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Alan Wright writes: > > Dave Story writes: > > > > In fact, it is one of the best things about deco on a computer: since > > hanging deeper reduces the pressure gradient, you are less likely to > > bubble. With tables, you cannot hang much below the indicated depth > > because (most) tables do not take the slowed offgassing into account. I strongly agree with this. Although you might get by hanging 10 feet or so deeper when using tables you are then guessing. This is another case where using nitrox to reduce your equivalent air depth is a big advantage. > I don't think anything I said disagrees with this. I think my tone was too > strong, for which I apologise. I think what I should have done is asked the > following questions. I didn't detect any problem with your tone. But in some of your early posts I think you may have just mis-stated your intentions with regard to substance. In particular I thought you were questioning whether a computer would correctly figure decompression if you did stops deeper than required. Or maybe just something got lost in the translation. Your tone seemed fine to me and no apologies were needed. > Do you plan the whole dive including stop depths and times before you > enter the water and then follow it during the dive, or do you make a rough > plan and then follow the computer during the dive? Ideally the whole dive should be planned in advance, but this is rarely possible or necessary on a recreational cave dive like most of the dives I do. On most air or nitrox dives to moderate depths gas supply for decompression is not a problem, given the reserves allowed for cave diving. Even so it is common to hang an extra bottle for emergencies. Very often the extra bottle is O_2 or nitrox. I still try to have a good idea how long decompression will be based on tables and/or previous dives at the same cave. To give you an idea of the range of dives for this, my longest decompression came after a dive to about 165 feet for 45-50 minutes, with a fairly square profile. Decompression by computer (Aladin) was 131 minutes, including several minutes of padding. USN tables would call for less. The dive was turned after consuming 1/3 of my air, from slightly over filled 104s. I was just barely able to finish decompression on the remaining 1/3 or so after swimming out. No flames, please. I did have a full stage bottle in the water in case I had needed it. I wasn't into nitrox at the time or I would of course have had nitrox in the stage bottle and used it. Clearly this seat-of-the-pants approach is not adequate for more extreme dives and can't be applied to trimix dives. > Or maybe I should ask, would you be able to complete the deco if your > computer failed? Do you carry tables which use the same model as your > computer and would you know how to enter them half way through deco? I confess I carry USN tables, which are less conservative than my computer. I also use Buhlmann tables for estimating dive plans. But my Buhlmann tables aren't water proof. My use of USN tables is partly from inertia, i.e. they are what I have. But in part I like having less conservative tables for in-water emergencies. I will probably be suffering from a lot of round up if I ever have to use the tables to decompress. I'd rather have a minimal decompression plan which I'm more likely to be able to carry out even if there are problems. I can always pad it, conditions permitting. I don't think it is critical that tables for emergencies use the same model as the computer. On a multi-level dive the computer won't be close to the tables anyway. My first backup in case of computer failure is my buddy's computer. If I could afford it I'd carry two computers myself, preferably NOT identical computers, and decompress using the most conservative so neither is ever in violation mode. This doesn't contribute to task loading during the dive. Once I start decompression I can easily handle watching two computers. > ... The other comment refered to the fact that you may not be carrying > enough gas to complete the dive. Therefore you have to get to your stage > bottle. If it is hanging at 10' then you rise to 10'. You do what you have to do in that case. But I would not want to cut it that close. Why not hang the stage bottle deeper? Bill Mayne
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