forum.heinrichsweikamp.com

Hi !

Could somebody be so kind to explain the deco behavior of my ostc 2n:

I went down with Nx 26. At 30m @5mins change to 22/36. At 32 min change to Nx 26.

At 33.6 mins at 38 m Deco notice: 2mins at 38 m ?

Is there an explanation for this ?

Thanks a lot in advance

Did your ostc show this decostop during the dive, or is it only the desktop application (subsurface) which shows it? In the latter case, it might be a bug in the subsurface (or even libdivecomputer). If you provide a memory dump of your device, I can have a look:

http://www.libdivecomputer.org/contribute.html#memory

Yes, it showed my at my dive shortly after i changed the gas from Tx 22/36 back to Nx 26.

I was a little suprised when i saw the deco notice red letter of missing a deco stop in this situation, because it made no sense, doing a deepstop with 1min after changing gas from trimix to nitrox in this deep. And you see, I'm still alive

Okay, here we go: I took a memdump, read out the dives (it's dive 1 from the 19.10, which is the important one) an looged all.

The rest of the files in the next post due to restrictions...

Thanks a lot in advance !!!

here are the rest of the files.

I took a look at the .XML File.

Here is the sequence with the mistake attached as jpeg.

Hi,

just an idea and without a close look on your data: What are your GF-settings? Maybe the "jump" from 36% He to 0% He in almost 40m depth resulted in a too high tissue saturation with He for your lower GF - and therefore the OSTC calculated this deco-stop.

Regards,

Jörn

Tom Wrote:
-------------------------------------------------------
> I took a look at the .XML File.
>
> Here is the sequence with the mistake attached as
> jpeg.

As you already discovered, the 38m decostop is indeed stored in the ostc data. So it's not a bug in subsurface/libdivecomputer. They are just reporting what's in the ostc data.

Hi Tom,

Are you aware of ``Isobaric counter diffusion''? the basic rule is :

Never let the partial pressure of Nitrogen rise (significantly)

When you switch from TX22/36 to EAN26 at 40m, you switch from 42% to 74% N2,
which is an increase of 76% in PPN2!!!

See f.i. the book "Deco for Divers" by MArk Powell (very good book IMHO).

For this dive to 44m. two better option would be either
-- the cheap one: use 2 gases (Air first then EAN50 for deco), or
-- as you can afford trimix: TX22/36 as first gas then EAN60 at 17m for deco.

Best wishes,
--
Daniel Flipo.

Hey Daniel,

you are right, but at this situation there was no danger of an isobaric counter diffusion.

I changed for TX 22/36 back to Nx 26. HE is the faster gas, N2 the slower diffusing gas. I had Argon in my drysuit, so that there was no danger of significant rise of Nitrogen.

Deco for divers ist a very good book.

The gas-setup was Nx 26, at 30m change to Tx 22/36; change at 40m after the Tx stage was empty back on Nx 26, ascending to 21m, change on EAN 50....

I think EAN 50 is better than EAN 60, because you can start the deco at a deeper level, and the pressure difference in the oxygen window is higher than at 17m, so deco is more effective-

But back to the issue: Why showed me the OSTC a deco level ?!?

My GF settings are GF lo: 20% and Gf high 85%.

My friend has the opinion, that the ostc planned the first deco level deeper to get the max. ppO2 of 1.6 for deco. I think this could be the solution of the issue...

It is because of "Isobaric counter diffusion"

When coming off trimix you shouldn't breath a mix that has a higher % of nitrogen than 1.5 (IANTD 1.4) times the % you had in trimix

So with 22/36 you have 42% of N2 and the next mix should be max 63% N2 (Nx37)

If you have a higher % of N2 then your partialpressure is so high that He cant come out and you are ongassing nitrogen.

So the computer is OK and trying to save your ass!

(maybe a quick look at the book or a tx course?)

Tom Wrote:
-------------------------------------------------------
> Hi !
>
> Could somebody be so kind to explain the deco
> behavior of my ostc 2n:
>
> I went down with Nx 26. At 30m @5mins change to
> 22/36. At 32 min change to Nx 26.
>
> At 33.6 mins at 38 m Deco notice: 2mins at 38 m ?
>
> Is there an explanation for this ?
>
> Thanks a lot in advance


i think you set Gas Switch (CF55) at 2min
and you set MOD for Nx 26 at 38m

Bardass Wrote:
> i think you set Gas Switch (CF55) at 2min
> and you set MOD for Nx 26 at 38m

For those who follow the bitbucket code repository: I have now removed the CF55 stuff from the deco list and will show the gas change stops differently.

Isobaric counter diffusion is an issue with your gas setup here, but I think this was caused because you configured your travel gas also as a deco gas here. It's better to use two gases in the list (Both with Nx26, one as "First" and one as Decogas with change depth=38m) for a dive like this.

Regards,
Matthias

Hi Matthias,

heinrichsweikamp Wrote:
-------------------------------------------------------
>I have now removed the CF55 stuff from
> the deco list and will show the gas change stops
> differently.

so CF55 is not used any more with Beta 2.74 and the following?
Configure the Gasswitch stop with the decogas?

Regards
Rob

Hi Rob,

CF55 will still be used as before but the gas change stops will not be added to the decolist anymore.

Regards,
Matthias