Info History of the Safety Stop

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tursiops

Marine Scientist and Master Instructor (retired)
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There have a number of threads recently on ScubaBoard that refer to a Safety Stop, and some assertions as to how it came about and when.

Here is what I can find on this topic; it would be nice to flesh it out so myths and speculation can be minimized! And always remember the research has been about reducing bubbling, not reducing incidence of DCS. That is a different subject.



Origin of the term Safety Stop in early 1970s.

Andrew A Pilimanis, a researcher for the US Air Force, worked on bubbles in the blood, after diving, under a contract from the Office of Naval Research, in the early 1970s.
upload_2020-8-14_13-34-39.png

The open summary of part of his work was published in 1989 as part of an American Academy of Underwater Sciences (AAUS) workshop on the Biomechanics of Safe Ascents, https://repository.si.edu/bitstream/handle/10088/2717/proceedings_safeascents.pdf. His report is pp65-71. The abstract of that report says:
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He concludes:
upload_2020-8-14_13-35-57.png

This is the first known reference to a “safety stop,” and it was introduced to lessen the bubble scores of the divers, not specifically to slow the ascent to the surface. It was overtly a stop, in analogy to a deco stop, not an approximation to a slow ascent to the surface. It was applicable to diving the US Navy tables near their NDL limits.

Incorporation of Safety Stops in Diving Practice:

Diving Science and Technology (DSAT), a corporate affiliate of PADI, developed the Recreational Dive Planner (RDP) in the mid-1980s, PADI introduced it in 1988, and the collected researchers and authors finally published their complete report in 1994:

http://archive.rubicon-foundation.org/xmlui/bitstream/handle/123456789/4228/DSAT_1994.pdf

This was the first dive table to incorporate the safety stop, pretty much as suggested originally by Pilimanis; “mandatory” if near NDL (the three pressure groups above NDL on the RDP), but added the “any dive to 100 ft or greater” which was only hinted at by Pilimanis.

As an aside, the 1994 DSAT report on the RDP development also said something worth noting but often ignored:

“From the test results we further conclude that performing multiple dives over multiple days with the RDP is acceptable, and can be done with no greater risk than is encountered in many common practices of recreational divers. Even so, based on this evidence and the suggestions of other experts, we recommend limiting the number of full-time dives per day to 3 or at most 4, and suggest including a day with a reduced level of diving (or none) every 2 or 3 days.”​

PADI also began using the Pilimanis recommendations as early as 1984 in its training materials, according to their blog: History of the Safety Stop.

Donna Uguccioni wrote a Master’s Thesis in 1994 exploring the Pilimanis work further.
http://archive.rubicon-foundation.org/xmlui/bitstream/handle/123456789/3430/UguccioniThesis.pdf

She states her “study was similar to earlier work done by A. Pilimanis, except Piliomanis’s study used compressed air and controlled the open water dive profiles.” In contrast, her only controlled variable was randomly to add in a 3-minute at 20-feet safety stop on a collection of working dive profiles by scientific divers, all using Nitrox. Her test was direct ascents to the surface, versus descents delayed by the safety stop. Her focus was very clearly stated as being on the effect of slowing one’s ascent to the surface by using a safety stop; perhaps this is where the assertions come from that the reason for a safety stop is to slow ascents. Oddly, she never mentions what the actual ascent rates were, although she mentions both 60 and 30 ft/sec as being in use elsewhere. All her studied dives were to 100 ft using 36% Nitrox, so about 75 ft equivalent air depth. The bottom line is that she clearly showed safety stops usually reduced bubbling, never increased it, and this was most significant for cold versus warm water, for bottom times greater than 25 mins (NDL was 30 mins for her studied dives), for no currents versus currents, for younger (30 and less) versus older divers, for females, and for not using alcohol before the dive. So, to minimize your bubbling, following her results, you would be male and over 30, in warm water with strong currents, and use shorter bottom times.

One interesting comment in her work was this:
“Subjectively all of the divers involved in this study, when asked about how they felt after each dive profile (safety stop or direct ascent) agreed that they had a general feeling of well-being after the safety stop. Many felt refreshed and clear headed after a safety stop while some felt· out of breath, fatigued, or complained of a headache after a direct ascent. One possible explanation for this may be that carbon dioxide has built up in the diver during the dive. A direct ascent may not allow divers a chance to off-gas the carbon dioxide quickly enough, while a stop may offer more time to off-gas carbon dioxide.”​

Current Practice and Recommendations:

The 1988 RDP recommendation was to do a safety stop after any NDL dive within 3 pressure-groups of your NDL, or any dive to 100 ft or greater. With an increasing number of divers using computers, from which is it not really possible to ascertain whether you are within 3 pressure groups of your NDL, what do you do if you want to translate the RDP recommendations your computer use? One action is to adhere to the 100 ft or more recommendation; do your safety stop if your dive exceeds 100 ft. Another is to look at how many minutes you lose of NDL if you are 3 pressure groups away; this number decreases with depth, but at 60 ft and below, varies from 6 down to 3 mins at 90 ft. So, if your computer says you only have X mins of NDL left, and your BT so far is within about 3-6 mins of that, you just bought a safety stop.

Complicated? Yep, thus the recommendation in modern material is to do a safety stop after every dive; DAN says, for example:

"Complete safety stops on all dives that exceed 10m depth. Safety stops assist with reduction of excess nitrogen, which reduces the risk of DCI. They also slow your ascent rate, by forcing you to stop for a period of time. The rule of thumb is 3-5 minutes at 5-6 metres."​

Why 10m (33 ft)? Two reasons: you don’t take on much nitrogen when that shallow, and your cylinder is likely to run out of gas if you try and get to NDL, so you most likely will not be near NDL.

Dive Training Magazine tried to summarize things in its usual wordy way in 2015, but got involved with deep stops a bit. A Safe Habit: Safety Stops as Standard Procedure | Scuba Diving News, Gear, Education | Dive Training Magazine.

The Future

Shearwater now offers a SurfGF display on their dive computers, which can actually tell you your status of off-gassing; people are using this to decide whether or not even to do a safety stop. The point is, not everyone has a Shearwater computer, and a safety stop never hurts. For the time being, I doubt we’ll see the training and safety agencies switching to recommending SurfGF rather than 3 minutes at 15 ft.

Summary

It has been nearly 50 years since Pilimanis showed that a safety stop was a good idea for deeper dives near one’s NDL. It has been over 30 years since that work has been incorporated into the RDP. It has been 25 years since Uguccioni extended that research and showed it was applicable to a broader range of dive profiles. But we are still arguing about safety stops and how long and how deep! Maybe it is time to move on and just do them. It has only been a few years since SurfGF has been available. Ask me in 20 or 30 years if it has become a recommendation.
 
Well done! Interesting!
 
Wow. Thank you for sharing...I'm always learning around here.
 
Thanks @tursiops

I do a 3 min SS even if I don't need it. The last 15 feet of ascent is also over at least a minute due to the huge GF99 increase during the final ascent. Diving a Teric, I do my last stop to get me to the surface with a GF of about 80. My last 250 dives have had a surfacing GF max of 80 (15-80). I dive an Oceanic VT3 running DSAT and a Teric running 80/95
 
Thanks @tursiops

I do a 3 min SS even if I don't need it. The last 15 feet of ascent is also over at least a minute due to the huge GF99 increase during the final ascent. Diving a Teric, I do my last stop to get me to the surface with a GF of about 80. My last 250 dives have had a surfacing GF max of 80 (15-80)
You probably meant to reply to the other thread...
 
very interesting how many of today's best practices coalesced out of some combination of low-sample-size research, old rules of thumb, and debriefs from terrible accidents
 
My wife and I always have enjoyed the safety stop. We use those 3 minutes to waltz underwater. Nearby divers can sometimes hear us humming the music.

It's a good way to spend 3 minutes, and confuses the living daylights out of the students and new folks on the boat.

I suppose we could just hover in place. That might be better from a physiological perspective, but that last dance at the end of each dive ensures we board the boat happy and relaxed.
 
My wife and I always have enjoyed the safety stop. We use those 3 minutes to waltz underwater. Nearby divers can sometimes hear us humming the music.

I suppose we could just hover in place. That might be better from a physiological perspective...
Keep doing what you're doing. Light to moderate exercise during safety (or deco) stops increases perfusion, aka blood flow, through all the little nooks and crannies of your tissues. This speeds up the movement of excess nitrogen into the blood where it can safely make its way out of the body via the lungs.



 
https://www.shearwater.com/products/swift/

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