Shearwater Teric NDL?

[Joneill]

I'm tired of hearing this "quote".....
Firstly, when and where did Powell say this? I can't find it in his book. What was the context of his statement (if he made it)? Was he being flippant? Colloquial? Joking? Rhetorical?
Secondly, in common usage "deco dive" is one with mandatory decompression stops, hence all dives are not "deco dives." Yes, all dives involve some decompression....but for an NDL dive that is done with ascent rates.
All dives involve decompression but not all dives are deco dives.
This has been discussed endlessly on SB; do we have to do it all over again?

Here you go - Google is your friend… lol

The exact quote you are tired of hearing is at 3:32 in:

 

[tursiops]

Here you go - Google is your friend… lol

The exact quote you are tired of hearing is at 3:32 in:

He says "all dives are decompression dives."
He does not say, "all dives are deco dives."
I am saying exactly what he is saying: an NDL dive means your ascent is sufficient to decompress, and no mandatory stops are needed.

 

[Joneill]

He says "all dives are decompression dives."
He does not say, "all dives are deco dives."
I am saying exactly what he is saying: an NDL dive means your ascent is sufficient to decompress, and no mandatory stops are needed.

That is your interpretation. I take “deco” as just an abbreviation for “decompression” - so, to me, he did say all dives are deco dives and his explanation of why makes perfect sense. He was also very clear on some dives needing actual, discrete decompression stops prior to surfacing (so as not to break the M value).

To me, it’s more important to understand what he lays out as the line can be blurred. I’ve been on dives that were supposed to be NDL, but went to required stops (so “deco” by your definition) and then went back to NDL before the stop depths were reached.

In those dives, had we needed to head to surface at certain points, they would have been “deco” dives as we would have had to make stops - however, we had time/gas and as we worked our way up to shallower depths the required stops disappeared (the DM knew this would be the case - he just neglected to tell us in advance).

I’ll add that those dives (Tiputa Pass in Rangiroa, btw) are certainly not typical for me - I do not normally see mandatory stops pop up on my computers in my diving. I just do a 3 min or 5 min safety stop (depending on max depth).

 

[iliketopetsharks]

Shortening deco by diving deeper – The Theoretical Diver

thetheoreticaldiver.org

Interesting blog entry about the behaviour of bubble models. Granted, it's VPM-B and not RGBM, but the latter is shrouded in mistery

 

[scubadada]

...To me, it’s more important to understand what he lays out as the line can be blurred. I’ve been on dives that were supposed to be NDL, but went to required stops (so “deco” by your definition) and then went back to NDL before the stop depths were reached.

In those dives, had we needed to head to surface at certain points, they would have been “deco” dives as we would have had to make stops - however, we had time/gas and as we worked our way up to shallower depths the required stops disappeared (the DM knew this would be the case - he just neglected to tell us in advance)...

So, these were not no stop dives, they were decompression dives for which you satisfied the deco obligation before surfacing.

Remember, you can make a normal ascent directly to the surface from a no stop dive. You were not able to do that from the dives you describe. You spent sufficient time at shallower depths to complete your decompression requirement whether you were at your stop/ceiling depth or not. A decompression dive may have a very short decompression obligation, or it may be much longer, either way, it limits your direct access to the surface.

 

[Joneill]

So, these were not no stop dives, they were decompression dives for which you satisfied the deco obligation before surfacing.

Remember, you can make a normal ascent directly to the surface from a no stop dive. You were not able to do that from the dives you describe. You spent sufficient time at shallower depths to complete your decompression requirement whether you were at your stop/ceiling depth or not. A decompression dive may have a very short decompression obligation, or it may be much longer, either way, it limits your direct access to the surface.

Understood. My point was that these were not intended (on my part - the DM later told me he knew we might have small deco obligations that would clear before needing to be executed - normal on the profile they dive) to be dives with decompression stops and the line can be crossed a bit at times.

On those dives, the required deco stops cleared well before I ever ascended to the depths the DC had indicated that I needed to do stops at. To me that is a bit different in practical terms to dives where you plan to accumulate obligations and must execute timed stops at those specific depths.

Therefore, as Mark spoke to in his presentation, understanding that all dives are actually decompression dives and the science behind that mindset helps one to be prepared for what might happen during a dive and understand how to address in order to be able to safely ascend.

 

[scubadada]

Hi @Joneill

I appreciate your position. It is probably best not to be surprised by an unexpected deco obligation, but to plan it ahead of time with regard to gas supply and backup. An unexpected deco obligation is a variation on the trust me dive.

 

[Joneill]

Hi @Joneill

I appreciate your position. It is probably best not to be surprised by an unexpected deco obligation, but to plan it ahead of time with regard to gas supply and backup. An unexpected deco obligation is a variation on the trust me dive.

Understood.

As I said, this is not typical for me. These were group drift dives in strong current at times (you were basically along for the ride once you entered the pass). We were not briefed on this pre-dive and when I saw my NDL dropping below 5, I showed the DM. He waved it off indicating not to worry about it. When I spoke to him after the dive, he told me that a “temporary deco obligation” was common on these profiles, but would clear well before needing to be done so was not anything to be bothered by. I was then prepared for this situation on the subsequent dives.

Gas was not an issue (but I always keep a keen eye on my gas) - ~1 hour dives and still surfaced with about 800 psi (and could have surface sooner if needed).

Honestly, in the almost 300 tropical location dives I’ve done, there is no real gas planning or opportunity for backup - you dive your tank and plan your turn (if needed) and ascent accordingly based on gas usage. I’ve never seen a pony or doubles in use; some get larger tanks (AL100/15 L), but that’s the extent.

 

[scubadada]

@Joneill

Understood.

No stop dives are just that and deco dives do not have free access to the surface,

 

[dmaziuk]

Shortening deco by diving deeper – The Theoretical Diver

thetheoreticaldiver.org

Interesting blog entry about the behaviour of bubble models. Granted, it's VPM-B and not RGBM, but the latter is shrouded in mistery

Weinke listed 5 (?) "Gradient Reduction" factors in one of his papers, it's a PDF that I downloaded from somewhere, so it should be findable if you're interested. (Some of which, like reverse profiles or "accumulation of decompression stress" over multiple dives/days, have since been shown to be questionable at best.)

IIRC his original model was shown to generate schedules identical to VPM-B and after much laughter and pointing of fingers from all sides, he came up with the new and improved version that is more-or-less shrouded in mystery. More importantly, the various implementations, especially the "folded" ones for low-powered devices, could be deviating from VPM in even stranger and more mysterious ways.