Tank weighting

Hi all,
I am presently diving with a steel 12.2lt and a 3lt pony attached to its side ,with my dry suit and under suit i have to put on an extra 7kg of lead.This works out as ,(not including gas)

cylinder + pony =16kg
lead weight = 7kg

total 23kg.


This set up i have dived with after after doing a wieght check in the pool at 50 bar.
now down to the real question i have just got my self twin 7lt and a set of twin 10 lt and there wieght is as follows,

twin 7's =15kg
twin 10's =22kg

So if i dive with the 7's i would have to add an extra 1 kg where as if i dive with the 10's i would have take off 6kg.
I know nothing beats a proper weight check in a controled evioroment but is my maths correct as a rough starting point as i does seem a bit too simple,have i missed something here.Also only having 1 kg of ditchable wieght seems a bit worring with the twin 10's.

Look forward to your replies,

Regards,

MAL..

Becuse what's important is the buoyancy of the cylinders, not their weight in air.

For example, if those twin 10s that weigh 22kg displaced 22kg of water, they'd be neutral. If they only displaced 20kg of water, they'd be 2kg negative and you'd require less weight.

Nothing beats just jumping in the water with nearly empty cylinders and adjusting your weight.

Roak

Thanks Roakey,
I thought it was to simple and just been reading about displacment and the Ureaka guy (can't remember how to spell name).And also found a bouyancy calculator for cylinders on a web site and i'll see if i can now work out the maths.You are right you can not beat jumping in and checking.I was hoping if you found the correct wieghting with one set up you could find roughly what it would be using different tanks by just maths.
I thought it was not right when i only ended up with 1kg on the belt when i see divers with bigger setups with still tons of weight round there waist .Thanks for pointing out where i was going wrong.

Regards,

MAL..