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Jan, Firstly, why do you think that 26/25 is a better mix than 21/35 at 45m ?? I'm guessing that because you are using the maximum ppO2 of 1,4 you are gaining by having to do less decompression, right? Also, by using less helium, the decompression software gives you shorter decompression, right? Well, using 21/35 has benefits over the "custom mix" or "best mix". First off, what if you mix your 26/25, get to the dive site and find that the wreck isn't in 45m, but rather 47m? What if you get to 47m and see something interesting just off the wreck in 49m? You were already at the edge of what you considered to be a safe limit, but to see the interesting things, you'd have to push the limit even farther. How lucky are you feeling? If you were diving lower oxygen and higher helium like 21/35, then you could see those things without risking toxicity or narcosis. Also, if you're diving the same mix for a given depth range, you get to know the deco for that mix. There are lots of dives that I used to consider deep and complex that I no longer need tables for. Now, 25% helium has no advantage over 35% helium. Helium is a very friendly gas, much moreso than nitrogen. Now, I don't advocate diving to 45m on air, but if you used to do it, do the same dive with the same deco but using 21/35 instead of air and you'll feel much better after the dive. If your deco software says that you need less deco with 25% helium, it is lying to you. When I dive to 45m on 21/35, I tell my software that I dive it on air to get a shorter schedule. So you get no advantage with less helium. High helium mixes also lets you blow air on top for a shallower repetitive dive and still have enough helium to avoid narcosis. Now back to the oxygen. You are going to expose yourself to high oxygen on deco, especially if you're going to be diving over many days. Our lungs become inefficient when they get exposed to lots of oxygen, so it is now common practice to keep the O2 low when on the bottom (1,1 or even 1,0) and then go to 1,6 on deco, but take breaks to a low ppO2 mix. So, high helium is good. Low oxygen is just fine too. Standard mixes let you mix, calculate deco and have all the buddies show up to the divesite with the same mixes much easier than "best or custom mixes". Does that help? ss >From: Jan Werbi�ski <janwer@fo*.co*> >Reply-To: Jan Werbi�ski <janwer@fo*.co*> >To: <techdiver@aquanaut.com> >Subject: What ppO2 for bottom mix? >Date: Mon, 14 Jul 2003 11:18:25 +0200 > >What ppO2 should I use for typical series nitrox or normoxic trimix OC >diving? > >I learnt 1,4 is safe limit for a bottom mix and 1,6 for deco. Of course if >there are following dives then all limits on deco should be watched >carefully. Why some people advice me it's better to use 1,1- or 1,2? I >don't >know any reason for this. Maybe for CCR divers there are reasons but for >OC? > >What is the reason some folks recommend Tx 21/35 for a 45 meters wreck with >up to 30 minutes BT? I personaly prefer to use 26/25. All other divers have >different mixes so there is no need or possibility to be dependant on >buddy. > >What advantage have GUE standard mix for above dive for solo diver making >custom blends and profiles? > >Jan Werbi�ski O0oo....._[:]) bul, bul, bul >Strona domowa http://www.janwer.com/ [ http://pa54.zgora.sdi.tpnet.pl/jw/ ] >Nasza sie� http://www.fredry.net/ [ http://pa54.zgora.sdi.tpnet.pl/ ] > > >-- >Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. >Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. _________________________________________________________________ The new MSN 8: smart spam protection and 2 months FREE* http://join.msn.com/?page=features/junkmail -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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