12 meter procedure - any good?

[Rollin]

Hello all,
I was asked to teach decompression strategies in our local diving club. We have a national diving manual which is quiet recent and a ppt is provided. However, as I like to know exactly what I'm talking about, I took some time to dig into the matter further.
Now, my question. I have to explain my students that there are several ways to make a dive safer, like a safety stop, diving nitrox but keeping your DC on air (when going not beyond mod), lowering Gf-high and something called the 12m-procedure. (In the past version of the manual, this list also mentioned deep stops.)

When a dive took more of you then expected (exertion...), you could or should wait at 12 meters (36 feet) for five minutes, before continuing. This allows offgassing before entering the shallows.
But in my understanding from all the talks of dr Mitchell and dr. Pollok and the book of Powell, you should better continue to your stop or safety stop, as shallow profiles are probably better, and use your gas to make your indicated stops longer.

Or I'm I wrong and is this 12 meter procedure a good idea?

Thanks!

 

[Wibble]

Would be nice to mention normal Buhlmann with Gradient Factors and sticking to them.

For NDLs, you can do safety stops which could be extended if you feel it's required. Safety stops for NDL (no decompression limit) diving are normally done around 5m/16ft.

For decompression dives, you'll plan it using your preferred gradient factors -- 50:80 or thereabouts -- and follow that plan using the computer. Should the bottom phase be rather strenuous, then you'd extend your final stop by a few minutes, e.g. add a safety stop. The final ascent from the last stop (3m/10ft or 6m/20ft) would normally be done slowly, possibly 1m/3ft per minute.

If you're feeling rather more 'vulnerable' to decompression sickness, then adjust the GF-high down from 50:80 to 50:75 or 50:70 to extend the final stop on the computer. Lots of people have opinions on Gradient Factors, these numbers are used by at least a couple of the people mentioned above.

 

[tursiops]

Hello all,
I was asked to teach decompression strategies in our local diving club. We have a national diving manual which is quiet recent and a ppt is provided. However, as I like to know exactly what I'm talking about, I took some time to dig into the matter further.
Now, my question. I have to explain my students that there are several ways to make a dive safer, like a safety stop, diving nitrox but keeping your DC on air (when going not beyond mod), lowering Gf-high and something called the 12m-procedure. (In the past version of the manual, this list also mentioned deep stops.)

When a dive took more of you then expected (exertion...), you could or should wait at 12 meters (36 feet) for five minutes, before continuing. This allows offgassing before entering the shallows.
But in my understanding from all the talks of dr Mitchell and dr. Pollok and the book of Powell, you should better continue to your stop or safety stop, as shallow profiles are probably better, and use your gas to make your indicated stops longer.

Or I'm I wrong and is this 12 meter procedure a good idea?

Thanks!

I've never heard of such a "12m procedure." What is its source?

 

[boulderjohn]

Hello all,
I was asked to teach decompression strategies in our local diving club. We have a national diving manual which is quiet recent and a ppt is provided. However, as I like to know exactly what I'm talking about, I took some time to dig into the matter further.
Now, my question. I have to explain my students that there are several ways to make a dive safer, like a safety stop, diving nitrox but keeping your DC on air (when going not beyond mod), lowering Gf-high and something called the 12m-procedure. (In the past version of the manual, this list also mentioned deep stops.)

When a dive took more of you then expected (exertion...), you could or should wait at 12 meters (36 feet) for five minutes, before continuing. This allows offgassing before entering the shallows.
But in my understanding from all the talks of dr Mitchell and dr. Pollok and the book of Powell, you should better continue to your stop or safety stop, as shallow profiles are probably better, and use your gas to make your indicated stops longer.

Or I'm I wrong and is this 12 meter procedure a good idea?

Thanks!

I wrote this summary of evolving thinking on deep decompression stops (Evolving Thought on Deep Decompression Stops) with considerable help from Simon Mitchell. I wanted to do the same kind of article for NDL dives and I asked Simon to help with that as well. He would not do so, explaining that the research is not nearly as clear on that. So I set out on my own, read all the research I could find, and decided the research is not nearly as clear as with decompression stops.

About 20 years ago, a couple studies indicated an advantage for deep stops on NDL dives, but those studies have since been criticized for drawing conclusions not warranted by the data.

So, I can say with some authority, that there is no good evidence supporting a deep (12 meter) stop for NDL diving. On the other hand, unlike decompression diving, there is no research saying it does any harm. Many a multilevel diver spends way more than 5 minutes in that depth range as the dive draws to a close.

 

[boulderjohn]

diving nitrox but keeping your DC on air (when going not beyond mod), lowering Gf-high

The flaw in this is very common. It starts with binary thinking--there are only two possibilities:

1. Dive with nitrox with the computer set to air, or​

2. Dive with nitrox to the full NDL for that gas.​

​

Diving with nitrox with the computer on air mode is safer because it keeps you farther away from NDL. There is no requirement in scuba that you dive to the NDL on your computer. It is permissible to ascend when you are 5, 10, 15 minutes from NDL--whatever you want. Why not put the correct gas in your computer and ascend before things get close?

Lowering your GF high has the same result. If you lower your GF high, that alone accomplishes the same thing as staying away from your NDL. Why do both?

 

[inquis]

diving nitrox but keeping your DC on air

An old approach that works for tables and DCs without user-adjustable conservatism, but the additional margin is inconsistent. When possible, a better approach is to tell it the actual gas mix and increase the conservatism. Alternatively, adopt a higher minimum NDL while using the real mix.

 

[boulderjohn]

Summary on NDL ascents:

The ascent strategy commonly used in diving (30 FPM ascent with safety stop) has been developed for over 100 years, and it is pretty safe. Some people are currently arguing for longer safety stops, and that will certainly be safer. Research indicates that you can have all tissues fully saturated at that depth and still surface safely, so more time at safety stop depth will help get faster tissues to a safer level without allowing slower tissues to on-gas to a dangerous level.

Other strategies have not been well studied, and it would be almost impossible to do such studies because there are so many options. For example, the deep stop studies mentioned earlier that found an improvement based on a short deep stop did not compare it to an ascent with that same extra time spent at normal safety stop depth. Unlike a traditional safety stop, on a deep stop slower tissues can on-gas past a safe point.

PADI officials told me that when they researched the RDP, they used the then common ascent rate of no more than 60 FPM. They noted that there was no problem going slower than that. As mentioned earlier, multi-level diving makes for very slow, multiple stop ascents, and there does not seem to be a problem with them. But who really knows? There are too many options to study them all, and the traditional view seems to be working just fine.

 

[scubadada]

...So, I can say with some authority, that there is no good evidence supporting a deep (12 meter) stop for NDL diving. On the other hand, unlike decompression diving, there is no research saying it does any harm. Many a multilevel diver spends way more than 5 minutes in that depth range as the dive draws to a close.

Funny, I had never thought of it that way. Many of us frequently do dives where we go out deeper on a wall dive and then come back shallower, a prolonged deep stop.

 

[inquis]

The final ascent from the last stop (3m/10ft or 6m/20ft) would normally be done slowly

Boyle's Law supports this. If you're comfortable with the bubble expansion rate afforded by, say, 30 fpm at 30 ft (10 mpm at 10 m), you would need to be ascending at 8 fpm (2.5 mpm) or slower at the surface to not exceed that expansion rate.

For such rates, it's useful to think of the rate in terms of seconds per ft (or meter). An easy rule from the safety stop would be ascending at 10 seconds per ft or 30 seconds per meter.

 

[Rollin]

Thank you all for the advice and especially Boulderjohn for answering my question.

@tursiops : as is common in diving manuals or study material, no source is given unfortunately. It appeared for the first time in 2014 in this course as far as I can see. So it is relatively recent.
I read quiet a lot about decompression strategies and indeed never encountered this.