Minimal decompression question, 20m and 40m.

[Kitzy]

So I recently did fundies but due to circumstances wasn't able to finish it, I remember (Though I might be remembering wrong) being told to always take the shallower depth for minimum decompression.

Now looking at the beginning with the end in mind book, it says that at 20m you should go to 12m (9m a minute) then switch to 3metre a minute, however that would support on gassing, wouldn't it?

Shouldn't you instead go to 9m, and go 3metre a minute from there?

Also for 40m it's much the same, you'd be at 22m (9metre a minute) with the average depth of 20m, you can go to 21m and switch to 3 meter a minute after, but wouldn't 18m be better?

Or would this all result in ascend rates that are too quick ?

 

[happy-diver]

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[whynot]

Post this here: DIR

Or, ask your instructor; I'm sure that they'll answer.

 

[PfcAJ]

So I recently did fundies but due to circumstances wasn't able to finish it, I remember (Though I might be remembering wrong) being told to always take the shallower depth for minimum decompression.

Now looking at the beginning with the end in mind book, it says that at 20m you should go to 12m (9m a minute) then switch to 3metre a minute, however that would support on gassing, wouldn't it?

Shouldn't you instead go to 9m, and go 3metre a minute from there?

Also for 40m it's much the same, you'd be at 22m (9metre a minute) with the average depth of 20m, you can go to 21m and switch to 3 meter a minute after, but wouldn't 18m be better?

Or would this all result in ascend rates that are too quick ?

The min deco ascent schedules miss a key variable in answering your question: time at avg depth.

When you ask if something is better, it’s tough to say without knowing the whole picture.

 

[boulderjohn]

I asked GUE headquarters to give me a through explanation of these procedures back when I was planning to write an article on differing ascent procedures for NDL dives. I got a very thorough response, and I still have the emails with that response. Here are a few key portions related to your question.

• The one (and really only) decompression-related theory behind it is the belief that the final ascent to the surface should be done more slowly than the initial ascent.
• The halfway point for the transition is chosen not because of any relationship to deep stop theory but rather because any idiot can calculate half easily. (I was specifically told it is NOT a deep stop strategy.)
• The series of 3-meter stops to the surface has two purposes:
  • It slows the ascent considerably.
  • It mimics the process used for decompression dive ascents, so it is a consistent procedure.

I eventually abandoned my attempt to write an article on NDL ascent strategies because, as Simon Mitchell warned me when I started, there is not enough good research available. The common strategy of a slow ascent followed by a safety stop has been studied extensively and has been found to be very safe. Alternative approaches have not been studied enough for any meaningful comparison. A couple studies were done on deep stops a couple decades ago, but those studies are not highly regarded (except by DAN Europe, for some reason).

The overwhelming majority of NDL dives are almost certainly multi-level, and that shows the wide variety of possible ascent profiles that can still end successfully with a simple safety stop. Comparing those ascent profiles to see if any one is better than another is probably not a fruitful use of time and money.

 

[Ayisha]

...series of 3-minute stops to the surface has two purposes:

Series of 3-minute stops?

 

[OrcasC205]

Series of 3-minute stops?

Should be a series of 1 minute "slide and stops" where you leave each 10 foot (or 3 meter) incremental depth at the start of each minute of the ascent. If it takes you 30 seconds to ascend 10 feet, you pause for 30 seconds before starting the next step. Or if it only takes you 20 seconds to ascend, you pause 40 seconds, etc.

 

[Kitzy]

Thanks for the replies everyone, I just realized I'm a giant derp and 21m should be the next best depth (Dividable by 3m, shallower then 22m), if I'm not mistaken

 

[boulderjohn]

Series of 3-minute stops?

Obvious typo--meant 3-meter.