math behind divetables?

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andypandy

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I'm not sure I've chosen the right forum for this question but here goes...

I'm about to write a small diveplanner in Java for a computer science course and I was wondering if someone knew of the equations behind for instance the Padi divetable. I could just copy the numbers in to the program but as one of the constraints of the project is the size of the finished software I'd really prefer to use some kind of equation. I'm guessing that the Padi divetable must be based on such an equation, but is it public or some kind of company secret? Anyone know where I can find it?

Thanx in advance

Andreas Lindberg
 
Hi Andreas:

The only equations that were used in the PADI tables were the same as those for the Haldane method of exponential on gassing and off gassing. The constants that are needed, in addition, are the no decompression times for several depths.

I am not certain that this is what you were seeking?

Dr Deco :doctor:
 
Andypandy,

I had answered your other post and when I saw this one I said to myself, "Wheres my reply"?

This link will take you to an article on Understanding M-Values. I didn't have the time to read the whole thing, but it gets a bit deep. It might be easier to just put in the values. But take a look at it.

Hope it helps, and either way I love to see the finished product.

Take Care

Bill :) :)

M-Values
 
The article can be difficult to read the first time through, but it is worth it.

A couple comments that might help:

1. The M values in fsw are absolute pressures.
2. They are the partial pressures of nitrogren only, so to get your inspired N2 pressure you first need to add 33 feet, subtract H2O+CO2 pressure (see below) then multiply by the 0.79 fraction N2 of air (or the correct fraction for other breathing gas if used).
3. That article doesn't mention H2O vapor pressure. The gas in you lungs is fully saturated with water vapor. There is also some outgassed CO2. Those two gases combine for a partial pressure in the range of 1.6-2.0 fsw, which must be subtracted from total pressure before applying the fraction of N2.

Everything else is just applicaton of the exponential equation:
P = Po + (Pi - Po)(1 - e^(-ln2t/half-time))

Po = starting partial pressure of N2
Pi is inspired pressure (per items 2 and 3 above)
t is elapsed time.

The times in a deco table are from the above equation, but with P replaced by the M-values (or a smaller fraction for less than NDL columns) and then solved for time:
t = HT/ln(2) * ln [ (Pi-P)/(Pi-Po) ]

Hopefully, that's enough to get you started.

Charlie
 
You can find all the math for all models in
Basic Deco And Applications (2nd Edition).
Plus Technical Depth In Depth. Both Best Pubs.

For Haldane approaches, for given halftimes,
specifying NDLs will yield M-values. Specifying
M-values will yield NDLs. The tissue functions
(exponential) can be inverted to give "longest"
NDLs for given sets of M-values across the tissue
spectrum. For given NDLs, M-values build up
from depth (fastest to slowest compartments)
as you move up to shallower zone NDLs. The
cutoff for an M-value across the tissues and
NDLs occurs when the next shallowest depth
has ambient partial pressure (say N2 for air)
less than the next greater depth gas buildup
at its NDL. Thus short tissues control deep
exposures, and slow tissues control shallow
exposures. As you move thru the fit zone,
different compartments with different M-values
and halftimes control (again fast compartments
with large M-values at depth, to slow
compartments with small M-values in the shallow
zone).

Such is true for all Haldane (dissolved
gas only) treatments. And for N2, He, and
whatever else you are smoking.

Such does not occur in dual phase models the
same way, because there are no M-values. Just
bubbles whose cumulative volume is the control
point (phase volume limit point for arbitrary
mixes that changes with mixes). However, even
in phase models like RGBM, the fast compartments
couple the strongest to bubble gas diffusion
at depth. And the slower ones in the shallow
zone. Also Boyle expansion of the bubbles
gets included across all bubbles.

Bruce Wienke
Program Manager Computational Physics
C & C Dive Team Ldr
:)
 
You can find all the math for all models in
Basic Deco And Applications (2nd Edition).
Plus Technical Depth In Depth. Both Best Pubs.
Bruce Wienke
Program Manager Computational Physics
C & C Dive Team Ldr
:)

you mean

Basic Decompression Theory and Application
Technical Diving in Depth

?
 
you mean

Basic Decompression Theory and Application
Technical Diving in Depth

?

I doubt BRW is going to respond to you. That Post was made 31 October 2003 and BRW has not been on SB since 02 October 2009.
 
Hello marc.collin

Yes. I believe those are the book titles.

[1] Basic Decompression Theory and Application. Best Publishing

[2] Technical Diving in Depth. Best Publishing.

Good luck.
 
https://www.shearwater.com/products/peregrine/

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