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Fun with Decompression Models May 15, 2009

Posted by Chris Sullivan in Technical Diving.
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This is totally premature but I was messing with the Bühlmann/Schreiner equations for computing compartment pressure and came up with something halfway reasonable in MS-Excel which I thought I would show you. This illustrates a dive going to 185’ on air for 10 minutes, 110’ on Nitrox 32 for 15 minutes, and 30 feet on Nitrox 80 for 20 minutes. Beware that this is a rather unlikely dive profile I made up just for illustrative purposes and I don’t recommend you dive it. In fact decoplan recommends a 2-3 min deco stop at 40′ depending on the gradient factors, although without gradient factors it doesn’t require any.

All pressures are in Pascals. Standard atmospheric pressure is 101.3 kPa, or 101,300 Pa. The purple line with the square dots represents the initial state with all compartments at atmospheric pressure. It is well below 101,300 Pa because we’re only representing the partial pressure of Nitrogen (using 79% of the atmosphere) and there’s also a reduction for water vapour pressure of 6250 Pa (this is Buhlmann’s value, about 5% of the total, there are more conservative values used by others) which I’ve assumed to be present under all conditions.

The light blue line with the triangular dots represents the state after being at 185 feet for 10 minutes. The fast compartments on the left “fill up” quicker than the slow compartments on the right which hardly move. Note that I’ve ignored the time it takes to descend to 185 and assumed that the diver is instantaneously transported there.

The next depth, 110 feet, is on 32% Nitrox. You can see that now the fast compartments off gas quickly to approach the ambient partial pressure of the 68% Nitrogen at that depth while the slower compartments continue to load up. The crossover point is between compartment 2 (8 minutes) and compartment 3 (12.5 minutes).

Finally, we go to 30’ on EAN 80 and stay for 20 minutes. The partial pressure of Nitrogen is less than 40% at this point, meaning that it is less than at the surface. So compartments 1 and 2 both end up off gassing more Nitrogen than they started with, as you can see from the purple line with the star-shaped dots. This is why we love our highly enriched mixes on deco, although this extended stay really racks up the time on the CNS clock.

The highest point on the graph after this 40 minute excursion is compartment 5. The faster compartments to its left went higher but off-gassed quickly, the ones to the far right barely moved. Next step is to overlay M-Values on this graph to see at which point, if any, that it is safe to surface. With the maximum pressure at the end of the dive around 30% over ambient, I’m guessing we’re safely decompressed by the end. It might also be interesting to look at the M-Values at 8000 feet above sea level, to see how flying after diving might affect things.

After adding the M-Values, I’m going to take a look at including the ascent and/or descent rates in the model. I will email the spreadsheet to anyone who asks. WordPress won’t let me post it here as it doesn’t support Excel. It’s a PDF file so you will need Adobe Acrobat Reader to view it.

Deco Model

Dive Plan

Time            Depth               Mixture

10                   185                    Air

15                   110                    EAN 32

20                     30                   EAN 80

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