Puzzling. The weight check was done? Yes, people can vary greatly in natural buoyancy. If the weight check was done, it's not a matter of an instructor being more or less flexible (it is what it is). After the weight check, 5 lbs. should be added as the tank gets lighter as air is used up (whether aluminum or steel-doesn't matter)--therefore you are 5 lbs. negative anyway at the start. Could there be any other reason he says no more than 10 lbs.?
With a typical AL 3000 psi 80 cft (77.4 cf), 2500 psi is 64.5 cf. One internet source lists a cf of air as being .08 lbs, so if you start with 3000 and end with 500 psi you used 5.16 lbs of air. The problem is that often for a girl that only needs 10 lbs the training may not be using AL 80 cft's.
If it were a Luxfer AL 50 cft (48.4 cf), there is only 3.23 lbs used at 500 psi, and a 63 cft would be 4.2 lbs, so unless you have more info about the training dives than I see in this thread you are assuming a few things to claim "
After the weight check, 5 lbs. should be added..."
And how about your claim that "
whether aluminum or steel-doesn't matter".....
A Faber low pressure (LP) steel 80 cft is stamped with 2400 as its working max pressure, but it can be filled to 2650 psi as long as it is still rated for +10%. At 2650 psi it is 78 cf, but many would still considered a 2400 psi fill to be full, which would only be 70.64 cf, and surfacing with 500 psi would then only use 55.9 cf (4.47 lbs).
An OMS LP 50 cft is 50 cf at +10% (2650), but again is marked 2400, so the gas used from 2400 psi to a 500 psi end is only 35.85 cf (2.87lbs).
Wasn't that fun!
Anyway, a better rule of thumb is to "add enough weight to make up for the weight of the air used."
