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Alan Wright writes: > > Dave Story writes: > > > Or use a computer. Typically, I do at 15-25', depending on conditions > > and the presence/absence of a downline. I make sure I don't rise > > above 10-15', and just count my deco time at the average depth (15-20'). > > Most computers I've seen don't allow this latitude. If you rise above > the stop they go into error and you have to drop down, or wait for the > trough to pass. I'd like to know if the computer is compensating for > the amount of time spent shallow... I'm confused by this response. Dave is surely not saying that he is doing any part of his stop shallower than the ceiling indicated by his computer. He is talking about the shallowest (10 foot) stop which is where surge could be a problem. I think on his profiles very little deco time is at deeper stops, anyay. So using this plan there should be no time spent shallow. I don't know of any computers which don't allow stops to be performed deeper than indicated, although they penalize you for doing this in accordance with conventional theory. If you spend more than a short time above the indicated stop depth the computer is likely to go into error mode. Aladins simply suspend desaturation calculations while too shallow. Although there may be valid concerns about bubble formation messing up the model it is clear from a theoretical point of view that for the short time the computer lets you spend too shallow before going into error mode you would be offgassing. Hence suspending calculations is conservative. Once in error mode they lock up for more than 24 hours. This is a serious disadvantage if you want to use them for depth and time measurements for mixed gas dives, where deco time (using nitrox and oxygen) is shorter than the computer would require. One cave diver built a little decompression chamber for his computer to get around this problem. > On a dive plan where you have to rise to staged bottles you don't have > this luxury... But on a dive where you have to rise to staged bottles you are most likely doing a gas switch. Otherwise it would be okay to just keep breathing from your main tanks, since deep stops are short and you should have enough reserve gas to do it. If employing gas switches which are critical (because you are shortening deco time compared to staying on air) you wouldn't be using a computer to control the decompression at all. > On Buhlmann tables you have a stop every 3m above 24m If we > take a Buhlmann profile; say 45m for 40 mins. The deco on this is: > > Depth (m) 24 21 18 15 12 9 6 3 > Time (min) 2 5 9 18 34 > > This means that you will have been bobbing up and down at 3m for over > half-an-hour possibly by up to 2m in rough seas. I wouldn't trust the > repetitive group or the allowable BT for subsequent dives from a > computer. Actually, the dive would have to be damned important before > considering doing it in these conditions in the first place. A simple solution to this problem, if not using a computer, would be to use nitrox and decompress deeper, but with equivalent air depths at least as shallow as the stops the table calls for so no penalty is incurred. On the other hand, using a computer and air the resulting repetitive dive calculation should be, if anything, more reliable than using a table. Bill Mayne
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