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Fine Tuning a Decompression Algorithm

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Fine Tuning a Decompression Algorithm

Postby eDiver » Sat Oct 29, 2011 12:41 pm

I have been asked several time what kind of decompression algorithm is DiveNav using in its eDiving and divePAL simulators and my answer invariably was ...... "Buhlmann" ;)
But I realized that probably you wanted a more specific answer, and maybe even a comparison of our algorithm against the ones used in real dive computers.

Existing Data

We did some research and found a very interesting article published by ScubaLab: Digging Deep on 2009's New Dive Computers.
The data for this article is based on dives run by our friend Karl Huggins and his staff from the USC Catalina Hyperbaric Chamber for Scuba Diving Magazine.
This article describes the results of a head-to-head comparison among several dive computers during a series of 4 dives performed at the Chamber.

Our Tests
To test our own decompression algorithm we entered into divePAL the series of 4 dives (dive profiles and surface intervals) described in the original report. We used the recently released divePAL Nitrox because it allows to Plan and Analyze series of up to 5 dives.
Here below the profiles for the 4 dives:
Dive 1Image
Dive 2Image
Dive 3Image
Dive 4Image

.... to be continued ....
Last edited by eDiver on Tue Nov 01, 2011 4:22 pm, edited 3 times in total.
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Fine tuning a Decompression Algorithm - Part 2

Postby eDiver » Sat Oct 29, 2011 6:29 pm

Once the dive profiles were ready, we "dove" them with divePAL, checking for the No Deco Time at each pre-defined point (depth and time).

We then collected the data in a spreadsheet and compared them against the results presented in the ScubaLab article mentioned earlier.

I was not surprised to find out that our own implementation of the Buhlmann ZH-L16C algorithm was the most aggressive of the group as I did suspect manufacturers of dive computers wanted to be cautious with their own algorithm.
Here, as an example, the comparison results for Dive 2:

Image


Just few notes on how to read this graph:
- the axis on the left side is NDT in minutes for each point measured
- the axis on the bottom is dive time in minutes
- the axis on the top is depth in feet

The RED LINE represents our original algorithm.
Clearly it was the most aggressive of the group as it was providing the highest NDT value for every point being measured.

After analysing the results, we decided that we wanted an algorithm that will fit in the "middle of the road". But how can we achieve this?

...... to be continued ....
Last edited by eDiver on Tue Aug 25, 2015 2:44 pm, edited 2 times in total.
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Fine tuning a Decompression Algorithm - Part 3

Postby eDiver » Sun Oct 30, 2011 11:46 am

So, our goal was to come up with a decompression algorithm that will fit in the "middle of the road": not too aggressive and not too conservative either.

The Bulhmann ZH-L16C algorithm is a mathematical representation of gasses absorption and release by the human body when scuba diving.
This algorithm includes 16 compartments that are characterized by HALF TIMES and OVER-PRESSURE GRADIENTS (for a more detailed explanation on how this algorithm works, see the online class Introduction to Dive Computers). Half Times range from 4 minutes to 635 minutes and Over-Pressure Gradientes range from 12.7msw to 32.4msw

Over the course of few days, we run several simulation with different coefficients for the various compartments.
Initially we thought that it would have been enough to just apply a linear reduction on all the compartments, but we quickly realized that this approach was penalizing excessively the fast compartments (making them too conservative) and dive 1, while barely affecting the slow compartments and dives 3 and 4.
So we decided to drop the linear approach and decided to fine tune each compartment at time - one by one!
This approach would have been impractical for a real dive computer as it could have taken a huge amount of time and resources, but, in our case, using divePAL it was relatively fast as it takes only few seconds to change the coefficients and recompile the code and only few minutes to check the algorithm on the series of 4 dives.

At the end of the process we obtained a modified algorithm that is more in line with the algorithms used in recreational dive computers, and, more specifically, is mid-to-conservative for single deep dives and middle-of-the- road for series of dives.
Here below the results for the series of 4 dives described in the ScubaLab report:

Dive 1

Image

Dive 2

Image

Dive 3

Image

Dive 4

Image


So, now you have a better idea on how DiveNav decompression algorithm compares to the algorithms implemented in several dive computers.
If you want to check out our algorithm yourself just use divePAL (the Nitrox version), load the series of 4 dives (search for "chamber" as dive site) and analyze them.

One final thing: when you dive for real, follow YOUR dive computer as it knows exactly what your dive profile is!
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Fine Tuning a Decompression Algorithm - Part 4

Postby eDiver » Wed Dec 28, 2011 11:48 am

Several of our members indicated the interest of being able to change the conservativism of our deco algorithm.

In the past couple of weeks we have further improved our "fine tuning" capabilities; we added to divePAL the possibility to modify - on the fly - the pm0 coefficients of the ZH-L16C baseline algorithm. This feature is currently available ONLY in our "Developer version" of divePAL, but we are considering to make it available also in the - coming soon - Tech version of divePAL.

Thank to this new feature, it was then quite easy to further refine our algorithm and create 3 levels of conservativism: Liberal, Middle of the Road and Conservative.

Here are the results for the series of 4 reference dives.

Image

Image

Image

Image


This new feature will become available in the nextr rev. of divePAL
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Re: Fine Tuning a Decompression Algorithm

Postby usrytregre » Fri May 18, 2012 1:05 pm

Excellent discussion!!!
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Re: Fine Tuning a Decompression Algorithm

Postby eDiver » Sat May 19, 2012 7:21 am

usrytregre wrote:Excellent discussion!!!

Thank You
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Re: Fine Tuning a Decompression Algorithm

Postby searcaigh » Sat Jun 22, 2013 9:54 pm

Thanks for this explanation Alberto, I was looking for some information regarding the algorithm type used for my article on bluebuddy, all the information I need is here.
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