Posted by Walt on October 23, 2004 at 22:00:19:
In Reply to: Why 34 lbs? posted by Chris on October 23, 2004 at 20:47:57:
Chris' calculations are correct for the change in air mass. Minor problem with the garbage can approach - make sure you use salt water or adjust for the difference in density of salt versus fresh water (salt water weighs about 34 lbs per cu ft, fresh 32.4, or about a 4.7% differnce).
Alternatives: compute the fluid displacement of your tank - 2 ways, calculate the cylinder's volume (good old geometry) OR fill the garbage can to the brim, submerge your tank and overflow the water, then determine how much water was displaced in cu ft, multiplying that by the weight of fresh or salt water, depending on what you'll be diving in. This number becomes the bouyant force exerted by your cylinder, a constant that you can use with the numbers provided by Chris and calculated in the next step. Next you merely weigh your tank(s) full or empty on a quality bathroom scale - note the PSI and calculate the weight of the air inside (Chris's method), subtract it and that gives you the net tare weight of your tank - negative bouyancy, again a constant. Put it all together and you have your beginning of and end of dive weight attributable to the tank.
Then ask yourself, why am I diving with a 120 when I could be diving with a HP steel 80 if I improved my air consumption? After all, you'd still have about the same bottom time, as your N2 load is your limiting factor anyway unless you're sucking air or you're diving shallow, in which case a 120 is awfully nice. The steel 80 (or 72) is lighter and smaller, factors which will help you improve your DACR.
Finally, go back to Chris' observation and ask 'why do I have so much weight on my belt?'
A final suggestion (not requiring math) - go on a dive, but keep some (most?) of that weight in a pouch and take along a goody bag. When you do your end safety stop on an anchor chain (ideally with 500 PSI left), fully deflate your BC or reduce your dry suit to a comfortable volume, tie your goody bag to the anchor charin, and begin removing weight, putting it into the goody bag. Continue until you're neutral. Now you can give that weight to your dive buddy (or sell it), as you've determined what you really need. I'll bet you won't need 34 lbs.
Gosh why did it have to rain making me cancel my planed beach diving this weekend?