how to plan a dive without a dive computer as a crutch

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Posted by RaiderKarl on March 07, 2001 at 16:55:03:

For the weekend of March 17, my dive club will be visiting the San Jose Creek Beach of the Carmel River State Beach, known locally as "Monastery", due to the beautiful Carmelite cloister of Roman Catholic nuns and priests in the immediate vicinity. Local divers appropriately call this beach "Mortuary Beach" or "Monster Berry Beach" because of the coarse gravel-like sand there that make exits difficult, as well as the deaths that regularly occur to divers [the last being in January 2001]. Regular physical fitness self-justifies my applying the U.S. Navy dive tables for my deep dives. Since I dont want to blow all my dive computer's circuits in so doing, I keep it set in gauge mode, which effectively turns off the calculation features for the built-in algorithm installed by the manufacturers. I always plan my dives and dive my plans. For this upcoming dive, I plan to go to 150 feet. At that depth, my U.S. Diver high pressure steel 120 cubic foot tank carries enough air for a 20 minute exposure at that depth. This dive is deeper than the nitrox limits, so nitrox is out of the question. Twenty minutes is more than the Navy tables allow for a NDL dive, so I have to compute decompression for the eggression, and this preliminarily becomes an overhead obstructed dive due to the decompression stop ceilings. Descent time, re-ascent time, and decomp stops will all need to be factored in. I calculate 5 min 30 sec for the 30 ft/min re-ascent, 3 min for the 20 ft first decomp/safety stop, 2 min for the 10 ft second decomp/safety stop, 1 min 30 sec to make the original descent drop, leaving me with 8 available minutes, for a total maximum available "bottom time" of nine and a half minutes. Not very long, but long enough. I will round this down to 9 minutes, for simplicity's sake. This also factors in 250 psi of remaining air in my large tank. My breathing rate is 0.95 cubic feet per minute, slightly less than the average of 1 cubic foot per minute for male divers. Now, what could go wrong? Since this beach has a deep Ocean trench leading up to it, anything pelagic could swell up from the deep. A very large carnivorous fish (I hate the S-word) could pin me down temporarily on the Ocean floor while I wait for it to swim away. That could consume additional air and leave nothing for decompression. Therefore, obviously my 120 tank is too small to go to this depth safely for this length of time, and either I need to go with my double 100s, giving me 200 cubic feet of air, and resulting in 40 minutes of bottom time available, but more decompression loading, or I could cut my bottom time in half, to 5 minutes, and reserve the other 4 minutes for an unexpected emergency like that. Five minutes of bottom time puts me within the Navy NDLs, but I will go ahead with my planned decompression anyway, just to be sure. I wear a small wrist slate, on which I will write my time limit, and I will set my timer on my dive computer to buzz me at 5 minutes. That way, if I get narked, it should wake me up to reality. What about equipment contingencies? My Ziegle Tek B/C is fairly new, and it has functioned well for me in the past, with a lift capacity of 65 pounds. My ScubaPro D400 reg & Atomic D backup were annually serviced within the past month, and are working great too. My weighting system is divided into 3 units, 1/3 on my weight belt, and 2/3 on my B/C. I can ditch the weights immediately or in parts if I have to. An extra weight belt on the beach canvas would be a good idea in case I do have to ditch my weights, so that I can then exit the water, put on the extra weights, and return to my first safety/decomp stop of 20' if I missed it. Since I will be diving on a single tank, I need to bring a pony bottle with its own hose and regulator on it as a backup. My friends in the dive club will be my ad hoc beach support, since you never know when or if DCS is going to hit you, even on a conservative dive. I need to find an experienced buddy to go down with me, but if not, I am willing to dive this solo, since there is at least beach support. I understand for myself that due to the somewhat technical nature of this dive plan, I am on my own, and no one will be able to help me if I mess up, even though we have several divemasters in our club. I can live with that responsibility. I think I have pre-planned everything. No pushing a modern modified algorithm beyond the Navy limits by "compartment loading" on a "multilevel" dive with a dive computer being used as a crutch. Only the remote possibility of the flu could interfere with this planned deep dive now. In a perfect world, I would have a pair of D/Ms or A/Is on trimix going down to watch over me, but its not a perfect world. The purpose of this dive is to see what is down there at 150' and then come back safely. This is my favorite dive site, and I want to explore more of it, on air. For a second dive that day, I will limit myself to 30' or less and with a buddy, so that the repetitive dive becomes additional decompression time in effect, and I will plan that to consume 1/3 of my air at 30' or less, 1/2 of my air at 20' or less, and the rest patiently relaxing at 10 feet. After these two dives, I will note my perceptions, and watch to see if any minor or major DCS symptoms occur, and log it, before I dive any deeper in the future. The D.I.R. cave divers of the East Coast have given us much food for thought. I appreciate that. After all, I would never venture into a cave at all. Too dangerous! Even for a daredevil like me.

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