Deco Gasses

[ianr33]

Thanks Aquamaster. Just one question.



You are saying that the plan you described would incur a heavier CNS percentage, and that the same dive would have less of a CNS load because with 50% you start breathing a leaner deco gas earlier and deeper?

Using 50% you will only be breathing a pO2 of 1.6 for a minute or 2 (At 70 feet)
Using 100 % though you might have 10 minutes on the 20 foot stop at a pO2 of 1.6,thus the higher CNS %

 

[DA Aquamaster]

To add a bit more detail:

With DPlan and the 150' for 30 min profile and 100%, I get the following stops:

70' 2 min
60' 2 min
50' 3 min
40' 4 min
30' 6 min
20' 5 min
10' 9 min

Where the 70' to 30' stops are done on backgas with O2 at the 10 and 20' stops.

The OTUs for the bottom portion of the dive are 49 while the CNS clock percentage is 20. The deco portion of the dive adds another 28 OTUs and another 30% of the CNS clock with 23 of the 28 OTUs and 27% of the CNS clock occurirng on the 10 and 20' O2 stops.

In comparison with 50% you get:

70' 2 min
60' 1 min
50' 2 min
40' 2 min
30' 5 min
20' 6 min
10' 13 min

All of which are done on 50%.

The OTUs for the bottom portion of the dive are the same at 49 while the CNS clock percentage is the same 20%. The deco portion of the dive adds another 26 OTUs but only another 12% of the CNS clock.

In effect, with 50% you get on it sooner but the time spent at high PPO2s is at the shorter deep stops of only 1-2 minutes rather than at the longer 10 and 20 ft stops where the PPO2 is in the .7 and .8 range. The deeper stops are shorter on 50% but the shallow stops are slightly longer, but the CNS % accrued is still much lower.

 

[peterbj7]

IANTD publish tables assuming that your rich deco gas is 75%. I always opt for 75% or 80% and never pure oxygen, not because of extra mixing risk but for two other reasons. When the sea is rough I can breathe 80% well below the wave action, whereas pure oxygen puts me right in it. And I can breathe 80% for a long, long time, whereas my time on pure oxygen before an obligatory break off it is quite short.

You'll learn all this stuff when you start your course.

 

[DA Aquamaster]

Absolutely - the 33' MOD on 80 percent gives you a solid buffer on a 20' stop, especially if you do the last stop at 20' in rough seas compared to 100% where you are at a PPO2 of 1.61 at 20'.

And the 1.3 PPO2 of 80% at 20' is much more permissive in terms of the CNS clock than the 1.6 PPO2 of 100% O2 at 20' (0.56% per minute compared to 2.22% per minute).

And the difference in total run time when using 80% versus 100% for deco is seldom more than a few minutes - well within the back ground noise of different tables.

 

[ScubaDemon]

If your instructor will use IANTD (Buehlmann) tables for excellerated decompression, you will be using a EAN75 or greater from 6m. Some people use O2, I prefer to stay a bit off the 1.6bar ppO2. It all comes down to what makes sense for the dive you are doing, or whether or not you are going with DIR ;-)

Make sure to discuss the pro's and con's with your instructor, and buy the IANTD Encyclopedia (second to last version if you can get it) by Tom Mount. This book will answer MANY of your questions, and it is worth every penny!

 

[Jax]

If your instructor will use IANTD (Buehlmann) tables for excellerated decompression, you will be using a EAN75 or greater from 6m. Some people use O2, I prefer to stay a bit off the 1.6bar ppO2. It all comes down to what makes sense for the dive you are doing, or whether or not you are going with DIR ;-)

Make sure to discuss the pro's and con's with your instructor, and buy the IANTD Encyclopedia (second to last version if you can get it) by Tom Mount. This book will answer MANY of your questions, and it is worth every penny!

Why the second-to-last edition?

 

[DA Aquamaster]

If I recall Tom Mount wrote the first edition in 1998, the revised edition in 2003 and the latest edition in 2008.

 

[PfcAJ]

Does this one have stuff on aligning your chakras before the dive?

 

[ScubaDemon]

Why the second-to-last edition?

Because this edition still has ALL the tables in it! Later edition reduce tables to a minimum and promote the use of software.

 

[HowardE]

Because this edition still has ALL the tables in it! Later edition reduce tables to a minimum and promote the use of software.

Could it be similar software to the one used to generate all of those ridiculous tables?