Diving in using a litre cylinder with bar working pressure that weighs kg, and has a 400 gram valve gives the following buoyancy (positive weight means positive buoyancy, e.g. there's a lift.):
 

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When full	At 50 bar	When empty

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Water Type
Salt Water	Fresh Water

Cylinder Type
Steel	Aluminium

Working Pressure
232 bar	300 bar

Cylinder Volume and Weight
3 Litre 7 Litre 10 Litre 11 Litre 12 Litre 15 Litre 18 Litre	4 kg 4.5 kg 5 kg 5.5 kg 6 kg 6.5 kg 7 kg 7.5 kg 8 kg 8.5 kg 9 kg 9.5 kg 10 kg 10.5 kg 11 kg 11.5 kg 12 kg 12.5 kg 13 kg 13.5 kg 14 kg 14.5 kg 15 kg 15.5 kg 16 kg 16.5 kg 17 kg 17.5 kg 18 kg 18.5 kg 19 kg 19.5 kg 20 kg 20.5 kg 21 kg 21.5 kg 22 kg 22.5 kg 23 kg 23.5 kg 24 kg 24.5 kg 25 kg

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