Is there any scientific evidence that safety stop decrease DCS risks?

[nohappy]

Dear all,

I've been searching for awhile but can't find any study or paper for this question. Could any one provide any reference for me?

My questions are...
1. Is there any experiment trying to compare the risk of DCS with and without safety stop? Or we all just assume that safety stop would help improve safety "theoretically"?

2. According to Haldane's decompression model, it would consider that it is better to dive without safety stop because divers should increase gradient between tissue pressure and ambient pressure for better off-gasing. Is that assumption correct?

3. Does anyone know why do we choose 3 meter / 5 meter (or maybe 6) as the depth for safety stop but not 7 or 8 meter? Is there any reason or story behind this? Or is it just a magic number?

 

[tbone1004]

so the safety stop is basically a shallow decompression stop. Provided that you are off-gassing, then you are reducing the inert gas in your body tissues at a deeper pressure, thus reducing the risk of DCS.

The safety stop was originally studied in the 70's IIRC and was recommended at 10ft because that was the final decompression depth so it made sense. It was then lowered to 15-20ft.
Was that to conveniently coincide with the Workman M-value of 1.58? Was it to conveniently get closer to the max exposure of oxygen depth? Was it to coincide with the final half ata depth? or was it simply a bunch of old white guys sitting in PADI's conference room who said "the safety stop is 15-20ft, for 3-5 minutes"
I'm not sure, though I'd like to believe it's some of the more scientific ones, but I'm willing to bet it was the last one.

Shallower lowers the partial pressure of inert gas being inspired and the gradient across the cells of the inspired gas vs. what is inside the cells is what determines deco efficiency. Shallower the better, however that has to be balanced with bubble size so you want to stay on the deeper end of that. In that depth range, the 6m also happens to be much easier to maintain buoyancy than at 3m as well as keeping you largely out of the surge in mild weather.

Masters thesis on safety stops from 1994
Doppler Detection of Silent Venous Gas Emboli in Non-Decompression Diving Involving Safety Stops

 

[doctormike]

I'm not a decompression scientist, but my understanding of the reasoning behind the safety stop is that it's an operational speed bump rather than any sort of calculated decompression advantage. That is, we teach it to everyone so that its a little less likely that inexperience divers will just rush to the surface and forget about their ascent rate. Also, people who have incurred an unrecognized deco obligation through inattention, poor planning or just ignoring their tables/computer might have a little time to offgas a bit more before surfacing, where missed deco is more likely to be clinically significant.

1) The problems with that experiment (a prospective study) would be the ethical issues of randomizing divers into the stop and no-stop groups, and the huge number of divers that you would need to enroll in order to reach statistical significance, since DCS is pretty rare in NDL dives.

2) At 15 feet, I believe that all of your compartments are offgassing, although there may be a small amount of ongassing happening in very slow compartments that weren't saturated during the dive. Someone who knows more than me can comment on that. It is true that you would have a greater gradient at the surface than at 15 feet, but 15 feet is still OK for efficient offgassing. Ascent strategy is always a balance between controlling bubble formation and efficient decompression.

3) See tbone's comment above.

 

[scubadada]

History of the Safety Stop

 

[boulderjohn]

tbone1004 did an excellent job with his post, but I will address this point separately and add one more thought.

2. According to Haldane's decompression model, it would consider that it is better to dive without safety stop because divers should increase gradient between tissue pressure and ambient pressure for better off-gasing. Is that assumption correct?

Yes, you will off-gas faster without the safety stop because of the greater gradient on the surface, but that assumes it was safe to go to the surface. The safety stop makes sure it is safe to go to the surface. The longer you stay there, the more certain you are that you will not have a problem when you ascent those last few feet.

In normal recreational diving when staying within the standard limits, we should not need to do a stop if we ascend at a safe speed. The safety stop was supposedly added to deal with the possibility that the ascent was a little faster than it should have been. Since then there have been studies that indicated that the safety stop is more valuable than was initially thought. For example, the Morroni study (around 2002--don't have a link to it right now) attempted to compare ascent rates and decided that the safety stop was more important than the ascent rate.

 

[ofg-1]

No, there is no more scientific evidence for a safety stop than there is for climate change. All a hoax. Next thing they will want to tell you that some type of human activity is responsible for safety stops.

 

[kelemvor]

Next thing they will want to tell you that some type of human activity is responsible for safety stops.

Well, that's not a totally unreasonable thing to say. I mean, scientific evidence or not safety stops are definitely the result of human activity .

 

[nohappy]

Wow! Thank you all! I'm really appreciate for all your answers. Just want to clarify my question again and trying to make a small conclusion so far...

1. Besides any theories (no matter what bubble model tell us), I guess I can say that there is no firm evidence or experiment that can convince us that "safety stop" is better than "no safety stop". (Right?)

2. Allow me to re-phrase my question, if there is a time machine that I could go back to ask Haldane if safety stop helps, I thought he would say "no" because he didn't consider bubble formation during ascending. (Right...?)

3. I thought the M-value 1.58 is redefined by Workman from the partial pressure of N2. And the value 2 is defined by Haldane first because he was considering total pressure. And they should both refer to the same depth which is 10 meter. Which means that both Haldane and Workman believe that you can stay above 10m (but not 6 meter) indefinitely on air and surface whenever you want without getting DCS. (Am I wrong?)
Ref: Gradient Factors | Dive Rite

Btw, I also have the feeling that safety stop is more like a speed bump as doctormike mentioned.

 

[scubadada]

@nohappy

I don't believe your thinking is correct. I believe it is "safer" to spend some time at 10-20 ft before ascending to the surface from a no-stop dive. The safer may be very small as you are able to ascend directly to the surface from a no-stop dive, assuming your ascent rate was correct. I also believe it is safer to pad your shallow stop to some degree after satisfying your deco obligation. The magnitude of this safer may also be small. The safer of a safety stop or extra time after deco may be contributed to by the decompression algorithm you employ and how close to the edge you are that day, including personal factors such as hydration, thermal stress, and exertion. Regardless, more shallow time is certainly not going to hurt you, there may even be things to see and do

 

[Steve_C]

I think that you are looking for an exactness in the formulas that does not exist. Large margins of error are built into them to account for human variability and dive variability. You can take any of the rules/algorithms and fudge them a bit and probably do fine most of the time. The more you fudge them, and the more often you fudge them, the more the chance of problems but no guarantees. You might get away with it for 20 years. You might get bit on the next dive. Had a friend who fell asleep driving to work in the early AM on the right side of the road, woke up against the barrier in the middle. Had crossed 3 lanes of morning rush hour traffic without hitting anybody. Everything is probabilities. I don't recommend driving with your eyes shut. Some times you are lucky and sometimes you feel undeservedly unlucky.