Posted by Chris on June 08, 2002 at 21:41:05:
In Reply to: Re: hp vs. lp tanks posted by Kendall Raine on June 07, 2002 at 13:44:03:
I too have discovered that much of the available buoyancy data online is wrong.
However I don't recommend finding out on a dive.
Buoyancy is in fact very easy to measure, thus it is easy to adjust for before a dive.
I have measured all of my tanks, so I know how to change weights when I change tanks.
The most important measurement is the buoyancy at 500 psi, because this is where you will be after a long dive when you are doing your safety stop.
Example:
I know for tank A I need a 16 lb. weight belt.
Put tank A (at 500 psi) in a trash can filled with water, and weigh it with a scale.
Lets say tank A weighs 1 lb. under these conditions.
Thus tank A is 1 lb. negative.
Put tank B (at 500 psi) in a trash can filled with water.
However tank B floats, so I add a 8 pound weight (because that is what I have handy).
With the 8 pound weight this tank now weighs 3 lbs.
Thus tank B is 8-3 = 5 lbs. positive.
Thus I now know if I want to use tank B instead of A add 1 - (-5) = 6 lbs to my weight belt. Thus instead of 16 lbs, I would need to use 22 lbs with tank B.
Although there is a small difference in buoyancy between fresh and salt water, it is a second order effect.
In the above example tank A is a PST HP120 and tank B is a Luxfer Al 80.