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Posted by Walt on May 13, 2007 at 10:36:20:

In Reply to: Lift capacity BCD posted by I_AM on May 12, 2007 at 18:01:16:

Remembering that one of my old BC's had a range, thought it would be fun to compute this. If you have a 40# lift BC, it would displace .625 cu ft of salt water and .641 cu ft of fresh water, but the actual phycical dimensions and capacity of the bag would have to be the same, wouldn't it? (unless made of Spandex!)

Now, a .625 bag displaces 39 pounds of fresh water whereas a .641 bag displaces 41 pounds of salt water, but that is not the true range, as the bag's dimensions and volume don't change.

So, assume an average size of .633 cu ft; what lift doews that provide? In salt water, 40.5# and in fresh water 39.5# (or an average of 40#).

An easy way to determine this (apart from grueling physical measurements that would probably be wrong anyway) is to fill your BC with water, then empty it and measure the weight of the water (use fresh or salt of a known density). I'd recommend fresh, as you might as well clean your BC while you're at it. Since you're cleaning your BC you might as well get in a dive first.

If its .633 cu ft for a 40# lift, then oyu have your answer - its an average. If its less, demand some money back; if more, you owe them for the extra capacity.

Let us know your results.

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