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OSTC cR deco
Posted by: opiffe ()
Date: May 05, 2019 07:19PM

Hi all

I have been diving with OSTC computers for a long time now and I have always been happy with them and with H&W. First I used to dive with a 2N, and later, as I was starting CCR diving, I sold it to upgrade to an OSTC cR to have a redundant deco and PPO2 reading. It is now on the latest firmware (V3.01)
Everything was fine, until a few month, when I started to have the impression that my cR makes "over deco" calculation...

With the same dive profile, same gas, same GF and SP changed at the same time, same speed ascent, it gives me more stops that any other computer (the AP handset, other H&W computers, Shearwater, Garmin MK1...). All other computers give me the same TTS and stops (as close as 1 or 2 minutes), but my cR gives me around 25 minutes more for a 90m x 15 min dive for example... There is no link with the O2 sensors reading, as it makes no difference when real PpO2 is used to calculate the deco, or when I use fixed Set Points (anyway, the PpO2 reading is accurate and very close to the PpO2 read on the AP handset).
Of course on « little dives » with air diluent, not too long and not too deep, I can see no deco time decreasing faster than on other computers, but it has no real effect on my dive. Maybe that is the reason why I did not immediately noticed the deco difference, which become greater as the depth and time increase (and the diluent gas changes to a richer in He) ?

Using it on OC dives too, I noticed the same thing : « no deco time » decreases faster, and TTS in longer compared to any other computer.

As I have the chance to live in the Carribean for a few years, I dive in hot and clear waters, then it is not a big issue (except my buddies eyes when I show them the time we will have to stay still at 6m when their own deco is finished winking smiley ), but it will be when I go back to live in France, with colder waters !

I just can't explain that difference. Did I missed a setting (although there's only a few on cR, compared to other H&W computers), is there a native issue (what I really don't think, as I have heard of no other user complaining about that, unless someone who reads me can testify to that ?), or maybe an issue with ONLY MY computer? Does anyone know if there way to check it (hoping without sending it back to H&W if not absolutely necessary?)

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Re: OSTC cR deco
Posted by: Ralph ()
Date: May 06, 2019 06:46AM


can you please provide a list of your computer's settings, and a dive profile in terms of diluents, SP, bottom depth & time to run some crosscheck tests?


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Re: OSTC cR deco
Posted by: opiffe ()
Date: May 06, 2019 02:45PM

Hi Ralph
Here is one particular dive for which the difference is obvious:
90m x 16'
dil 10/50
SP 30/85
I have the dive imported on subsurface, I can provide you the file if you want.
Or maybe, if only I can acces to a file from the cR root to give you all the settings of my computer?

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Re: OSTC cR deco
Posted by: Ralph ()
Date: May 06, 2019 09:14PM

Right, let's see - these are my settings:

90 meters, 16 mins BT, fresh water (Salinity 0%)
Diluent 10/50 (130 meter)
GF 30/85
SP 1,30
Loop max. 100%
last stop 6 meters
clean tissues

Ascent Speed 10 m/min, final ascent 1 m/min

Descent Speed 34 meters for Multideco as best possible match to the OSTC, whose deco calculator bumps down to bottom depth in zero time, so the OSTC will calculate with a little bit more deco obligation than Multideco.
Calculation at sea Level (0 meter), the OSTC is at 1.000 hPa (the OSTC takes the current surface air pressure as reference).

Here's the stops i got, first column stop depth, seconds column Multideco stop time, third OSTC stop time, 4th Delta if there is one:

48 1 - -1
45 1 1
42 1 1
39 1 1
36 1 1
33 1 2 +1
30 1 2 +1
27 2 1 -1
24 2 2
21 2 3 +1
18 4 4
15 4 5 +1
12 6 6
9 8 9 +1
6 34 40 +6 (see below)

Considering that the OSTC is faster down and thus had taken up a bit more deco, that's pretty aligned.

The length of the last stop is heavily infuenced by the surface pressure. There had also been done some fine tuning of the deco code after release of the V3.01, my latest V3.03 development version delivers 35 minutes instead of 40 minutes for the last stop and the rest on equal, so that's a close match then again.

Can you reproduce that?

best regards,

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Re: OSTC cR deco
Posted by: opiffe ()
Date: May 07, 2019 03:34PM

Hi Ralph

My results are not exactly the same as yours (a little longer both with multideco and OSTC, though with the same settings). Indeed, the deco are close when I compare my MultiDeco runtime with my OSTC runtime.

One thing I noticed these last days as I import my dives from OSTC on subsurface. When I look at the real dive profile compared to the deco stops shown, it appears that the last deco (3m by 3m instead or ceiling) ends earlier that the time I really finish the dive and ascent to surface.
Of course, I do not leave my last stop (at 6m) until OSTC shows "240", but there is always a difference between end of last stop (visible only on subsurface imported profile) and my last ascent (when I read "240").
I looked back for a few month to compare between air, normoxic and hypoxic dives: the deeper (and then of course the longer) the dives are, the longer this difference is. From 3 or 4 minutes in a 60m x 60minutes total dive time to 15 minutes for a 90m x 110 minutes total dive time.
The stop / ceiling shown corresponds +/- to the other computers deco...

Is there a way to send you some file from my computer that would allow you to check if there is any issue?

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Re: OSTC cR deco
Posted by: Ralph ()
Date: May 07, 2019 06:51PM

Hi, pls. apologise, i'll come back on you as i'm a little bit too busy in the moment...

What i spotted on your screen shot is the SD setting of 120/90. That activates a _secondary_ conservatism mechanism: calculate any tissue Saturation with 120% (i.e. factor 1.2 more) loading speed, and tissue Desaturation with just 90% (factor 0.9 applied to Buhlmann table values). That definitely will make your indicated deco longer. When using GF 30/85 you can put SD on 100/100.

The sync between ceiling going to 0 and TTS switching to NDL has been fine tuned lately, it's already in the development Firmware but not yet released officially.


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Re: OSTC cR deco
Posted by: opiffe ()
Date: May 07, 2019 07:15PM

OK !
I thought that I had to choose between the GF OR the % of sat/desat !
I used to dive with % of sat/desat in OC Air et Nx dives, and GF for CCR dives. But I had not understood that the % applies to the bühlman at the same time that GF and then add more stops !
I try it this afternoon, and I'll let you know.

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Re: OSTC cR deco
Posted by: DennisA ()
Date: May 08, 2019 09:51AM

It used to be like that, but it was changed a few versions ago....

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Re: OSTC cR deco
Posted by: Ralph ()
Date: May 08, 2019 06:45PM

The truth is, the Sat & Desat settings were acting in the past like this:

Without GF enabled, they were used during the dive and while calculating surface desaturation. Remember at the end of the dive when reaching the surface, the tissue supersaturation is at peak value and the offgassing is calculated in the OSTC's surface mode.

With GF enabled, they were not used in dive mode, but still used in surface mode - without this been made visible. So choices were to either throw them out in surface mode if GF is enabled, or make them visible and consistently working in the same way in both GF and none-GF mode. The decision was taken to do the second option. Internally there are two sets for SD values, so you can have one value pair that is active when having GF switched on and a seconds one for use when GFs are disabled. In case GF is activated the default values for SD are 100/100.


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Re: OSTC cR deco
Posted by: opiffe ()
Date: May 09, 2019 03:27AM

DennisA Wrote:
> It used to be like that, but it was changed a few
> versions ago....

I should have read all the new versions note confused smiley . It was not a computer issue, but MY mistake.... And now that I understand it, I am reassured.
Now that I am aware of that, I will keep diving in GF mode, with a 100/100 pure Bühlmann!

First dive like that yesterday. No deco dive, but the NDL were similar on all the comuters, including my cR. I don't know when will be my next deco dive, but I will keep in touch to let you know the result.


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