Which ascent profile to use when?

[rjack321]

We've been doing the same 10 feet/min from 1/2 depth that GUE teaches in DIR-F ( and is taught in essentials), but have been adding a 6 minute ascent from 20 feet (if we know we are close to the boat etc and not drifting).

Are you budgeting for this in your gas calculations or do you plan on acclerating through this slow section in an emergency?

BTW, Curt and I had a long discussion about the Marroni work/paper on another board. Its under "diving medicine-stops vs. pause and their lengths"

 

[StSomewhere]

FYI and to no one in particular:

The Essentials profile should be done on EAN32, not air.

 

[rjack321]

Other hyperoxic helium mixes work too. Actually, they work better than 32% for dives where you have more than 3 stops (i.e. greater than 60').

 

[StSomewhere]

Sure, He adds another element.

But BigTuna's question was specific to what Essentials covers. EAN32 for standard rec depths and times, which means depths at or shallower than 100 ft and within the MDL. This simplifies a lot of what is discussed (pauses vs. an actual deep stop, etc.) in that pdf.

I was just pointing out that implicit in the "newtonian" Essentials view is the assumption that the diver should be diving the standardized EAN32, and not air.

John

 

[rjack321]

Oops, yes you were on-topic, I was the one getting distracted.

 

[limeyx]

No contradiction really. Note that the slowing of ascent in the Essential profile starts at 50% of max depth. The ascent in the Marroni paper that came out so poorly was a 10fpm ascent starting right at the max depth. Big difference. (The profile taught in many DIR-F classes, 10fpm ascent from 80% max ata is much closer to the "bad" profile in the DAN Europe study, since the ascent is slowed at a depth before offgassing starts. A simple run of bubble model programs such as VPM or RGBM will easily show that.)

Were the Essentials profile applied to the Dan Europe study, the ascent from 82' would have slowed at 40', the same point the ascent is slowed in the dir-diver.com ascent. The point where the Essentials profile DOES deviate from Dan Europe findings is that they found either a deep stop, or a 6m/20' or both would provide even less doppler bubbling.

Officially this was not "taught" in my DIR-F but just presented as a set of ideas to get us thinking, and something to consider. I have not dove with a computer telling me what to do since then

What was actually presented in my case in DIR-F was *pauses* at 80% ATA, the stops not starting until 50% of depth (approx for recreational dives)

 

[limeyx]

We've been doing the same 10 feet/min from 1/2 depth that GUE teaches in DIR-F ( and is taught in essentials), but have been adding a 6 minute ascent from 20 feet (if we know we are close to the boat etc and not drifting).

Are you budgeting for this in your gas calculations or do you plan on acclerating through this slow section in an emergency?

BTW, Curt and I had a long discussion about the Marroni work/paper on another board. Its under "diving medicine-stops vs. pause and their lengths"

Interesting point regarding accounting for it in RB calcs. Answer is "kinda"
We plan RB for 1m at 20 and 1 min at 10, so obviously not enough to cover 6 min ascent from 20.

It's really an "if we did not hit RB, and we know we're close to boat/beach/exit point, and there's not a raging current, and we are not OOA and ..." well, you get the point.

Obviously we'd get to 20, check our gas, check our situation and the guy running deco makes a decision, the other guy can always veto that with a big "Thumbs" sign.

in a real emergency, I would probably blow off even the 50 and 40 stops if we hit OOA at rock bottom on a 100ft dive. Then hang at 30 or 20 for longer so we are closer to the surface.

Obviously my thinking might change once I've been hit over the head with a more advanced diving course!

 

[rjack321]

in a real emergency, I would probably blow off even the 50 and 40 stops if we hit OOA at rock bottom on a 100ft dive. Then hang at 30 or 20 for longer so we are closer to the surface.

Realize that the chances of your team having an OOA are virtually nil, and that the chances of some random dude coming up to you and asking for gas are higher. And that in this latter case, your new buddy will probably be trying to get up to 15' to do a safety stop. YMMV

I extend some shallow stops in good conditions on the fly too. Always bumping up against the minimum stop time seems too rigid to me - and more aggresive than necessary.

 

[limeyx]

Realize that the chances of your team having an OOA are virtually nil, and that the chances of some random dude coming up to you and asking for gas are higher. And that in this latter case, your new buddy will probably be trying to get up to 15' to do a safety stop. YMMV

I extend some shallow stops in good conditions on the fly too. Always bumping up against the minimum stop time seems too rigid to me - and more aggresive than necessary.

Right. I allow enough gas to do a full RB ascent with 1 min stops starting at 50% depth for two divers with 1cf/min breathing rate. If the OOA happened exactly at our RB pressure, then this in theory (if we keep breathing hard) allows us to surface with zero psi in tanks (not a good thing obviously)

so I'd have plenty to get a standard 15 foot/3 min stop ascent done

If I hit OOA earlier in the dive (and it was not some other guy coming up to us) then I would hope that I'd just be in a state of mind to do a controlled normal ascent. Of course, when it happens for real, YMM wil *always* "V" !

 

[BigTuna]

My purpose in this post is to clarify some points related to the key Marroni paper (cited in an earlier post) for non-technical divers (like me!) reading the thread. And I have some questions at the end.

In brief, Marroni thinks that DCS rates can be reduced if ascents are planned so as to manage bubbling in neurological spinal tissue. This is because about 2/3 of DCS cases seem to involve the spinal cord.

The following quote from the Marroni paper's abstract gives study's approach for "conventional" air ascents:

"In spite of many modifications to decompression algorithms, the incidence of decompression sickness (DCS) in scuba divers has changed very little.... The most serious signs and symptoms of DCS involve the spinal cord, with a tissue half time of only 12.5 minutes. It is proposed that present decompression schedules do not permit sufficient gas elimination from such fast tissues, resulting in bubble formation. Further, it is hypothesized that introduction of a deep stop will significantly reduce fast tissue bubble formation and neurological DCS risk....introduction of a deep stop significantly reduced Doppler detected bubbles together with tissue gas tensions in the 5 and 10 min tissues, which has implications for reducing the incidence of neurological DCS in divers."

Standard recreational, non-technical ascents on air such as Profiles 1 and 2 in my original post are not designed to manage fast tissue bubbles. Marroni's paper demonstrates that simply slowing the ascent rate to 30 fpm and adding a deep stop to the existing safety stops would make these profiles "good." (Maybe "better" would be a better word.)

So here are characteristics of an ascent profile (on air) consistent with the Marroni findings: 30 fpm ascent rate, deep stop, safety stop, and total ascent time comparable to neurological tissue half times, or about 12 minutes. Presumably the deep stops would add most value for dives to depths where the fast tissues are taxed significantly, which would be dives below 60 to 80 feet, or thereabouts.

How deep should the deep stop be? Marroni doesn't specify this, having selected a specific depth for a specific dive depth. I don't have a firm handle on this, but a good choice seems to be 1/2 the dive's maximum depth. From my playing around with V-Planner, this puts the stop somewhat above the depth at which off-gassing starts.

Here are some questions for you deco experts concerning recreational, "Marroni"-style deep stops:

GLOBAL QUESTIONS:
- How would you design an optimal Marrioni-style ascent?
- For what depths and durations would you use it?
- How would you modify the design for recreational nitrox diving?

OR...NITTY QUESTIONS:
- Is 1/2 max depth a good choice for the deep stops?
- Is it reasonable to drop the deep stop for dives shallower than 60 to 80 feet?
- Is a deep stop useful if the fast tissues haven't been significantly taxed, that is, if the bottom time is less than, say, 1/2 of the NDL time for the max depth?
- What's an optimum choice for the time at the deep stop? (Marroni used 5 min for both the deep and the safety stop.)