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>From uunet!opal.com!owner-techdiver Tue Jan 4 18:41:22 1994 >Return-Path: <uunet!opal.com!owner-techdiver> >Received: from csfb1 by phantom.fir.fbc.com (4.1/SMI-4.1) > id AA16611; Tue, 4 Jan 94 18:41:15 EST >Errors-To: owner-techdiver@opal.com >Received: from uunet.UUCP by csfb1 (4.1/SMI-4.1) > id AA28658; Tue, 4 Jan 94 18:41:13 EST >Errors-To: owner-techdiver@opal.com >Received: from argali.opal.com (via fulton.opal.com) by relay2.UU.NET with SMTP > (5.61/UUNET-internet-primary) id AA10282; Tue, 4 Jan 94 16:16:16 -0500 >Received: from localhost (daemon@lo*) > by argali.opal.com (8.6.4/jr2.9) > id QAA28760; Tue, 4 Jan 1994 16:19:22 -0500 >Precedence: bulk >Errors-To: owner-techdiver@opal.com >From: <uunet!gte.com!jheimann%scsd.dnet> >Date: Tue, 4 Jan 94 15:22:36 -0500 >To: "techdiver@opal.com"%BUNNY.dnet@gte.com >Message-Id: <9401042022.AA08015@bu*.gt*.co*> >Received: from ns.gte.com (cliff.gte.com [132.197.8.9]) > by argali.opal.com (8.6.4/jr2.9) with SMTP > id QAA28744; Tue, 4 Jan 1994 16:19:16 -0500 >Received: from bunny.gte.com by ns.gte.com (5.65c/IDA-1.4.4) > id AA15155; Tue, 4 Jan 1994 16:14:53 -0500 >Received: from SCSD.DECnet MAIL11D_V3 by bunny.gte.com (5.61/GTEL2.19) > id AA08015; Tue, 4 Jan 94 15:22:37 -0500 >Status: R > jheimann@sc*.gt*.co* wrote: >Wrolf recently posted in favor of "poor man's mix," which is trimix blended from >air and helium. While he is correct that this mix is easy to blend and analyze, >I would disagree that it is an attractive bottom mix.... Summaryizing John's points pro poor man's mix: * no high pressure O2 * can compute full composition from O2 analysis alone * can adjust mixes in the event of errors. John's VERY IMPORTANT negative about poor man's mix: >Now for the bad news: it's hard to get enough He in the mix to get an adequate >reduction in narcosis. If you keep the FO2 in the mix at or above 16%, which is >a very good idea since otherwise you can't breath the mix at the surface, then >the maximum FHe in the mix is 24%. Now the maximum operating depth of 16/24 >trimix is 255 fsw (assuming max PO2 of 1.4 ATA). At this depth, the equivalent >narcotic depth (END) is 185 fsw (by coincidence, you get the same answer whether >or not you assume O2 is narcotic). Cosidering the operational complexity of >trimix diving, a 130 fsw maximum END is more desirable. > >I should note that it is possible to make more user-friendly mixes by mixing >nitrox and He. For example, 18/50 mix can be blended from equal parts of EAN 36 >and He. If the nitrox is accurately blended and analyzed first, then the FHe >can be computed (just as with poor man's mix) directly from the analyzed FO2 in >the final mix, and adjusted accordingly. Depending on how you make or buy your >nitrox, you may not need any clean tanks for this approach. > Thanks for responding John! The critical point here is, do you absolutely need to have a bottom mix that you can breathe on the surface. If you carry 50/50 Nitrox then you can use that to get below the "minimum operating depth". Let's say that you set for Equivalent Narcotic Depth of 132 fsw. Basically what you are then doing is diving on 132 fsw of air + however much He you need for that depth. Let's say you are diving to 260 fsw. Then you need 260 fsw - 132 fsw = 228 / 33 = 6.91 ATA He, on the bottom. The net of this is that you for 260 fsw, setting END to 132 fsw, you get 44% He, 12% O2, 44% N2, i.e. 44% He + balance Air. The minimum operating depth for this mix is 12 feet. Mixing is easy - for a 3000 psi tank, add 1311 psi He, and top off with air. (Call it 1300 psi). Analyze for 12% O2. If too low, let out some mix and add more air. Start low - it is harder to add the He, since presumably you have a compressor, but your He tank may be at a lower pressure that your fill pressure. Your pp O2 will always be 1.05 ATA, if you are setting END = 132 fsw. The disadvantage for decompression over pp O2 1.4 ATA roughly equivalent to 8 feet difference in depth (I mean approximate, depends a lot on your tables). You need never worry about your CNS O2 clock again :-). So all we are left with is the operational aspects of diving with a bottom mix that you cannot breathe on the surface. If you are carrying Nitrox 50/50, for decompression from 70 feet to 20 feet, then you would could use that on the surface swim and initial descent. Your backup would be your O2 - I would assume that you would carry that on your back too, but depend mainly on surface supply. Is this acceptable? On blending nitrox and He - yes, I also realize that this gives the best of both worlds. Is this economically feasible though? In order to use most of the nitrox, you would need to set up banks. This could be very feasible for dive stores, but not practical for in the field. Wrolf
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