RD lost deco gas planning

[rjack321]

Sorry to hog all the bandwidth on "DIR" stuff, but what the hell work is really boring...

Lost deco gas planning in RD...
Assumption:
EAN50 has a 50% time advantage over backgas deco
O2 has a 50% time advantage over backdeco (careful, O2 is not assumed to have a 50% advantage over EAN50)


Dive 150ft, 25mins
21/35 backgas
25mins deco on EAN50 + some deep stops

With all gases the profile (for me) would look like this, 13mins shallow, 12mins deeper:
10-5
20-8
30-2
40-1
50-1
60-4
70-4
80-1
90-1
100-1
110-30sec slide.

Assume lost EAN50 now in the context of the 2x advantage over backgas. A backgas version of this profile would have 50mins of deco shaped exponentially shallower. This is one technique but completely ignores the possibility of any EAN50 50% time advantage.

Instead you can do a bit of a hydrid, each sharing gas for the entire duration of the deeper stops where the "o2 window" (theoretically) exists. I.e. I do my complete stop on your bottle of EAN50, then you do your complete stop time while I am on backgas.

So each person would do:
10-5 on backgas
20-8 on backgas
30-2 + 2 on backgas
40-1 + 1 on backgas
50-1 + 1 on backgas
60-4 + 4 on backgas
70-4 + 4 on backgas
80-1
90-1
100-1
110-30sec slide.

Somewhere about the 30ft stop we'll assume the bottle runs out.

Net, this is the equivalent EAN50 deco:
10-5/2 = 2.5
20-8/2 = 4
30-2 + 2/2 = 3
40-1 + 1/2 = 1.5
50-1 + 1/2 = 1.5
60-4 + 4/2 = 6
70-4 + 4/2 = 6
80-1
90-1
100-1
110-30sec slide.

Ta da, the sum of that time is equivelant to 24.5 mins of EAN50 deco, essentially what you would have done on EAN50 if you hadn't lost it. A prudent diver would use the EAN50 until its gone, then depending on when it ran out pad the shallow stops fairly generously depending on how much backgas you have remaining. If you brought enough rock bottom you should have a fair bit in reserve still and can do the "full time" (4 & 8 mins in this example) of the shallow time. The EAN50 has only a modest deco advantage over backgas on the shallow stops anyway.

The same sharing 50% time advantage principle can be used on O2. If you need to I can elaborate on that in another post.

 

[Cave Diver]

Sorry to hog all the bandwidth on "DIR" stuff, but what the hell work is really boring...

We have plenty of bandwidth. Hog away!

 

[rjack321]

Me and my posse has never actually lost EAN50 on a dive before. Limeyx did lose his O2 once on an O2 deco dive (no EAN50 along). Oring extruded on the LP hose. Its was all mis-shapen and not going to reseat. I did the "full time" on O2 (10mins I think). Then he did 7mins on O2. Net he did the equivelant of 12mins of O2. 10mins backgas/2 = 5 + 7 mins on O2. We were a long way from anywhere and doing a little extra O2 time was prudent.

We just did this schedule modification on the fly from first principles.

 

[Cave Diver]

I haven't ever looked really closely at RD but the bits and pieces I've seen seem to run similar to the article that Doppler wrote on decompression several years ago. It's definitely something I'd like to see more postings on, from an informational standpoint.

 

[rjack321]

I haven't ever looked really closely at RD but the bits and pieces I've seen seem to run similar to the article that Doppler wrote on decompression several years ago. It's definitely something I'd like to see more postings on, from an informational standpoint.

Doppler's version IMHO is a little more like "If you try this you'll notice a pattern" which is good for some folks. I needed to be told the pattern before I could "see" it. Now it just comes naturally, at least down to about 210ft. Below that I don't have enough experience to rattle a functional profile off the top of my head. I have to work it out/add it up ahead of time on a peice of scrap paper. I have done some simulations in Vplanner to see the impacts of changing gases etc but I don't feel the need to "confirm" a plan close to what I've done before with software anymore.

I suspect each explanation appeals to different folks for different reasons.

 

[limeyx]

Rainer on the other thread asked about why UTD do not do 1.5x on the 50 and up stops in the case of a lost deco bottle (assuming you consume the 50% at 70,60) and potentially chalked it up to the superhuman properties of AG

BUT think about this

#1 -- you are no longer on an S-curve for that bottle once you lose it, you are back to exponential (of course with some benefit of being on the 50% gas to in theory reduce bubbles)
#2 at 30,20,10 you are really using gradient not PPO2 assuming you dont have O2. Look at the PPo2 of 50% at these depths.

So consider this

Deco schedule on 50% for 30 @ 150 (starting at 70)

4,4,2,2,3,10,6up (31 mins)

if you lose the 50% at the begin, you have 30 mins deco (on the bottle) or to look another way, 60 mins on BG

We say the 50% bottle effectively halves the deco of a segment (70-30 or 70-surface), but that's an average over that whole range

so the lost bottle schedule is now like this

70,60 = 10,10 (5 on bottle, 5 on BG at each stop)

so the "total credit" to your 60 mins

70 foot: (5*2) + 5 (15 mins per diver)
60 foot: (5*2) + 5 (15 mins per diver)

so when you arrive at 50 feet, you effectively already have 1/2 the deco done. Yes you can argue that it was done "in the wrong place" (i.e. 10 mins of BG deco at 70,60 doesn't really mean 10 mins because the gradient was low). But that's still not going to make me hang out at 50,40

so now you do the rest unmodified (remember we "should" be doing something more like fobbonacci/exponential now not S-curve)

so now you do 2,2,3 on BG

Total credit: 30+7 = 37 mins

now you go to 20, and do say 15 mins
so you have total credit 52 mins.
Now you do 6 min up for a total of 58 mins

yes you did a lot of it "in the wrong place" and yes, the 40,30 stops are shorter than buhlman (but remember in theory you have way fewer bubbles due to the 50%)

All I did was the "alternate" 20 foot stop where I just didnt count the 6 min ascent as part of the 20 foot stop, so effectively adding 5 mins to the 20 foot stop.

This gets even better if you have O2 and then again extend the 20 foot stop by 5 mins. You also use less BG this way, and are shallower longer so closer to help.

Just something to think about.

Apologies for the (almost) "Malin-length" post

 

[Rainer]

It's the not extending the shallow (20-10') stops part that I don't really get.

You're really doing nothing for yourself on a quick (no extra time), low PPO2 (all backgas) 50-30' section on this method. Why not do at least 1.5 (if not 2) x time at 20' to make up for it?

That 10 minutes of time I did at 70-60' on 18/45 (backgas) really isn't helping the deco much. I'm not as thrilled as you to call the deco half over at that point.

I'd be interested to hear from those who have run either of these "lost deco" profiles. Ever hear any stories about how people felt after either?

Yes, this is all a lot easier/cleaner if you can switch to O2 at 20'.

 

[limeyx]

It's the not extending the shallow (20-10') stops part that I don't really get.

You're really doing nothing for yourself on a quick (no extra time), low PPO2 (all backgas) 50-30' section on this method. Why not do at least 1.5 x time at 20' to make up for it?

That 10 minutes of time I did at 70-60' on 18/45 (backgas) really isn't helping the deco much. I'm not as thrilled as you to call the deco half over at that point.

I'd be interested to hear from those who have run either of these "lost deco" profiles. Ever hear any stories about how people felt after either?

Yes, this is all a lot easier/cleaner if you can switch to O2 at 20'.

sure you can extend the 20 foot stop by 50% if you want to, but all the crap in the middle -- no probably not.

I bet you dont get hardly any benefit from 50-30 on 50% anyway, and on a T2 dive even most of the 30 foot stop will be on backgas anyway.

I have so far not talked to anyone actually having to execute one of these profiles out of necessity but I have had talks with people who have done some pretty whacky experiments.

Bottom line is yes if you want to extend the 20 foot stop, you could talk me into it, but I wouldn't really extend anything else in between.

 

[limeyx]

It's the not extending the shallow (20-10') stops part that I don't really get.
......
If you wanted to extend the 50'-surface (by 1.5 x time), I'd have less issue with UTD's method. For me, as is, it places too much emphasis on the O2 window (which admittedly, the GUE method under-emphasizes).

really ?

 

[rjack321]

If you wanted to extend the 50'-surface (by 1.5 x time), I'd have less issue with UTD's method. For me, as is, it places too much emphasis on the O2 window (which admittedly, the GUE method under-emphasizes).

1) Extending the shallow time by 1.5x on backgas is now assuming that you were getting a 50% time advantage from the EAN50 up there (shallow). The time advantage is derived from the average over all stops the gas is used for. EAN50 is only "used for" the 70-30ft segment - Tech1 level EAN50 dives are actually the exception. You've taken the exception and made it the rule that the 50% time advantage is derived from the entire 70-surface segment while that is not assumed for deeper dives.

2) Streetdoctor's profile has more "O2 window" emphasis than I've ever seen. Limey and I did a profile like that once and I felt like crap. He thinks he was fine but was asleep on the couch so I don't know how he knows Maybe is was because of the shape, maybe because it was the 2nd dive of the day. In any case, I don't think its fair to say UTD emphasizes the O2 window more or less than GUE anymore, it depends on the circumstances now.

All that said I would have no problem padding the shallow backgas stops in this lost deco gas scenario. Even though I'm unlikely to actually do an EAN50 only dive much anymore.