Buoyancy difference between 12 and 15L steel cylinders

[drcolyn]

I am in the process of upgrading from 12 liter 232 bar to 15 liter 232 bar Faber steel cylinders and would like to know by how much (if any) I must adjust my weight belt so that I am still neutral at the end of a dive.
The following factors force me to optimize my weight adjustment.
I do underwater photography in cold and shallow water (mostly in 2 to 12 meter) so I must not be too light. The dry suit makes this even more tricky.
I do shore entries which result in a bit of a hiking trip sometimes so every kilo overweight counts heavily against me during entry/exit.
Does anybody have actual measured buoyancy data for these type of cylinders or other practical advice (apart from getting a boat - not in the budget).
I will also get an 18 liter 232 bar Faber steel in the process but doubt my a ability to handle that one on the rocks.
Description Empty weight kg
12 L 232 Bar 13.60
15 L 232 Bar 16.70
18 L 232 Bar 22.30

Regards
David

 

[Scared Silly]

Fifth grade math - It is the difference in the cylinder weight less the weight of the additional air.

weight change = (difference in empty cylinder weight) - (difference in air volume) * (fill pressure) * (density of air)

= (16.70 - 13.60)kg - (15L-12L) * 0.001 m^3/L * 232 * 1.2041 kg/m^3
= 3.1 - .83
= 2.26

So you can take approximately 2kg off.

 

[drcolyn]

Fifth grade math - It is the difference in the cylinder weight less the weight of the additional air.
So you can take approximately 2kg off.

Hi Scared Silly
Thanks for this info - yes one would think that after having taken Fluid mechanics and Thermodynamics up till my 4th year that I should be able to do the maths myself but sometimes things works a bit different in practice.
I saw this table on the net with actual measured buoyancy (both full and empty) for steel cylinders and though it was difficult to make exact comparisons with all the pressures and volumes being in imperial units it did seem to indicate that between a 10 and 12 liter steel there was a decrease in negative buoyancy when empty.
Even the buoyancy calculator found at UK Scuba - Cylinder Buoyancy Calculation indicates a 0,4kg increase in positive buoyancy from 12 to 15 liter steel.
But I like your answer and will gladly part with 2kg on my belt since I gained 3kg on the tank allready.
Regards David

 

[Hatul]

I considered using steel for pony tank and decided against it for theoretical reasons. The pony air is rarely used and with the small volume I was concerned corrosion may affect the air inside. It could be an irrational fear, but why take the chance.

Adam