Safety Stop - 3 min or 5 min?

[Basking Ridge Diver]

Per the Naui Air Tables -
Precautionary Decompression Stop.
This is a stop at five meters (15 feet) for three minutes as a safety precaution when you have not exceeded the Maximum Dive Time. You should perform such a stop at the end of every dive. It is also known as a Safety Stop.

Deco for Divers - "Bubbling is controlled by the difference between the tissue tension and the ambient pressure - however off-gassing is controlled by the difference between the tissue tension and the inspired inert gas partial pressure. This difference, which is is known as the inert gas gradient, determines the amount of off-gassing. If this tissue compartment has a 5 minute half-time then the tissue will go 50% of the way from the current tissue tension to the inspired inert gas pressure in 5 mins."

Based on this wouldn't it be better served to recommend a 5 min safety stop for all no stop dives (1 min @ 6 meters and 4 min @ 3 meters) to get the fast compartments to reduce by 50%?
Why is the recommendation 3 mins, does it matter?

 

[TMHeimer]

My guess is the 3 minute recommendation is based on actual testing of divers, their "silent" bubble reduction, etc. due to the safety stop. I'm sure someone will elaborate on the history of it.

 

[diver 85]

Now that I'm over 60(YO), I do 5 min stops........No reason to base that on, I just do it......

 

[scubadada]

Q: 3 or 5 minute safety stop? A: Yes

When I was certified to dive in 1970, there was no SS, we made a direct ascent to the surface and did it at 60 ft/min. I don't know when the safety stop was instituted but would love to hear the history from someone who knows it.

I believe the SS was first recommended at 10 feet, same as the last decompression stop. At some point it became about 15 feet, maybe because it was easier to hold and gave leeway from the surface if there was fluctuation. I have often heard the SS was instituted mainly to make sure that divers with an excessively fast ascent actually slowed down/stopped, before surfacing. The SS would probably make up for the error in ascent. I would not be at all surprised if the 3 minute recommendation was somewhat arbitrary. If 3 minutes is good, 5 minutes is better? (more conservative). I doubt there is any harm a longer SS would do. I am skeptical that the SS duration has anything to due with tissue half lives. There will be different tissues controlling the dive dependent on multiple, multiple variables.

I do a 3 minute safety stop after all my dives and generally add on the same amount of time after one of my short deco dives after clearing the deco obligation. I arbitrarily add on a minute or two sometimes if I really pushed the limits or, more commonly, have something fun to watch or just want to hang around, (pun intended).

I made up most of the above based on my memory and/or beliefs. I would love to hear from those who really know the history and/or the science behind the safety stop

Good diving, Craig

 

[NAM001]

I don't know the history of it but I could make an educated guess. at proper ascent rate no SS was necessary. given that most exceed the ascent rate a stop for a safety factor was instituted as a backup or extra bit of insurance to cover high ascent rates.

Like I said I have nothing to say this is true but people being people it sure sounds reasonable. If you did a 20 fpm ascent you would have no need for one. of course the deeper you are before ascending the more potential you have for a problem. So given the worse situation of a 120 ft dive and a 100 ft / min ascent the safety stop would be a good idea to do. Follow the rules and no stop would be needed.

It will be interested to hear what others think is the reason.

 

[Patoux01]

If I recall correctly, there's a study lying around somewhere with a Doppler test at the end of dives with and without safety stop (same ascent rates) and the ones without safety stop had a higher bubble level. Now we know that bubbles don't necessarily imply DCS, but I think that's one of the reasons to do it.

Also there's those claiming that it's for beginners, which I don't really believe in...

However, I'm the first to skip them when I feel there's definitely no need for it or that it is hazardous to do it.

 

[NAM001]

I would have to definately say that any stop no matter where is benificial. the meat of the matter is wherther it is needed for the dive. The deeper the dive or the longer the dive the more need for a stop. I dont know about the bubble test you are refering to but if no stop resulted in 100 bubbles and a 3 min safety stop resulted in 95 bubbles and a 5 minute stop made 90 bubbles, the difference is there but moot.

As far as the beginners comment goes there could be some substance in that as beginners have much less depth control that more experienced / skilled divers do. With that no one can label one form another. so make a rule to cover the worst scenereo and that works. overkill perhaps but no recommendation is ever made without ballancing the legal aspects.

If I recall correctly, there's a study lying around somewhere with a Doppler test at the end of dives with and without safety stop (same ascent rates) and the ones without safety stop had a higher bubble level. Now we know that bubbles don't necessarily imply DCS, but I think that's one of the reasons to do it.

Also there's those claiming that it's for beginners, which I don't really believe in...

However, I'm the first to skip them when I feel there's definitely no need for it or that it is hazardous to do it.

 

[Phil_C]

OK - I know this has changed over the years, but I have recently started to wonder about safety stops myself - so currently who teaches what?

For no decompression dives -
CMAS - 3 minutes at 5 metres
PADI - 3 minutes at 5 metres?
BSAC - no safety stop per se but ascent rate not to exceed 15 metres/minute then from 6m to the surface to take 1 minute (I need to check this is what they currently say but it is what my slightly older manual says).

And secondly which agencies teach decompression diving at the recreational level? - CMAS and PADI seem not to, but decompression diving is covered by the BSAC 88 (basic) tables and introduced at the entry level open water qualification. - Phil.

 

[scoobydrew]

In the dive centre I work for, we ask people to do 5 minute safety stops just to add conservatism.

After reading Deco for Divers, I try to do at least a minute at 20 feet followed by 3 minutes at 15 feet

 

[Skulmoski]

I had a quick look at the US Navy Diving Manual and could not find the term "Safety Stop." It seems that decompression stops are calculated. Indeed, the no-decompression limit is defined in terms of ascending directly to the surface:

"No-Decompression (No “D&#8221 Limit. The maximum time a diver can spend at a given depth and still ascend directly to the surface at the prescribed travel rate without taking decompression stops." (Chapter 9-3).

And..

"No-Decompression Dive. A dive that does not require a diver to take decompression stops during ascent to the surface. (Chapter 9.3)."

I might conclude that according to the US Navy, safety stops are optional whether it is 5 or 3 minutes.

GJS