No Deco shown on a 45m dive (BT 22mins)

[Rebel]

Hi,
I was after some ideas as to why my OSTC 2N running in L16-GF OC (GF set to 25/80) mode on a 45m dive with a bottom time of 22min never went into deco (never displayed any stops). Gas's selected were EAN24 (Back gas) and O2 (deco gass). I've downloaded the profile (Diving Log 5.0) and the deco profile is also not shown there. Any ideas what I might have misconfigured for this to happen? This was the first time I changed the back gass to anything other than air, so my first suspiscions are that I did that incorrectly. Since then it has been running fine (deco is showing as it should).

Geoff.

[scubatinoo]

Gauge-Mode (Bottom timer)?

[Bardass]

firmeware ?
why are you using GF for this type of dive ?

With GF, you must have this plan :
22:00 45.0m Deco:
21 m : 1 ' 24 '
18 m : 1 ' 26 '
15 m : 2 ' 27 '
12 m : 3 ' 29 '
9 m : 4 ' 33 '
6 m : 4 ' 37 '
3 m : 6 ' 41 '
0 m : --- ' 47 '
TTS = 27 '

without GF, and CF11 = 120, CF12 = 80, this plan
22:04 45.0m Deco:
9 m : 3 ' 26 '
6 m : 5 ' 29 '
3 m : 8 ' 34 '
0 m : --- ' 42 '
TTS = 22 '

[scubatinoo]

Why shouldn't he use GF?

[Bardass]

because it is not useful without helium mixtures

[tiefunten]

@Bardass: It might not be necessary, but it is ok, too...

[PeterK]

@Rebel: Can you repeat this dive profile with the simulator? You also may want to check the gas settings again?

VLG
Peter

[gorcio]

@Bardass - could You elaborate more on why GFs are only to be used with TMX/helium dives ? Somehow I haven't noticed restriction for using GFs in air/nitrox dives ...

[sailor]

Hi,

GF was implemented in order to customize Buehlmann for Helium-mixes.

You can use GF for air or nitrox but it might increase the TTS with very little or no increase in safety margin.

My half cent, Reiner

[swissdiving]

wanna know about GF?
Read this:
A new slant on GF by K. Gurr

Of course there is a lot of other stuff to read. E. C. Baker is a good start.
Clearing up the confusion about deep stops by E. C. Baker

Have fun

[Rebel]

@Scubatinoo - the computer was definately running in L16-GF OC mode, it was verified after the dive (ie. not in guage mode).

@Bardass - Firmware is v2.00.

@PeterK - Gas settings were double, tripple checked. G4 set to EAN24 (selected as first gas) and G3 was selected and set to 100% O2. I'll try running the simulator again and see if it repeats the observation.

I have since dived around 10 more deco dives with it (all on air as backgass with either 100% and/or 50% as deco gasses) and it has worked flawlessly. My only thoughts are that somehow setting my backgass to anything other than air, or using G4 as my first gas somehow was wrong?

[JeanDo]

@Rebel

Firmware 2.00 is known to have a bug in gas management. I think it works better if you used Gas#1.
But anyway, please upgrade to the lastest stable release 2.01.

Regards

[gorcio]

swissdiving Wrote:

> Of course there is a lot of other stuff to read.
> E. C. Baker is a good start.
> Clearing up the confusion about deep stops by E.
> C. Baker


Thanks. Read them quite a while ago, and re-read them now. Still can't find anything in Erik Baker's work stating that it was done for helium dives. He only refers to 'deep' dives which is a bit arbitrary.
I am not discussing an issue of TTS, as it's obvious that using GFs on air dives would increase it.
My impression from reading the paper was that GFs were implemented as a prothesis to mimic bubble gas models ...

[swissdiving]

Salut Gorcio,

I tend to agree with you.

I also considered switching to GF Bühlmann for additional safety. From reading E. C. Baker I was not able to see clearly that the additional deep stops would be an advantage in the case of my dive profiles. Most of the time I dive air with a maximum depth of app. 40 - 50m without going to much in to deco; or should I say, the dive profile does not require a deco stop while returning leasurely to the surface.

GF has to be considered with every dive profile individually and as I understand have the greatest value with dives to depths beyond the "recreational" dive limitations (depth and time).

However this is my interpretation. I am sure there would be plenty of people who may disagree with me.

[PeterK]

@JeanDo: What bug in FW 2.00 do you mean? Do you refer to the issue with the defaults in gas 4, 5, and 6 or is there another issue?

[swissdiving]

Peter,

I suspect he is referring to this:
FW Upgrade 2.01

[Rebel]

gorcio Wrote:
-------------------------------------------------------
> Thanks. Read them quite a while ago, and re-read
> them now. Still can't find anything in Erik
> Baker's work stating that it was done for helium
> dives. He only refers to 'deep' dives which is a
> bit arbitrary.
> I am not discussing an issue of TTS, as it's
> obvious that using GFs on air dives would increase
> it.
> My impression from reading the paper was that GFs
> were implemented as a prothesis to mimic bubble
> gas models ...

The implemantation of GF as a way to include the bubble moddeling (e.g VPM-B) is my understanding also.
"A recent tool which provides a simpler solution between Buhlmann and the bubble models has been to use Gradient Factors on a dissolved gas model which in turn modifies the M values of the controlling compartments (Baker). This has the effect of combining a bubble+dissolved gas model style decompression profile" - 'A New Slant on Gradient Factors by Kevin Gurr'

However in the article 'Clearing Up the Confusion About "Deep Stops" ' the author uses a 13/50 Trimix gas in Figure's 1-3. And in 'Understanding M-values' the author is using a 15/40 trimix gas in Table 5. But I cannot find any reference in any of these texts that states that Gradient Factor compensation is only relevant for Trimix or for dive profiles only in the ranges considered in the examples of the articles (100m and 250ft respecively). What has been said in Kevin Gurr's article is that GF need to be adjusted for differnt depths/exposure times. So for a 30m dive a 20/80 GF is unecessary (100/100 i.e. no GF is appropriate).