Best Safety Stop Depth

[quietstorm]

I write to ask your opinion about the optimal depth to take a safety stop for NDL diving. I continue to see and read opinions that vary from 25 feet to 15 feet.

Why do many organizations and instructors and people and Scubaboarders and dive computers recommend a safety stop at 25 feet? while a large contingency of others say 15 feet?

For example, my Tusa Sapience IQ 800 wrist computer (Swiss algorithm) calculates the safety stop to begin at roughly 20 feet, but will allow the diver to remain at 26.5 feet, for the 3 minute safety stop interval.

Let me digress for a moment, before you answer, to see whether I am on the right track in trying to answer this question myself. While getting certified many years ago with BSAC (they had a branch in San Diego once upon a time), we learned that we exist at 1 bar of atmospheric pressure at the surface, increasing by 1 bar for every 10 meters of descending below the surface. Thus, we are at 2 bars of pressure at 10 meters, 3 bars of pressure at 20 meters, and so on. Welcome to Boyle's world.

We then learned that air is comprised of roughly 80% nitrogen, 20% oxygen, which increase proportionally as pressure increases. These are partial pressures of gases that increase as we descend, and decrease as we ascend. Welcome to Dalton's Law.

As we descend, pressure increases. Nitrogen gas is dissolved and absorbed by our blood and tissues. As we ascend and pressure falls, the situation goes in reverse. Nitrogen wants to leave our tissue and blood and come back into gas (but not too fast there young Nitro:no ). Hello Henry's Law of gas solubility.

So what does this all lead to? I surmise that since the greatest increase/decrease in pressure and partial gases occurs at 1 bar/10 meters, the optimum safety stop is just slightly above 1 bar/10 meters.

Does my theory hold water? What exactly is the best depth for a safety stop for a NDL dive?

 

[Cacia]

I think 15 ft is better than 25 (hopefully you've done the deeper one minute stops at ten foot increments)

I usually do 3 at 20 and 1 minute at 10 just because that is my habit. We usually have the 15 crowd and the 25 crowd and I like to split the difference to maximize keeping an eye on the folks at 25 who are sometimes low on air. I am often in an unoffical capacity of "helping keep an eye on people". Reality is that staying in the water longer and making sure they reach the ladder and de-fin, etc is my little niche.

I am not sure my answer follows any one formal recommendation, but I guess my point is that sometimes it is multifactorial: divers present, current, surface conditions to a small degree. (and I hate waiting on the surface, I'd rather not *see* that fiasco, I like the peacefulness of being the below the surface scout)
Getting on last has real advantages, one being the least amount of post-decompression thrashing and physical exertion. I think I've decided that is more important than five feet one way or another? (plus, it's safer for my camera) what's that saying? slow is smooth, smooth is fast

 

[stu_in_fl]

my computer requires between 20ft and 10ft to count down the 3 min safety stop.

In OW courses most are taught 15-20ft (as most OW students wouldn't be able to really hold it to 10ft and not risk popping up to the surface). You'll find the more technical crowd use 10ft (but they are expected to be able to hold it at 10 without moving)

of course, if diving a little deeper, there are also the "deep stops" that are now quite popular - generally half depth for a couple of minutes on the way up, then do your 15ft stop (or some take it even most slowly with 1 minute stops every 10feet or so.)

 

[wreckchick]

Holding a 10' stop in the ocean with any wave action at all is problematic. Even people doing planned decompression diving often extend the 20' stop and skip the 10' stop.

My ideal depth is 19' (reality it's 20' but my Suunto gets angry with me if it's not above 20'). I also feel a lot better with deep stops and a very slow ascent from SS depth to the surface.

I'm with Catherine in that I actually prefer to hang under water while everyone boards the boat. I almost always surface directly onto the ladder with zero surface time.

Rachel

 

[fweber]

A shallower SS is going to provide better off gasing. However, in any sort of sea state, 10 feet can be down right uncomfortable and difficult to maintain. So, most folks settle on 15-20 feet or 4.5-6 meters. I've not met many people using 25 feet as a safety stop depth. It sounds sort of deep to me. You would still have nearly a 50% reduction in ambient pressure as you ascend from that depth. I would think that somewhat shallower would be closer to optimal.

 

[shurite7]

I've read 10' is ideal for both safety stop and last stop for deco. However, when the sea state is a bit rough 10' is nolonger ideal. Therefore 15'-20' is used.

Recently Dive Training magazine had a nice article on safety stops. It wouldn't hurt to contact them to obtain the article(s) and of course DAN has some good info on safety stops.

Chris

 

[Cacia]

I've not met many people using 25 feet as a safety stop depth. It sounds sort of deep to me.

We get new divers who crowd the line on ascents from deep wrecks...and there will almost always be a group stuck a bit too deep. Usually the weakest divers, in my experience. Often they are the ones who don't really know better and also the ones that think its fine that they have only 200 psi left, and in the back of a traffic jam without good buyancy control. Often they are not even with our group. Yesterday, I discreetly signaled to another dive guide at the "top" of the deco stop cluster that unicycle man only had 200 psi. he said "yea okay", but took no action. So...I just hang and be "available" just in case. I think any experienced diver who sees this is in a position to not tread on anybody's toes, but at the same time not just swim off. If you are not in an official capacity, tread lightly but keep your eyes peeled. I really wanted to stick him on my long hose, if he was with us, I would have. Doing so on another operator's diver causes some friction, people get defensive. ... life gets kinda grey.

One of my friends is pretty harsh: he briefs if you don't get back on with 500 psi, or skip the safety stop you cannot do the second dive. And he doesn't pay any Instructors who don't bring them back with 500 psi. He actually goes around quietly gauging tanks as he switches the gear over...he is the captain and says he gets an idea of "what is going on down there" by doing this. If your computer is yelping, you better have a good story because you will be interogated. (mine is usually that I freedived for the buoy and forgot to take it off)

 

[Henryville]

Just my opinion, but I think the best safety stop is a 6 minute ascent from 20 feet/6 meters.

 

[Rainer]

Just my opinion, but I think the best safety stop is a 6 minute ascent from 20 feet/6 meters.

Given your background, why not recommend min deco for MDL dives? What is a 6 min ascent from 20' buying the average *recreational* diver?

 

[Cacia]

I have a question for you also. If you happen past a diver (not on your team) and you happen to see his gauge is low, what do you do? Do you ignore that or do you give some attention-from-a distance?

hey, I could post that in DIR!...kidding. really.