Best Safety Stop Depth

[halocline]

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

Sorry dude, but there isn't a diver in the sea that can hold a continuous rate of ascent that slow, plus wave action totally screws the idea.

Maybe what you really mean is this far more practical advice: a 3 minute stop and then as slow an ascent as possible, stopping along the way, so that it takes you a total of 6 minutes to get to the surface from the beginning of your safety stop. If that's what you're recommending, I would enthusiastically agree.

 

[rockjock3]

Sorry dude, but there isn't a diver in the sea that can hold a continuous rate of ascent that slow, plus wave action totally screws the idea.

Maybe what you really mean is this far more practical advice: a 3 minute stop and then as slow an ascent as possible, stopping along the way, so that it takes you a total of 6 minutes to get to the surface from the beginning of your safety stop. If that's what you're recommending, I would enthusiastically agree.

You're right. This morning we could only do 5 minutes from 20fsw to the surface. That was with 2-3 foot seas, doubles, and 2-stage bottles hanging. Yea, I bet 6 minutes would be impossible.

 

[rockjock3]

To the OP. Here is another school of thought for NDL divers in Rec limits.

Ascend to half your depth at 30fpm then 1 minute there. Then you ascend at 10fpm till you get to approx 20feet. At 20feet you do your 3 minute safety stop.

 

[Charlie99]

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

The optimum depth varies with the profile of that particular dive.

Decompression (and that is what a safety stop is, whether or not we choose to call it a deco stop) is a balancing act between going shallow to offgas, and staying deep to avoid bubbling. This tug-of-war between going shallow vs. staying deep is the fundamental conflict to be resolved in optimizing decompression profiles.

If you are going to do one and only one deco/safety stop that, for a square profile dive that the optimum safety stop depth will be deeper for a short deep dive than it will be for a moderate depth longer dive.

If you have done a multilevel dive and have spent several minutes at intermediate depths, then your safety stop should tend to reflect that shallower depth, ignoring the earlier, deeper portion of your dive.

If you want to get a reasonably good understanding of why these statements are true, two good articles to read would be the M-value and the Deep Stops articles by Richard Pyle. The M-values article is good general background. The logic behind the deep stops is similar to that for choosing the optimum safety stop depth. If you are coming up from a short square profile deep dive, then the fast compartments are the closest to the limits. You need to stay a bit deeper than normal to avoid unecessarily creating bubbles in those compartments. You don't need to worry about a 25' stop adding additional loading to the medium and slow compartments.
OTOH, when coming up from a medium depth NDL dive --- 40-60', the controlling compartents are the slower ones. One can safely go to 15' or even 10' without excessive bubbling, and the shallower depth will better offgas the medium speed compartments.
http://www.dive-tech.co.uk/resources/mvalues.pdf and
http://www.dive-tech.co.uk/resources/deepstops.pdf

Charlie Allen

 

[TSandM]

Excellent post, Charlie, and you saved me all that typing . . .

 

[Cacia]

Charlie99 rocks.

 

[nereas]

NAUI currently teaches that for air and nitrox NDL diving, the safety stop should be 1 minute at 1/2 of your maximum depth, followed by a 30 ft/min slow ascent to 15 ft, with a 3 min stop there.

Most dive computers have a 20 to 10 ft safety stop range built into them, and they will tell you to stop for 3 mins within that range. But if you skip this, they will not go into penaly mode. The stop is optional.

If you run V-Planner programs for your NDL air and nitrox dives, it will tell you that you do not need a safety stop in most circumstances. That explains why the stop is optional. It also defines NDL, which means that you can by definition ascend to the surface at any time.

Given those popular choices, what should you do?

I suggest that to be as safe as possible, you stop for 1 minute at 1/2 of your maximum depth on every dive, then proceed from there like a tech diver, stopping every 10 ft, for 1 minute at each stop. In this manner, your safetly stop will be adjusted for the depth of your dive.

For a 100 ft NDL dive, this means a 1 minute stop at 50 ft, then a 1 min stop at 40 ft, then a 1 min stop at 30 ft, then a 1 minute stop at 20 ft, and finally a 1 min stop at 10 ft. This gives you 5 mins of offgassing for a 100 ft dive, which is recommended by many of the training manuals as well.

For a 75 ft NDL dive, this means 1 min at 40 ft, 1 min at 30 ft, 1 min at 20 ft, and finally 1 min at 10 ft. This would be like a 4 minute safety stop.

For a 50 ft NDL dive, this means 1 min at 25 ft, 1 min at 15 ft, and 1 min at 10 ft. This is very similar to your normal 3 minute stop.

There is nothing sacred or magical about 15 ft. It is not a cure-all. There are better ways to ensure your safe off-gassing besides choosing a single depth for all your safety stops. Stopping simply gives your body more time to off-gas at a rate slower than if you ascended directly to the surface without a stop.

Since you seem like a perceptive person QS, you may want to customize your own safety stops, to get more safety out of them.

 

[stu_in_fl]

Most dive computers have a 20 to 10 ft safety stop range built into them, and they will tell you to stop for 3 mins within that range. But if you skip this, they will not go into penaly mode. The stop is optional.

true - it is a recommended stop rather than mandatory, but if doing another dive after you will be penalized for missing the stop.

 

[lamont]

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?

If you are down at 4 ata / 100 fsw / 30 msw and breathing air, then the ppN2 of saturated fast compartments is (4 ata * .80 ) 3.2 ata / 22 msw / 72 fsw. Below that depth you have no oversaturation in any of your tissues and will not be bubbling at all. Above that depth is when you'll be driving an overpressurization gradient. What does that all mean? Well, you should scoot off the bottom fairly promptly and then as you start getting above 70 fsw / 22 msw you should start to think about slowing down. Zipping up to 15 fsw / 5 msw doesn't make a whole lot of sense to me since you're driving quite an overpressurization gradient there. Technical divers will also typically start their deep stops roughly 1.5-2 ata off the bottom, not 2.5 ata off the bottom.

Of course you need to reserve enough gas to do 1s from 1/2 max depth...

 

[nereas]

...you need to reserve enough gas to do 1s from 1/2 max depth...

Exactly!