Manual calculation for accelerated deco

[lowviz]

Nice obfuscation.

You know that you are wrong. Do the math.

 

[boulderjohn]

I think that I see your concern. (Please pardon me for speaking out of turn)

Yes, at altitude your stop sequence will become 'compressed' with respect to depth and you will have to add stops at the lower end of your schedule. These aren't really 'deep stops' (ala' Pyle) but rather necessary additional stops that you have to add to get you off the bottom properly.

@boulderjohn

The result of this is that you will end up needing more stops when diving the same depth if at altitude. The stops will be more numerous and closer (with respect to depth) than an equivalent dive to the same depth at sea level...

@Dr Simon Mitchell, can you explain why none of the various software algorithms have done what lowviz says must be done?

 

[victorzamora]

He explained that as you get higher and higher in altitude, more and more factors come into play. For example, the incompressibility of water vapor droplets in your breathing becomes a factor. He said there are only about 6 people in the world with the expertise to plan such a dive. The decompression stop needs for a decompression dive at that altitude would be enormous.

This is one of those things that'll certainly never apply to me, but is super interesting nonetheless. I'd love to see what a "simple" deco dive at that altitude would look like planned by one of those 6 folks.

 

[Dan_P]

Okay, I'm going to stick my hand up on this one.

I'm not comfortable diving to altitudes like 5km, at all. I figured it'd never be an issue, permafrost and all.
So, yes, I took it to heart when RD was called out because I was thinking maybe 2km, 3km at most.

But 5.000m? I have no idea, and I've never been there.
There you have it.

I wouldn't dive to that altitude, and if I did need to dive at that altitude, I'd probably want to contact someone other than myself for advice on the decompression side of things.

If you want to pin that on UTD, we'll have to agree to disagree in so far.

Also, again, apologies to all involved. I didn't explain myself well enough at several points, but more importantly, I did bring into the discussion a matter that was incontextual, and which caused significant distraction from the matter at hand (a matter I'll agree is most interesting, as Victor points out).

Heart, sleeve, and goodnight.

Best Wishes and Happy Holidays from Scandinavia.

Safe diving

 

[boulderjohn]

Just for grins, here is a comparison of a dive plan in Bulmann, first at sea level and then at 2,000 meters, both in fresh water. (I converted to feet (6562) becauses that is how I am comfortable doing it.)

ZHL code by Erik C. Baker.
Decompression model: ZHL16-C + GF
DIVE PLAN
Surface interval = 1 day 0 hr 0 min.
Elevation = 0ft
Conservatism = GF 50/80
Dec to 200ft (3) Trimix 18/45 60ft/min descent.
Level 200ft 26:40 (30) Trimix 18/45 1.24 ppO2, 76ft ead, 95ft end
Asc to 100ft (33) Trimix 18/45 -30ft/min ascent.
Stop at 100ft 0:40 (34) Trimix 18/45 0.71 ppO2, 29ft ead, 40ft end
Stop at 90ft 1:00 (35) Trimix 18/45 0.66 ppO2, 24ft ead, 34ft end
Stop at 80ft 3:00 (38) Trimix 18/45 0.60 ppO2, 19ft ead, 29ft end
Stop at 70ft 2:00 (40) Nitrox 50 1.53 ppO2, 32ft ead
Stop at 60ft 3:00 (43) Nitrox 50 1.38 ppO2, 26ft ead
Stop at 50ft 3:00 (46) Nitrox 50 1.24 ppO2, 19ft ead
Stop at 40ft 6:00 (52) Nitrox 50 1.09 ppO2, 13ft ead
Stop at 30ft 8:00 (60) Nitrox 50 0.94 ppO2, 7ft ead
Stop at 20ft 10:00 (70) Oxygen 1.59 ppO2, 0ft ead
Stop at 10ft 20:00 (90) Oxygen 1.29 ppO2, 0ft ead
Surface (90) Oxygen -20ft/min ascent.


MultiDeco 4.14 by Ross Hemingway,
ZHL code by Erik C. Baker.
Decompression model: ZHL16-C + GF
DIVE PLAN
Surface interval = 1 day 0 hr 0 min.
Elevation = 6,562ft (c)
Conservatism = GF 50/80
Dec to 200ft (3) Trimix 18/45 60ft/min descent.
Level 200ft 26:40 (30) Trimix 18/45 1.20 ppO2, 72ft ead, 91ft end
Asc to 100ft (33) Trimix 18/45 -30ft/min ascent.
Stop at 100ft 0:40 (34) Trimix 18/45 0.67 ppO2, 25ft ead, 36ft end
Stop at 90ft 2:00 (36) Trimix 18/45 0.62 ppO2, 21ft ead, 30ft end
Stop at 80ft 2:00 (38) Nitrox 50 1.57 ppO2, 34ft ead
Stop at 70ft 2:00 (40) Nitrox 50 1.42 ppO2, 27ft ead
Stop at 60ft 2:00 (42) Nitrox 50 1.28 ppO2, 21ft ead
Stop at 50ft 4:00 (46) Nitrox 50 1.13 ppO2, 15ft ead
Stop at 40ft 6:00 (52) Nitrox 50 0.98 ppO2, 8ft ead
Stop at 30ft 9:00 (61) Nitrox 50 0.83 ppO2, 2ft ead
Stop at 20ft 12:00 (73) Oxygen 1.37 ppO2, 0ft ead
Stop at 10ft 24:00 (97) Oxygen 1.08 ppO2, 0ft ead
Surface (97) Oxygen -20ft/min ascent.

Notice the ascents are identical through the 40 foot stops. After that the 30, 20, and 10 foot stops get an additional 1, 2, and 4 minutes respectively, for a total of 7 extra minutes at the shallow stops.

 

[lowviz]

I have no doubts whatsoever that this dive could be accomplished safely at elevation using a mandated 10' stop sequence. That is not the issue.

The question is, could this dive be accomplished just as safely by scaling the stop depths to shallower levels (due to altitude), setting the stop times to an equivalent ocean depth, and adding any necessary stops to the deep end? I *think* that this is what @Dan_P was struggling with.

It is an interesting concept.

 

[victorzamora]

Calculate the stops for a dive to 40 meters at sea level.

Apply Dr. Mitchell's suggested correction to those stop depths at 0.5 ATM elevation.

You really plan to ascend from the bottom to the first calculated stop?

One issue I just thought of today: these deco algorithms force you into stops at 10ft increments. You'd have to do some pretty aggressive fiddling to get an algorithm that'll give you stops in increments that would output what you're wanting to match up with your "atmospheric correction" (for lack of a better term).

One thing that I found interesting was that the profile that John posted was one dive at two altitudes. What you're seeking is two dives done at two altitudes. I'll see if I have time to play with that when I get home.

From what I have now, though: John's dive had more shallow stops but similar depth deep stops. With your "atmospheric correction" the deep stops would be deeper on the high-altitude profile, and it'd more closely emulate a deeper dive done at sea level...similar to the atmospheric correction you're talking about.

 

[victorzamora]

I have no doubts whatsoever that this dive could be accomplished safely at elevation using a mandated 10' stop sequence. That is not the issue.

The question is, could this dive be accomplished just as safely by scaling the stop depths to shallower levels (due to altitude), setting the stop times to an equivalent ocean depth, and adding any necessary stops to the deep end? I *think* that this is what @Dan_P was struggling with.

It is an interesting concept.

So 10ft is 30.3% of a sea-level atmosphere. It'd be 8ft stop increments in the "atmospheric-correction" model.

 

[lowviz]

@victorzamora Yes, exactly. I await the decision of the experts.

Now if someone were to rewrite the software for VPM allowing for atmospherically corrected depth...

 

[Dan_P]

If Buhlmann was right about the first stop depth being identical on an ascend from a similar dive at sea level and 2000m, he was preconceiving and accurately accounting for factors conflicting with - or taking into account - concepts such as m-values and gradients affecting dissolution versus gas mechanics, and at which ratio, all to a degree that not just casts a shadow upon us all today, but indeed, most likely, tomorrow as well.

In other words, due to the increased rate at which ascending at altitude - compared to similar ascend at sea level - would impact tissue supersaturation, (because of increased relative pressure differential) there is no way except factors outside now current understanding of the mechanics of slow-tissue on-gassing to explain why such models would not impose an earlier (deeper) first stop time.
This imposes an implicit paradox, as this knowledge was unavailable at the time of formulating the relevant models - and at the same time, implies a complete and unlimited application of a dissolved gas-paradigm to questions decompression.

All this would imply reliance on not only knowledge secured at no time before the NAUI-study, but also dissolution gas concepts to be true and accurate on their own.

I dare argue, this is unlikely.

If M-values alone are a valid determinator for stop depths, those stop depths would be deeper on an altitude dive than a sea level equal, due to increased impact of relative pressure differential at altitude.