This thread is relevant to the OP's query: http://www.scubaboard.com/forums/ask-dr-decompression/342-80-20-100%-o2-decompression.html
Just Started Diving Again and Found out I'm an Idiot ?????? Need advice.....
- Thread starter burjee1
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johngayle
Registered
I have a friend who uses 80 % and 40% for deco. Most use 100% and 50% if you put those gases in vplanner and compare deco is the same runtime.
PfcAJ
Contributor
There's more to it than just 'runtime'.
waterpirate
Contributor
I don"t dive in holes in the ground, so to me " where you do it " is one of the other factors to consider. The other is the depth and the profile. I like to get on a deco gas early on deep dives, like a 30/30 and ride that up to 40fsw and get on 70. YMMV but for mid atlantic open ocean it is what makes sense to me.
Eric
OK, this is actually more interesting than it seemed at first viewing. My initial reaction was not dissimilar to dumpsterdiver's - What the heck does that mean? But, a message for me is maybe I can look at numbers differently.
There are some evident 'jumps' in absolute efficiency: the first comes in simply going from air to an enriched air mix such as 32% for deco - 1.0 to 1.16. A second comes from going from 32% to 50% - 1.16 to 1.32. IOW, I get an equivalent incremental bump with each. And, yes, I do understand the proportional differences in going from 1.0 to 1.16, compared to 1.16 to 1.32. So, if I look at percentage relative improvement, the two jumps are 21% to 32% (a 16% improvement in relative efficiency), and 32% to 80% (an 18% improvement in relative efficiency).
Thinking about it by simply looking at the chart, I don't get a great deal more from using 100% as my second gas compared to 80% - I get some increase in efficiency but not a huge amount.
I have long used 50% and 100% as my two deco gases. Easy to blend, easy to use, standardized, the MOD labels already pasted on my bottles are set for those mixes, etc., etc. My dive buddy has suggested, from time to time, using 80% as our second mix. I told him I didn't see the point, although his suggestion of making the switch a bit earlier had some appeal. I recently did a dive to 245' where one member of our team wanted to go with 30% and 100%, in order to get off helium earlier, so we did. The run times were similar to a projection with 50% and 100%, and the practicality made sense. So, I started thinking whether some different mixes might be worth trying.
Simply going by the math is not necessarily the overriding factor in selection. I may stay with 50 and 100% for convenience (I don't have to 'blend' 100%). But, now I think I will present the table as a discussion point to future tech students, simply to get them to think about their mixes a bit more.
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