Safety Stops

[Tavi]

Hi all,

When we dove in Grand Bahama our DM told us to do safety stops at 25ft for 1 min and 15ft for 4 min. The one dive I did over 100ft the stops were 35ft 1 min, 25ft 3 min, 15ft 5 min.

When I asked about stops on the shallower dives being more than 3 minutes. The explanation I got was that all of the research wasn't in when the Industry (manufacturers) adopted the 3 minute recomendation.

3 minutes is printed in logbook pages, 3 minutes is programed into my suunto cobra, sure I can keep track of another minute or two, I would rather add a little extra safety measure.
I was wondering what others thought of this.

What do you do for safety stops on recreational dives?

Thanks, Tavi:tree:

 

[SubMariner]

Recreational diving acknowledges a safety stop for 3 minutes at 15' as the "standard". If you do a deep dive, the safety stop is pretty well mandated; but not for shallower dives. Then it's basically "just a good idea", adding in more "safety" by offgassing while still under pressure. (At 15' you're at about 1.5 ATM.)

If you are using the PADI tables, take a look on table 3. There you will see that a longer safety stop ("Emergency Decompression")is required if the No Decompression Limit (NDL) is exceeded. How long the stop is extended is determined by how many minutes you have exceeded the NDL.

It appears that this particular operation has taken it upon themselves to add in the extra safety stops a la decompression diving. To me it seems like overkill.... esp when you consider that you would be using more air at these "stops" than if you were just hanging at 15'. What happens if someone runs out of air at one of these "extra stops"? Do they have any additional air source for them to use?

BTW, it wasn't the manufacturers that decided on the 15' safety stop; it was the training agencies. The equipment people could care less.

~SubMariner~

 

[Tavi]

SubMariner,

No there is not an additional air source available other than your buddys, When we dove Theo's wreck (103') I mentioned that it sounded like the dive and stops would last longer than my air.(this was my first dive this deep)I was told to let them know if I had less than 1500psi before the turnaround point in the dive plan. I was okay there. I was asked if I was okay several times after that, Including on the line at safety stops. My air went below 500psi after 4 minutes at the 15' stop an Instructor was right next to me, no problem I still returned to the boat with air. If someone goes to less than 1500 before the turn around point the group returns imediately.

If the Equipment people could care less, why dont they make the recomended (not the mandatory ones)safety stops user programable?

Thanks Tavi:tree:

 

[The Chairman]

Hey Tavi, & All,

Manufacturers have a vested interest in our safety… if we die and can’t dive, then we can’t buy more equipment from them. Suunto uses a modified bubble gradient in it’s tables. 3 minutes is the MINIMUM amount they will ask for on a safety stop. IF you get into the “Deco” zone, you will see Suunto’s computers bring you up with many small stops along the way, all based on that modified bubble gradient. I feel that most deco/safety stops in the 50 ft zone are probably counterproductive. You are still probably on-gassing nitrogen, and that does not reverse itself(although this does vary, depending on your tissue nitrogen load) until you hit around 33 ft and does even better at the 15 ft range. I have yet to take a DCS hit using my Suunto, but I do it with both eyes wide open. Many factors, including dehydration, exertion, lack of rest, lack of fitness, etc. etc. can and will affect your body’s ability to handle the nitrogen in an efficient manner. In those and other scenarios, it is better to be ultra cautious! Remember, dive computers only calculate, they can not measure your nitrogen load. Its what we refer to as a SWAG (Scientific Wild Ass Guess), but it will have to do for now.

As for your buddies down south. Kudos to them for being so cautious with their precious cargo!!! Glad to have you back safe!

 

[Walter]

..........I'll almost always make a 3 minute stop at 30 ft and a 5 minute stop at 15 ft. Depending on how deep and/or how close I come to my NDL I may add additional stops or additional time to the stops.

As for "deco/safety stops in the 50 ft zone are probably counterproductive. You are still probably on-gassing nitrogen, and that does not reverse itself(although this does vary, depending on your tissue nitrogen load) until you hit around 33 ft" I believe that's an over simplification of the issue. Keep in mind we are dealing with several compartments with different half times. It is possible to have one compartment on gassing while another is off gassing. Actually, that's a basic concept with a modified Haldanian model, but it's true even with a Haldanian model. On a deep dive, the 5 minute compartment will be saturated after 30 minutes, while a 120 minute compartment still has a long way to go. A deeper safety (or deco) stop will allow the fast compartment time to off gas while slow compartments are still on gassing.

This past sunday, my stops were: 70 ft - 1 minute, 60 ft - 1 minute, 50 ft - 1 minute, 40 ft - 2 minutes 30 ft - 4 minutes and 15 ft - 8 minutes.

I feel deep stops are helpful in certain situations and it never hurts to increase the time (air supply permitting) on shallow stops.

We discussed this in October in the Ask Dr. Deco forum. http://www.scubaboard.com/showthread.php?threadid=201

WWW™

 

[The Chairman]

Hey Walter,

Indeed it was a gross oversimplification of the total on/off gassing theory. Not only are there diffent gasses at issue here, but also different tissues, and non-ideal curves as to how those on/off gassing rates change at pressure. When I dive deep, I tend to rely heavily on my Cobra. Perhaps too heavily, but thats a personal choice. My Cobra and I have only been into the "Deco zone" twice. Both times saw brief slowdowns at the deeper levels (measured in secs, not minutes) and it kept me for over 6 1/2 minutes at 15 foot. I guess I could go and look at my dive profile on the Suunto program, but thats how I remember it.

I guess my point to him, was that I trusted my Cobra, as it makes a continual calculation based on the depths I dive. It is not going to just pop in arbitrary stops at various depths, unless the model calls for them. It does, however, always ask for a safety stop (good dive computer) at fifteen feet or so.

 

[rcohn]

If you want to increase the margin of safety you need to be concerned about bubble growth not just the dissolved gas load. Deeper stops and slower ascents minimize the gradient between the dissolved gasses and any micronuclei present allowing the gas be released through the lungs rather than pumping up the existing bubbles.

DAN is now beginning to recommend slower ascent rates, they are currently testing 10 ft per minute, and have begun talking about deeper safety stops. Note that deep safety stops provide a practical way to slow the overall ascent rate. So a stop in the 50 ft range (or perhaps even deeper) on a deep dive can be very beneficial.

Submariner notes that the training agencies set the safety stop recommendations. However, the agencies do not in general research diving physiology, they rely on the Navy, NOAA, DAN, NASA, etc. As new information becomes available, the recommendations will eventually change, but it can be a slow laborious process. It sounds like the DM is trying to stay ahead of the curve (as many of us do who use deep safety stops) and provide the safest reasonable profile. I assume he/she set the tank pressure where the ascent would begin based on allowing sufficient gas for the planned stops.

Hopefully, Dr. Deco may have some more information to add to this subject.

Ralph

 

[Tavi]

Hi All,

Thanks for the responses, I do love my Cobra, but it sounds like adding the extra stops and extending the 15'stop is a good Idea.
As far as the dive operation we dove with, I appreciate their extra efforts to be safe. There were 2 days we could have done a second dive(and they could have made more $). But they called it a day after one dive because of strong currents,rough seas and seasick divers on the boat.

Looking forward to more opinions, Keep 'em coming

Tavi:tree:

 

[Dr Deco]

Dear readers:

Those slow ascents are important. (Sorry about getting back on this issue. I was away at the Aerospace Medical Association meeting and a short vacation.) DAN is performing a study, I believe, on this question of ascent rates. Originally, they were slow, as recommended by JS Haldane.

The safety stops were looked at by Dr. Andrew Pilmanis several decades ago and reported in an earlier Dr Deco FORUM. Slow ascents retard the growth of nuclei already formed.

Additionally, I continually mention, nucleation is important. This processes occurs when one climbs ladders with gear on, or lifts tanks on board, etc.

This effect was first noted during the Second World War when selection tests were performed on aviators. It was desired to determine who was or was not susceptible to DCS for the high altitude bomber crews. Originally, the tests consisted of putting the candidates into the altitude chamber and holding them (while seated) at altitude for several hours. This was time consuming and was not as specific as desired. Thus, they added activity at altitude (the decompression phase of the session) to the study. This consisted of stair stepping, or marching in place or lifting small cylinders. The incidence of DCS increased dramatically. This occurred in individuals all of whom had the same amount of gas in their tissues at the initiation of depress. That is, all were saturated at sea level. Gas uptake was not an issue - - only nuclei formation.

Give yourself, and your computer, a break by avoiding those activities for which the computer has not control. I am speaking of vigorous musculoskeletal activity.

Dr Deco

 

[TexasMike]

I had always wondered why 3 minutes was the time versus 2, or 4, or 5.

Then my advanced o/w instructor explained that 3 minutes was the theoretical time it takes for blood to make one complete circut through your system.

He also stressed making your ascents as slow as possible for much the same reasons that Dr. Deco explained above. Essentially, the slower the better.

--Twxas Mike :mean: