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My point with the below comment is that it would be a strange mix to have in your tanks (50/21). Maybe there are applications for this but probably not very practical for most. Having a 50/17 mix or a 30/21 mix might be more common. I used the 50/21 mix because I wanted to use for illustration purposes mixes that had the same O2 content and just the N2 content had changed. The point of the email was not so much the mixture, it was that the statement that decoing on O2 vs. 50/50 in a single deco gas situation is not always correct. Art. -----Original Message----- From: Joel Silverstein [SMTP:joelsilverstein@wo*.at*.ne*] Sent: Wednesday, January 12, 2000 6:29 PM To: Paltz, Art; 'Oystein Mehus'; Manos Manoli Cc: Tech Diver Subject: RE: I think I don't understand Art you wrote ...... > >Say you are using a helium rich mixture, say 50% He, 21% O2 (want to keep >the O2 the same when comparing. What you would be doing with this mix in >your tanks I don't know. > There are a number of reasons you might have this mix in your tanks but the most common reason would be the site you set out for today got blown out and you ended up on a shallower wreck. Back in November of '92 we all went out with sets of 17/50 trimix for a dive on the U-Who only to wake up in the middle of snotty seas and opted not to continue out to that wreck. We were already committed to the day for diving and opted to go to the USS San Diego for some fun dives, but with only one set of air on board (yeah we used to use air for 65-120 fsw dives). Via Cell Phone Billy Deans got Dr. Hamilton to make us tables to use that mix that day on a 120 fsw site. Mind you this was long before desktop software. The point is there will come a time when the "perfect mix" is not available and you need to make due with what you have. Although 21/50 is not an optimal gas for a 130 fsw dive site it does have some merits. A PO2 of 1.01 atm and P-air of 3.55 atm. The clarity at 130 fsw is about 50 fsw/air which means lots of fun looking for artifacts. The 21% oxygen makes the deco time quite reasonable (vs 16%) whereby you can do anywhere between a 20-60 minute dive with no more than 15-60 minutes of deco with the OEA50%. (run on Abyss. Add in 100% oxygen and it drops to almost a range of 10-50 minutes respectively. For the most part once the oxygen is introduced the decompression time starts to drop quickly, and missing a few minutes either way rarely makes a whole lot of difference (don't read that to mean you can cut your time bend-free). As George has indicated he has had good experience experimenting and testing decompression models and profiles on himself. We all do that to some extent each time we dive. The empirical data that gets accumulated while one works a set of tables is invaluable to the ultimate decompression profile. For the most part I have used either Hamilton Research Trimix Tables or Abyss software for eight years now, with some very good results. The key is to keeping good notes, and understanding how the model works with you. Where I cut some time and add some other, that may not work for you or someone else, but it does for me. As long as you accept the fact that DCI is not an accident, that it can occur on most any dive where decompression is incomplete (incomplete for the gas load not necessarily completing the table in hand,) then you can mess with this stuff all day long. The proof is in the diving. I just would caution those who are new to decompression diving that a 60 or 100 minute deco is not a walk in the park, especially in the ocean. It's easy to toss around long or short deco times from the comfort of the boat deck, but when the sea kicks up, shorten the bottom time and get the flock out of the water. All decompression tables work until they don't Regards, Joel D. Silverstein http://www.trimixdiver.com http://www.nitroxdiver.com -------------------------- Technical Diving Resources -------------------------- -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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