Unrealistic Deco Time

[WetLens]

Thanks for everyone's input. As you may have noticed, I did post this in Sherwood's thread area - they haven't entered the fray as yet.

Doc: I know you are not trying to dog me. I certainly did not take it that way. It is just that I cannot fathom an algorithym telling me to stop at 46' for 4 minutes on a dive that had an average depth of 47'. Unless I completely misunderstand the interface on my computer, it was telling me that I needed to go to 10'.

I downloaded a trial version of GAP-Diveplanner and ran the day's dive sequence - squared off a bit and always on the conservative (deeper/longer) side.

RGBM at +2 conservative gave me (interestingly enough) 4 minutes at 10'.
BuhlmannC-GF with Conservatism set at 100% - direct ascent okay.
90% - 1 @ 20; 5 @10
80% - 1 @ 20; 14 @ 10
70% - 1 @ 20; 25 @ 10
60% - 1 @ 20; 44 @ 10

From there, I began taking away minutes from my bottom time. These models followed a predictable path - each increment of minutes taken off (I went in 5 minute increments) subtracted a reasonable number of minutes of deco obligation. At 60%, I could make an 18 minute dive to 60' with no deco obligation. Everything beyond that build deco obligation in a rational progression. This makes sense to me.

Going into deco at 47', 68 minutes into the dive with 4 minutes at 10' makes sense.
One minute later - 45 feet; 69 minutes- to have 15 minutes at 10' does not make sense.
Two minutes after that - 42 feet; 71 minutes- now 24 minutes at 10' ???
Two minutes after that - 13 feet; 73 minutes- now 43 minutes at 10' Come on.... that is not a rational progression.

I can follow the model. I understand this stuff. I don't understand how the Wisdom handled the situation.

 

[WetLens]

#1 - I need to get a life... I am spending far too much time on this for a beautiful day in NC.

#2 - DiveNav: Thanks for the input. I read Baker's paper. Good stuff. I have a fairly good understanding of the material covered and knew much of it from prior reading. I was an active instructor while dive computers were being introduced, listening and reading to anything I could get my hands on to better understand how I was getting to spend so much more time underwater. Agian, thanks.

If I had more details on your dive profiles (not just max depth and average depth) AND surface intervals ..... I could easily replicate your dives with our simulator in few minutes (with a time warping feature just for this) ..... but I am quite sure it will kill me if I would ascend from the fourth dive as You did
Our simulator is based on straight Buhlmann ZHL16C algorithm (quite close to what is used in your dive computer i believe).

#3 - Want data? I got it. I also carry a Sensus Ultra data collection device. I have all of the dives in 10 second intervals.

I did download V-Planner. Similar results to GAP-Diveplanner with 6 minutes at 10' (+2 conservative) vs. GAP's 4 minutes.

 

[DiveNav]

.........
Going into deco at 47', 68 minutes into the dive with 4 minutes at 10' makes sense.
One minute later - 45 feet; 69 minutes- to have 15 minutes at 10' does not make sense.
Two minutes after that - 42 feet; 71 minutes- now 24 minutes at 10' ???
Two minutes after that - 13 feet; 73 minutes- now 43 minutes at 10' Come on.... that is not a rational progression.....

Actually .... it could make sense .... when the leading compartment is changing (going to a slower one) while ascending.
Remember that the ZHL16C formulas are not linear at all

Alberto (aka eDiver)

 

[DiveNav]

#1 - I need to get a life... I am spending far too much time on this for a beautiful day in NC..

me too .... as soon as I am done I will fire up my Triumph Rocket 3 and will go for a ride on the Ortega HWY looking to challenge some SoCal Harley's riders

.....#3 - Want data? I got it. I also carry a Sensus Ultra data collection device. I have all of the dives in 10 second intervals....

Data for the 4 dives in question (including SI). I have the Sensus too and I think I can import your files then run the data into our simulator and produce compartments loadings as per ZHL16C (that it should not be too much different from what is inside your dive computer)

 

[Doc Intrepid]

Like the rest of you guys, I seriously need to get a life.

But two other thoughts come to mind (just because I like messing with this stuff). While trying to find out which specific algorithm was programmed into the Wisdom, I noted this little gem:

"Deco Calculations: Calculates decompression stops to 60 feet"

from:
Scuba.com Video: Sherwood Scuba Wisdom 2 Dive Computer Instructional Video - Computers

My thought is that if the Wisdom computer is calculating deco stops down to 60 fsw, then logically it would display them at the depths they need to be held, rather than displaying all of them as being required at the 10' stop. (I've no idea HOW they would be displayed, but like Ian said, the computer should not necessarily be displaying every deco stop as being required at the 10' stop - although I agree that required stops generally display at standard 10' intervals.)

Second, (again, I'm not familiar with the Wisdom specifically,) but WetLens if you used the same computer for the three earlier dives, then it likely was not calculating it's parameters based on one discrete 47' dive - rather, it factored your 4th dive parameters together with the residual nitrogen loading from the first three dives - in effect one long dive calculation. But I don't know what relative weighting it would have used for the rate of N2 offgassing during surface intervals, etc.

The only explanation (that I can see) for the geometric deco obligation progression is that the Wisdom was somehow recalculating for omitted deco when it added nearly 40 minutes of deco to the 10' stop over a ascent of 3 minutes duration. I'm not able to offer any rational explanation otherwise.

It would be good to hear from one of the Sherwood techs on this, if/when they weigh in.

And now....it's Harley time!

see you guys later!

 

[Doc Intrepid]

I find that really hard to believe. Have never seen a computer that wanted a 46' stop. 40 feet or 50 feet,yes,but never 46'

Also when a computer first goes into deco the initial stop is always going to be at 10 feet. Any dive requiring 4 minutes in the 40-50 foot range is a full on deco dive,not an NDL dive that slipped over the "limit"

I have an Aeris Atmos pro and have seen it do weird things a couple of times. I once did a 60 foot NDL dive followed an hour or so later by a 130 foot deco dive.Computer got very upset and started asking for insane amounts of deco. (Perhaps 40 minutes when V planner wanted 10)
My take is thats I was using the computer way outside its design parameters and it really did not know what to do. Suspect the computer in the OP had the same issues??

Ian, you could be completely correct. The only plausible reasons that I could see that the computer might have called for 4 minutes at (somewhere between 50' and 40') are first that the 4th dive followed three earlier dives in excess of 70' for in excess of 70 minutes each, which would have resulted in a pretty good N2 load - which it may have calculated as one long N2 exposure (in other words, treating it as 'a full on deco dive'); and second, somewhere in some of the sales literature it mentioned that the Wisdom algorithm incorporated 'deep stops'. (However they define that term.) I took that to mean that the computer might recognize that a deco stop of 4 minutes at say 40' might shave off a considerably longer hang time at 10' - but not knowing what the algorithm is, etc. makes this sheer speculation on my part.

It could also be - as you suspect - that the computer is simply not well set up to calculate repetitive dives that wind up with relatively (for recreational diving) longer deco requirements. No way to know without some additional info...

 

[Thalassamania]

The Deep Stop on the Wisdom II is three minutes, it is optional, the computer does not penalize you if you ignore it.

 

[Doc Intrepid]

Thanks, Thal.

(What algorithm is it using?)

 

[TSandM]

I'm with you, WetLens. This makes very little sense to me, and I can't really put together a deco strategy that would give you 45 minutes at ten feet.

I know that, as I go into deco with V-planner running, it will start by giving me a 10' ceiling and increment time to a certain, fairly short point (I think it's around 10 minutes, but I don't watch it that closely). Then it will lower the ceiling to 20, and give me a short stop there and a longer stop at 10. The longer I stay down, the lower the ceiling, but the stops are always graded longer as you get shallower.

The only reason I can think of to increment to 40+ minutes at 10 feet is that the program only has a forced 10' stop built into it, and the only way it can handle increasing deco obligation is by incrementing that stop. (Which would make sense, in a way, with recreational diving, because you're going to be gas-limited on deco and therefore probably better off doing it as shallow as possible.) The algorithms used in recreational diving computers are not designed to calculate true staged decompression profiles, so my guess is that this is some kind of a kludge. Why it incremented so fast as you ascended is another good question.

 

[WetLens]

DiveNav: Data attached in spreadsheet format. I included all dives leading up to the day's events. I ignored prior day's dives in the simulations I built, but thought you might want them. Thanks for taking an interest.

Edit: Oops, file was a bit too big. Dropped repeating cells and deleted temperature column to get under 400k.