in which one is called for and is required. This applies in all extended range diving. I stated that research has made nothing more than a "recommendation" that a safety stop be made for divers diving within recreational dive limits... and this is factual.

Different than yourself, I do concern myself with the safety of other divers and I take that very seriously. Because of this, I know that the likelihood of a DCS hit for someone who dives within recreational limits and follows their computer or tables has virtually zero chance of a DCS hit if they ascend at the correct rate of 30 ft per minute, with or without a 3-5 minute safety stop.

If someone does get DCS without doing a 3-5 minute safety stop, it is likely they would have gotten DCS even with one on that particular dive.

Happy Diving.

 

[dumpsterDiver]

If someone does get DCS without doing a 3-5 minute safety stop, it is likely they would have gotten DCS even with one on that particular dive.

Happy Diving.

Wow I didn't know that.

 

[Thalassamania]

The reverse - both the incidents were found to be DCS by the DDRC. Both happened in the UK so the incident report etc is publicly available.

I'd be interested in a web accessible reference, the diving docs over here whom I talked with about those cases basically said, "Sull Bhit."

 

[scubafool]

If someone does get DCS without doing a 3-5 minute safety stop, it is likely they would have gotten DCS even with one on that particular dive.

That is a most interesting statement.

 

[MikeFerrara]

Different than yourself, I do concern myself with the safety of other divers and I take that very seriously. Because of this, I know that the likelihood of a DCS hit for someone who dives within recreational limits and follows their computer or tables has virtually zero chance of a DCS hit if they ascend at the correct rate of 30 ft per minute, with or without a 3-5 minute safety stop.

There have been lots of cases of divers being bent while within their tables or their computers. Table and computer models vary quite a bit and so do people. I'm not sure what the "chances" really are but they are definately non-zero. Combine that with multiple dives/day and multi day trip and it gets even more "fuzzy" which is why it's recommended to avoid table/computer limits...dive conservatively. All a safty stop does is add some conservatism by adding a bit of in-water decompression.

If someone does get DCS without doing a 3-5 minute safety stop, it is likely they would have gotten DCS even with one on that particular dive.

Happy Diving.

I think we would have to look at individual profiles in order to say that.