on air dive with Buhlmann (no GF) then supersat = 1.17 is proposed

[edhage]

Matthias proposed to send this email to the forum so more people can react.

I calculated a profile according to Buhlmann (no GF yet) and it seems that the pressures in some compartments exceed the allowed pressure.
This is for a profile that is recommended (simulation-mode) by the OSTC plus computer. Also when I put the dive in Subsurface, it recommends a profile that is very similar to the OSTC computer.
So basically since both computers give very similar results which both result in pressures larger than allowed, I thought there may be an error on my side.

Thus I looked at the code (https://bitbucket.org/heinrichsweikamp/ ... /p2_deco.c). Only difference is that you do not integrate over a small delta-time, but use the p2sec table (with basically pre-integrated values between two seconds). When I use your method (which is much quicker than using a delta-t =0.1 seconds when integrating of course ) I get the same results.

Basically it is good that using the integral dp/dt = -log(2)/halftime*(p(t) – pamb) or -(p(t)-pamb)*e2sec give the same result. But the same results is still that pressures for some tissues become larger than allowed (super-saturation).

I made an example and described it in the added pdf document: It concerns a dive on air to 51 m, 12 min bottomtime, first deco-stop on 6 meter (5 min), second stop on 3 meter (6 min). No gas-switches.

Could you be so kind to help me, and calculate the pressures in the 16 compartments when just arrived at 6 meter and when arrived at 3 meter. Since I use your calculation-method I think the results should be the same.

I have the results and the complete code in an attached Word-file (code can be copied and saved as an m-file to be used in Matlab or Octave, since you program C you can read it ). Also my results (table) with tissue-pressure at these depths is shown. @6 meter than compartments 2,3 and 4 are supersaturated. @3 meter compartments 3,4 and 5 are supersaturated.

Since the code is quit simple (just calculate the tissue-pressures and plot them in graph with Buhlmann-curve) I am really puzzled .

Like to hear from you.

PS the supersaturation has also been calculated for the 3m stop and for tissue 4 it is 1.17 . More than allowed according to the Heinrich Weikamp SW.

[Ralph]

Hi,

wasn't able to understand in-depth your way of calculation, but maybe some hints help:

- the OSTC calculates ppN2 a depth using the gravity constant of 0.9... instead of the 1 bar/10m

- also the ppWaterVapor (a constant value) is subtracted,

==> both of this measures reduce the ppN2 a bit, reducing deco obligation a bit.

- very important: the off-gassing of the tissues is calculated while the ascent, with ascent speed set at 10m/min.

==> In fact, when the OSTC gives a stop depth of e.g. 3m, it doen't mean you are allowed to "plopp" to 3m immediatly. Instead, it means: if you now ascent with 10m/min, with the tissues loosing a bit of pressure while on the way up, you will become cleared for 3 meters while being under way.

When you look at the half-times of the fast tissues, it is obvious that these tissues change pressure quite a lot while every 1 minute / 10 m of ascent.

BR
Ralph

[edhage]

Hello Ralph,

The gravity constant what you mention is simply the METER_TO_BAR constant. I used the same parameter as in the Heinrich formula (1/10.19716)

Also I used ppWater = 0.06270 (same as in the code)

So this is not different.

Code: Select all

BAR_TO_METER = 10.19716; 
ppWater = 0.06270; % from heinrich weikamp computer (Buhlmann says 0.063)
psurf = 1.013; % [bar]
frac = 0.79; % Stikstof
pamb = frac* ( (diepte/BAR_TO_METER + psurf) - ppWater); % [bar] partiele ambient

I calculate with dt = 0.1 second (And also an example with the 2 second table from Heinrich) so this accounts for any ascent speed (In the added pdf you see the profile, I ascend also with 10 m/min, and account for all the travel time), still I get supersaturation 1.17.

You say it is obvious that the tissues pressure change is quit large at ascent. Yes, this is not my point. My point is that I dive with > 1 supersaturation is proposed which is dangerous.