Desaturation

[Tom]

I was diving with a friend who used the VR3 (Bühlmann ZHL16, Software: 3.03aC) and I the MK2 with software version 1.80. We had exactly the same dive profil with the same gases (Trimix, Nitrox, O2 > 56 m, 60 min).
After the dive I had a desaturation time of 4,33 hours and my buddy more than 20 hours.

That's a significant difference. Any idea what causes the difference?

Tom

[JeanDo]

Hello Tom,

You dived 60 min at 56 m ?
Can you post your exact gas list, CF#11/CF#12/CF#29, OCR/CCR.


Thanks,

[Tom]

Hi,

I had a bottom time of 15 min at 56 m until ascending.

Decotype > L16-GF OC,

Trimix 18/45
EAN 50
O2

CF#12 > 90%,
CF#13 > 60%,
CF#29 > 6m

Hope that hepls

Tom

[JeanDo]

Tom, with 1.83 alpha, I have the following traces:

Bühlmann ZH-L16C-1a 2002 with GRADIENT FACTOR decompression:
* GF_low (CF32) = 30 %
* GF_high (CF33) = 90 %
* deco offset (CF16) = 1.0 m
* last stop (CF29) = 6 m
* dive started at Pamb = 900 mbar
Gas#1 = 18/45 at 0 m
Gas#2 = 50/0 at 21 m
Gas#3 = 100/0 at 6 m
Gas switch delay (CF55) = 0 min
[...]
15:02 CNS= 7%, Deco:
24 m : 1 ' 18 '
21 m : 1 ' 20 '
18 m : 1 ' 21 '
15 m : 1 ' 22 '
12 m : 2 ' 23 '
9 m : 3 ' 26 '
6 m : 11 ' 29 '
0 m : --- ' 40 '
TTS = 27 '
Depth = 56 m
Bottom time = 15 min
Bottom gas consumption (CF56)= 30.0
Ascent gas consumption (CF57)= 20.0
Gas#1 18/45: 3400
Gas#2 50/00: 500
Gas#3 100/00: 400
[...]
37:30 ascent, depth=0.3 m, GF=87%, CNS=13%
37:34 ASCENT DONE TO 0
37:34 CNS 13%
37:34 NoFly 3:49
37:34 Desat 6:23

Your numbers should be (very) slightly different (Ambient pressure, actual GF settings, and mainly you probably didn't do a square dive with an instant descent to 56m).

I have no idea of the desaturtion limit used by VR3, btw.

Hope this helps,

[Tom]

Hi Jean,

thanks for your comprehensive message.

Yes, it wasn't exactly a square dive but a pretty straight descent to 56 m. Your desaturation figures are as similar to my real figures so that I think they are comparable.

I am just wondering about the huge difference between the MK2 and the VR3. With the same Model (Bühlman ZH L-16 OC) results should be similar. I also believe that my buddy didn't make a mistake in setting the VR3 as all dives we made together showed a similar difference.
My dive buddy told me that he set a moderate level for the safety.

The question for me is whether the calculation of the desaturation ist correct in the MK2.
Out of my diving experience the desaturation an no fly time seems a bit short despite the long desaturation time of the VR3.

Tom

[JeanDo]

@Tom

Actually, this simulation uses GRADIENT FACTORS (you wrote GF), and I used the default 30/90 settings. If you have different GF_low (CF#32) / GF_high (CF#33), let me know.

If you really use pure "ZHL16 OC" mode, please give me the CF#11, CF#12 values.

Regards,

[Tom]

Hi Jean,

I used the Gradient Factors 30/70 with CF#11 = 110% and CF#12=90% (default values).

Tom

[dmainou]

Hey tom,

If I read your post correctly your computer and your buddy's pulled you out of the water with very similar stops whilst using similar settings and a similar algorithm. However his computer had substantially more desat time than yours.

If this is correct then shouldn't you be looking at the desat and no fly cf's rather than GF CF's?

D

Ps the other thing is that the vr3 algorithm is not published and doesn't claim to follow dr buhlmann to the T.

Therefore your friend's computer may be providing a padded result whilst the ostc will be giving you the outcome as per the model. You can of course consciously padd it. However, you have the advantage of knowing where the line In the sand is.

[Olivier]

Tom, CF 11 and 12 are useless with the GF model. They are only used with the pure pure "ZHL16 OC" model - for which GF Lo and Hi are useless...

[Tom]

Hi dmainou,

yes, after the discussion in this forum I have been looking at the desat and no fly cf's, wich are set zo default and the GF's.
I couldn't find any wrong setting. As mentioned before my friend set a moderate level for the safety in the VR3.

So I would understnad slight differences, but a 5 times longer desaturation on the VR3 seems strange.

Tom

[dmainou]

You may find that if you dived together for a week the desat values are similar bu if you dive for a day the vr3 gives you 24 hrs. (I'm speculating )

Read the vr3 manual you may get some insight.

D

[dmainou]

"VR3 manual page 11

Decompression Algorithms
VR dive computers use a derivative of the Buhlmann ZHL 16 algorithm modified to
assimilate the latest thinking in practical micro -bubble avoidance for enhanced
decompression safety."


They use a derivative of Buhlmann ZHL16 which is not the same as the ZHL16 or ZHL16GF.

They should be quite similar but it's padded in certain areas such as the no fly and minimum desat time... we'll never know exactly

[Tom]

Hi Dmainou,

no, the VR3 always hat this huge difference in desaturation time for all dives together with the same dive profile.

Tom

[_StranGe_]

Hello !

I have recently learnt Trimix diving and I have noticed that desat times are drastically reduced when I dive the same dive with trimix compared to air. For instance, 20 minutes at 45m with air diluant (GF CC algo) will give me around 24h of desat while 20 minutes at 60m Tx 17/40 diluant (GF CC algo) will give me around 6h of desat... There might be something wrong or is it because ppN2 gets below 0.8 during deco so it accelerates the desaturation ?

The tissue saturation graph even shows more He saturation than N2 sometimes (once out of the water).

I have version 2.01.

Any idea ?
Gilles.