Decompression Stop Questions

[craig chamberlain]

Hi

I am a TDI extended range diver. i have some questions about decompression and decompression stops or more accurately the physics behind them.

1) Why when making a deco stop must you remain at 9m for example and not 8.5m for example? Surely when you are at 8.5m you are off gassing just as you are at 9m?

2) How do you select a deco mix or deco mixes? when i did my courses i was told that we will use 52% o2 as that was what was available so how do you select mixes?

3) When diving with trimix as helium is absorbed when you ascend it is released just as nitrogen is so is it the same principle of residual nitrogen levels as residual helium levels and can they also be used to calculate the length of deco stops?


Craig Chamberlain

 

[Dr. Lecter]

I'm not quite sure what to make of this. In addition to Extended Range, do you have training/experience in advanced nitrox and deco procedures?

1 is somewhat concerning (do you not understand the theory, if not the full-blown math, behind the calculation of the stops?) but depending on how you have your computer/table-generating software set up, yes, minor variances are probably NBD (until they are--see comment on 3 below).

2 is both more concerning (really? you don't understand the CNS and O2 clock issues, or how your deco curve and your available deco mixes might work together?) but at the same time a pretty lively subject of perfectly valid gas planning debates.

3 is outright terrifying (though I may be reading it wrong...you're asking if you can calculate deco stops for trimix using the same calculations as a nitrox mix?). Are you currently diving mixes with helium in them? That... might not end well.

Hi

I am a TDI extended range diver. i have some questions about decompression and decompression stops or more accurately the physics behind them.

1) Why when making a deco stop must you remain at 9m for example and not 8.5m for example? Surely when you are at 8.5m you are off gassing just as you are at 9m?

2) How do you select a deco mix or deco mixes? when i did my courses i was told that we will use 52% o2 as that was what was available so how do you select mixes?

3) When diving with trimix as helium is absorbed when you ascend it is released just as nitrogen is so is it the same principle of residual nitrogen levels as residual helium levels and can they also be used to calculate the length of deco stops?


Craig Chamberlain

 

[nimoh]

I am not all that familiar with TDI Extended Range Diver (I am TDI Advanced Nitrox/Decompression Procedures), but I'll take a stab at this question.

1. I don't think a half a meter makes a big difference in your decompression, however, the TDI protocol for gas switches involve team members checking your MOD before switching and if you are at 8.5m and they are at 9.5m, this is rather difficult. IMHO if your plan is to stop at 9m, why not just stop at 9m and try to stay there?

2. I am not a big fan of trying to select the perfect mix for a dive. I have my goto mixes of 32%, 50% and 100%, and plan my dives with v-planner using those. I do also use 24% to go to past 100', but that is only temporary until I complete the trimix class.

3. I think I'll let someone that has completed trimix training answer this one.

 

[mala]

1-the shallower the quicker the off gassing
depth depends on plan and gas available.

2-you can use whatever deco mix you want-the weaker the mix the deeper it can be breathed-the richer the mix the quicker offgassing happens.
you can have more than i deco gas
3-the principle is the same.
the 2 elements have different properties so no they cant.

 

[craig chamberlain]

Yikes.

1 is somewhat concerning (do you not understand the theory, if not the full-blown math, behind the calculation of the stops?) but depending on how you have your computer/table-generating software set up, yes, minor variances are probably NBD (until they are--see comment on 3 below).

2 is both more concerning (really? you don't understand the CNS and O2 clock issues?) but at the same time a pretty lively subject of perfectly valid gas planning debates.

3 is outright terrifying (though I may be reading it wrong...you're asking if you can calculate deco stops for trimix using the same calculations as a nitrox mix?). Are you currently diving mixes with helium in them? That... might not end well.

ok -- so, i do understand the phsics of diving but as they must be 15, 12, 9, 6, metres i do not understand - surely 14, 11, 8, 5 does the same job?


and i think that the cns clock is not used practically to dertimind best deco gas?

and no i am not diving trimix.

 

[Dr. Lecter]

ok -- so, i do understand the phsics of diving but as they must be 15, 12, 9, 6, metres i do not understand - surely 14, 11, 8, 5 does the same job?


and i think that the cns clock is not used practically to dertimind best deco gas?

and no i am not diving trimix.

I edited my post because I think it started overly harsh. That said:

Whether it matters that the stop is a Xm or Xm-1m will depend on how conservative your planning was and what inert gas you're off-gassing.

You don't think O2 toxicity concerns, compared to the length of the deco obligation sought to be cleared (and potential repetitive dives), play a role in selecting what mix(s) to use?

 

[ajduplessis]

The very first thing you need to understand about any form of deco is thats it not a pure black and white activity. The various algorythms for deco cant calulate what happens on a dive, temp, workload, metal & physical stresses etc. They are guidlines only and should be used as such. You will hear a lot that you should feel good when you get out of he water. If not, change the way you decompress. Find what works for you!!! Thus to anwser you, the 9m stop is a recommedation, do what you feel works. But please, the work that was done on the algo's are not quick guesses. A lot of science/ math is involve with a lot of divers that lost their live to help guide us today.

Deco gasses again is what works for you and you can pritty much do as you please as long as you understand the impact (cns, icd, gas planning etc) and what the rest of the team is doing. Once again, there are guidelines for deco gasses based on the profile. I am not familiar with you agency but believe that 50 is the course recommedation.

Helium comes out of solution very, vey quickly. Thus by the time for your next dive there will be no traces of helium in you tissue/bloodstream. This is the simplest way to your question.

I would also seriously recommend finding a mentor/instructor to guide you going forward. Your questions here will raise some eyebrows as the basics should have been covered in your extended range course. I have seen to many divers following a DC and not truely understanding the impact of what they are doing and ending up hurt!!! Decompression diving is serious stuff, make sure you know what you are doing. PLEASE!!!!

 

[nimoh]

Hi

I am a TDI extended range diver. i have some questions about decompression and decompression stops or more accurately the physics behind them.

1) Why when making a deco stop must you remain at 9m for example and not 8.5m for example? Surely when you are at 8.5m you are off gassing just as you are at 9m?

2) How do you select a deco mix or deco mixes? when i did my courses i was told that we will use 52% o2 as that was what was available so how do you select mixes?

3) When diving with trimix as helium is absorbed when you ascend it is released just as nitrogen is so is it the same principle of residual nitrogen levels as residual helium levels and can they also be used to calculate the length of deco stops?


Craig Chamberlain

a little off topic from your main questions, but is 52% permitted for extended range diver? I thought Advanced Nitrox certification was necessary for any nitrox mix over 40% O2

 

[craig chamberlain]

a little off topic from your main questions, but is 52% permitted for extended range diver? I thought Advanced Nitrox certification was necessary for any nitrox mix over 40% O2

i am tdi adv ntx, deco proc, and er

 

[nimoh]

i am tdi adv ntx, deco proc, and er

thanks for clarifying...I'll shut up now