Manual calculation for accelerated deco

[_Slacker_]

If M-values alone are a valid determinator for stop depths, those stop depths would be deeper on an altitude dive than a sea level equal, due to increased impact of relative pressure differential at altitude.

Your stops are in 10 ft buckets. Say the algorithm spits out a 93 ft ceiling at sea level and a 95 ft ceiling at altitude. The implementation still spits out a first stop at 100 ft.

 

[lowviz]

...//... If M-values alone are a valid determinator for stop depths, those stop depths would be deeper on an altitude dive than a sea level equal, due to increased impact of relative pressure differential at altitude.

1) The increased impact is countered by decreasing the stop depths in order to make the dive fall in line with an equivalent sea level dive.

OR

2) You can futz with the stop times using a standard interval sequence.

 

[leadduck]

In boulderjohn's particular example the deepest stop didn't change, but that's not true for every case. When I plan a 40msw air dive with 28min bottom time and GF30/80 in subsurface then I get a first stop at 18mfw at sea level, but a first stop at 21mfw at 3000m elevation (same depth, bottom time, gas).

 

[rossh]

@victorzamora Yes, exactly. I await the decision of the experts.

Now if someone were to rewrite the software for VPM allowing for atmospherically corrected depth...

You can set deco step spacing in V-Planner / MultiDeco to anything you like...

And VPM already works internally on absolute pressures, there for the elevation dive plan is easily calculated.

 

[lowviz]

OK, then. I have some work to do...

 

[victorzamora]

You can set deco step spacing in V-Planner / MultiDeco to anything you like...

Cool! And good to know!

 

[leadduck]

Hello Lowviz,

Sorry, I think I understand what you are getting at.

Scaling stop depths, and stop time (by calculating a sea-level equivalent dive depth, and then decompressing according to the sea level table for that new depth) is the crux of typical standard approaches to altitude diving - such as the guidelines published in the US Navy Diving Manual. The formulae for that approach are similar to the ones Kev has presented. For example:

Equivalent depth (fsw) = Actual depth (fsw) x (1 ATA / Atmospheric pressure at altitude)

Altitude stop depth (fsw) = Sea level stop depth (fsw) x (Atmospheric pressure at altitude / 1 ATA)

These approaches are promulgated in the US Navy Diving Manual.

Simon M

just for reference: this is called the "Cross correction" after E.R.Cross, also used for example in PADI Altitude Diving for RDP NDL time correction. AFAIK it's a conservative approximation. A real altitude table cut from the same model would show fewer and shorter stops.

 

[EFX]

Regarding decompression; I want to understand how to calculate accelerated decompression. In other words; using Buhlmann's tables I can calculate decompression stops for a dive if back gas is air. In case of Nitrox; I simply use EAD with Buhlmann's tables to figure out deco stops. But in real world; we use rich mixes (EANx50 and higher) to accelerate decompression at shallower depths (12, 9, 6 meters). Simply I can use computer software like MultiDeco to calculate the profile, but I'm eager to know exactly how to calculate this acceleration by hand. Thanks so much.

I'm close to releasing my spreadsheet that will show TC pressures for nitrogen and helium. It will calculate deco ascent schedules if one is needed using two different nitrox deco gasses. You'll be able to select imperial or metric units and fresh or salt water. You'll be able to see the calculations in the VB code of the ss. I'll post it in the advanced forum and here.

hanymamdouh, see the thread "Tissue Compartment Gas Loading Spreadsheet" in the technical forum.