Imported post
Mark you may have misunderstood me. What I meant was the whole profile on Air or Nitrox, not just the deco phase.
35m 30mins .21
9m 3
6m 9
3m 19
9m 3
6m 38
So take the original 9mins off and the 3m stop has taken about 1.5 times as long to clear
9m 71
Take off the original 3 mins and the 6m stop has taken very roughly twice as long to clear.
This is pretty much what you would expect to see considering the gas expansion going on and the effect it has on the tissue / inspired differential.
Trimix profiles are just about impossible to estimate. Before a Trimix dive you are saturated with Nitrogen at .79 bar. Trimix at the surface has a PPN2 of < 0.79 so if you breathe trimix at the surface you off gas N2 and on gas He. Once you descend it all gets very complicated very quickly as the He on gases faster than the N2 off gases.
[b said:
Quote[/b] ]The problem with pulling the last stop deeper than 5m is that it is theoretically possible to on gas a slower compartment to the point that it exceeds its M-Value on final ascent.
[b said:
Quote[/b] ]The rate of on gassing at 6m is not worth talking about. On air it goes off the scale let alown 80% Nitrox. You would have to be at 6m for several hours before significant on gasing occurs.
I will do my best to explain it.
Keep in mind you have 16 half-times, 16 M-Values and 16 pressure differentials to think about, any of which can control the decompression. Lets say that on reaching 6m two of the compartments are close to their surfacing M-Value, call these C1 and C2. The faster compartment C1 is a little over M and is controlling the indicated stop. The slower compartment C2 is a lilttle under M. At 3m both C1 and C2 would be off gasing. But at 6m C1 is off gasing while C2 is still on gasing. Once C1 has off-gased below M, the computer indicates it is safe to ascend, however C2 is now almost at M and still on gasing. In the time it takes to reach 3m C2 exceeds M and an additional stop is indicated.
You are unlikely to see this behaviour in a VR3 or any other computer or software package that models all 16 compartments. It can show up on models quantized to 8 or 12 compartments due to the increased gap between the compartments.
This is all theoretical. The biggest fudge factor is our personal physiology, possibly the last word in adequate decompression.
6m was thought to be the maximum depth you could become completely saturated at and still surface without decompression. Shortly after the 88s were compiled this was revised to about 5m. Personally I make the last stop nearer 5m than 6m, although on one dive last year the sea was snotty enough to persuade me that 7m was a better option than bouncing in the swell.
Regards
Matt