Decompression Theory Question???

[Diver0001]

log natural of 2.

The equations more naturally work as log naturals and power of e.

If you work with doublings or powers of 2, then you have to convert the exponential time constants to halftimes by the ln(2) factor.

It just so happens that because of things I studied (against my will and better judgement) I can follow what you mean. But I'd be willing to bet that the vast majority of people think you're speaking in tongues.

R..

 

[Diver0001]

(which is also the 86.5 minute time constant compartment ).

errrr... huh? where did you get that number (86.5) from?

If I did the reverse engineering of the PADI table correctly, then PG Z is at 95% of the 60 minute compartment M value, then each letter group down from there moves 3% of the distance to the sea level saturation point.

Check. I should have known that you had the answer to this....

The controlling compartment for a deep dive (actually any dive deeper than 40') will be a compartment faster than the 60 minute compartment upon which the pressure groups are based. That's why the NDLs for deeper than 40' are at pressure groups less than Z. The faster compartment will have reached its M value at NDL time, but the 60 minute compartment is at less than its limit.

Ergo, long repetative shallow dives are more dangerous (on the tables) than repetative deep dives.... ?!?

R..

 

[Charlie99]

errrr... huh? where did you get that number (86.5) from?

60 minute halftime divided by 0.693 to get a tau or time constant of 86.5 mintues.

2^(-t/60) * START will give the amount left in a 60 minute halftime compartment which is decaying towards zero, from a starting pressure of START. When t=60, the exponent is -1 and 50% is left. If t=120 the exponent is -2 and 25% is left.

e^(-t/86.5) * START, or the equivalent expressions exp(-t/86.6), or exp(-0.693t/60), or exp(-ln(2)*t/60) all will give the same answer for any t as the 2^(-t/60) equation.

Ergo, long repetative shallow dives are more dangerous (on the tables) than repetative deep dives.... ?!?

As with most things having to do with deco, there probably aren't any real studies or hard facts. But you can easily observe that for a series of repetitive deep dives the tables are much more conservative than a decompression program that runs the equivalent full model -- if the model has the fast compartments offgassing at the same halftimes as they ongass. In a series of 40-50' dives, the computer and the tables will be much closer to each other. Some computer such as the Oceanics, however, have thing called "surface credit control" which is just another way of saying assymettrical ongass and offgassing haltimes. Once you have surfaced, the offgassing halftime of the fast compartments is slowed to 60 minutes. Were you to pop directly to the surface without a safety stop, then the Oceanic computers would be nearly as conservative as the table on repetitive deep dives. But what happens in real life is that most prudent divers are either doing multilevel diving, or are making a series of deep stop, slow ascents and safety/shallow stops. Even computers with assymettrical offgassing on the surface will have symmetrical (fast) offgassing of the fast compartments during this ascent and stops.


Another thing that doesn't show on tables, but does show up on computers is the effect of extended safety stops. It is easy to extend a safety stop a few minutes and add a large safety margin to a deep dive where the most heavily loaded compartments are the fast ones, but doing the same length of extended stop after long shallow dives does relatively less offgassing of the slower compartments that are most heavily loaded in that case.
That's why a couple minutes of deco from a drop down to 100+' doesn't concern me as much as being within NDL but approaching limits after doing a couple of repetitive dives to 50-60' range.

 

[Bismark]

my head isstarting to hurt........:11:

 

[Diver0001]

60 minute halftime divided by 0.693 to get a tau or time constant of 86.5 mintues.

There's that constant (0.693) again. What is that?

R..

 

[Jai Bar]

Hi,

This is really a great site (perhaps the best I've seen) if you want to understand decompression theory:
http://www.deepocean.net/

There is also in the site a nice program (and source code) that one can use to compare the various algorithms and so on.

 

[MikeFerrara]

There's that constant (0.693) again. What is that?

R..

It's the natural log of 2

Meaning that e^0.693 = 2
e = 2.718

 

[Diver0001]

Ah for the decay factors.

 

[rawls]

There...You now know how to answer your divemaster question