Safety stops and deco

[TwoBitTxn]

What is behind the magic number of 15ft for a safety stop and the final deco stop?

I have found that it is easier to hang between 15 and 20 feet than 10 and 15 feet due to wet suit compression. Is there a significant difference?

Tom

 

[syruss32]

I would like to know more about this too.

I have a new computer and the doc shows it with a safety stop at 20 feet for x minutes. I thought it was 15 feet?

Is there some algorythm that determines this? Since I haven't acutally used this underwater I don't know what it will actually display. I should know after tomorrow.

 

[Rick Murchison]

The safety stop is just that - a safety stop - it is not required by the table algorithm. It comes from a discovery during the doppler studies of years ago when it was found that there were asymptomatic bubbles in many divers who were diving within table limits. Among other things, the researchers found that a stop of about three minutes between 20 and 10 feet followed by a very slow ascent to the surface all but eliminated these asymptomatic bubbles entirely, so that stop was adopted as an additional safety precaution along with some revision to max NDL bottom times. 15 feet was chosen because it's halfway between 10 and 20, easier to maintain than 10, and still not too deep. After all, for most of us, if we're hanging on a line at 15, our feet are at 20 anyway.
(Standing by for the "horizontal safety stop benefits" post)
Rick

 

[TwoBitTxn]

Thanks Rick,

My Genesis shows the final deco ceiling at 15 ft. I'll keep it above 20 ft but like I said I have an easier time of it around 17' or so.

Tom

 

[Diverl]

I am sure that NetDoc or Dr. Deco can give you a very in depth explanations as to how they came up with these numbers, my understanding is as follows and if I am wrong I hope someone will correct me:

The greatest pressure change happens between the surface and 33 feet, move from 1 atm to 2 atm. Because this change is so significant it is beneficial to give your body some time to adjust to the pressure change and I assume that the most appropriate place would probably be the midpoint in the pressure change.

14.7 psi / 33 feet = .445 psi/foot.
14.7 psi / 2 = 7.35 psi = midpoint in pressure change
7.35 psi / .445 psi/foot = 16.5 feet

A stop at 15 feet is roughly the midpoint in pressure change between the two depths allowing your body to adjust to this rapid change in pressure and gives you body a chance to off gas some at a safe rate.

 

[Dr Deco]

Dear Readers:

No, I cannot give you a better reason for the 10, 15 or 20-foot stops. The work referred to by Rick Murchison was performed by Dr Andrew Pilmanis in the 1970s as a part of this PhD thesis research. A Doppler probe was lowered into the water from a boat above and, at these depths, measurements were made of the precordial bubbles. As mentioned above, the Doppler bubbles were reduced when the in-water, shallow stops were added.

People then started to advance the idea that this might be a good idea for all dives. Why the depths were chosen I do not really know. I believe that the idea went between [1] the depth must be deep enough to give a real pressure and [2] yet not deep enough to cause any substantial gas loading. In addition, it was necessary that the stop be deep enough that wave action would not render stop depth unmanageable.

We return to Doc's three golden rules:

• slow ascents -- because they prevent the over expansion of tissue micronuclei ;
• safety stops -- because they are a variation of the slow ascent and further prevent the growth of micronuclei above the Laplace limit (where they turn into decompression bubbles);
• prevention of straining after reaching the surface -- as musculoskeletal activity breeds tissue micronuclei .

Dr Deco

 

[Diverl]

Apparently Rick is fast with the keyboard and got his reply in before mine. I also like his answer better than mine.

Thanks for the info!

 

[Liquid]

Generaly,

A safety stop should be done between 6-3 meters (10-20 feet I belive). So it's ok to stop 15-20 feet. The reason for this depth for the final stop is that 6 meters is the "no decompression" depth. Up to 6 meters you are safe from decompression, but if you get shalower than 3 meters, the effect of the stop is reduced., and it is quite hard to stop there, becouse of the wetsuit.

And as was said already,
A safety stop is just a safety measure. It is not something you must do or elses. It's just to get a better safety margine, as not all of us are in the same shape/age/weight /alcohol consumption/hunger/thirst condition.

 

[blacknet]

Hello,

Well it really comes down to who's standards you follow as for the safety stop 'magic' number. Table A may say 10' and table B may say 15'. What I like to do while diving in the gulf is hold the line at 30' 20' and 10', reasoning is various: a) simulated deco stops (always ends safer) b) get to unwind from the dive and work on bouyancy skills c) get to hand out and work on breathing skills and d) get to hand out and watch the marine life swarm around you!

BTW I do advocate that divers learn how to do deco stops and apply it to regular open water dives.

Ed

 

[Texass]

The safety stops I mentioned in a previous post were horizontal !!! As for their benificial qualities, the jury's still out. They did sort of combine the thrills of scuba diving and water ski-ing.
:whoa: