Cylinder buoyancy information

[60plus]

Hi
I am "new" to diving so don't yet have a log of ideal weights for different conditions. I have big lungs (48 inch chest even when slim) so am pretty buoyant when I breath deeply.
With a full 3 mm wetsuit, and Mares DS5 Boots I find I need 8 Kg on the belt and 3 Kg in the BCD pockets. With a 15 L cylinder I am slightly over buoyant when it is down to 50 Bar. With a full 15 L cylinder I can descend comfortably.
What is the weight of a steel 15 L cylinder with about 205 bar
What is the weight same cylinder with 50 Bar
What is displacement of a 15 L cylinder
So far I have only dived in seawater but might do some freshwater (or swimming pool) practice before my next holidays to the sea.
My instructors were able to sit comfortably in an armchair position. If I tried it my feet rose and I turned upside down. Is a little bit of ankle weight the answer or should I try to adjust my BCD and belt weights to a lower position. During "normal" diving, just swimming along looking at things on the bottom I have no problem keeping level and the feet up tendency is useful when I want a close look without my fins hitting the sea bed.

Cheers

 

[pasley]

Your question is good one, but misses two critical points - WHO MAKES THE CYLINDER and are we talking high pressure or low pressure. Example: Steel 80 Cubic Foot tank both made by the same company and be negative 1.7 pounds, to a negative 7.22positive or 2 pounds or neutral all depending on mfg. Other steel tanks can be positive (very rare), neutral (also rare) or negative by a fraction to as much as 7 or more pounds. So look up the specification for the tank you are diving. I carry a table with me on all dives just in case I have a tank other than my own (example the boat provided it or I am renting on a trip).
To get the answer you need see the manufa the table at: Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan or http://www.indianvalleyscuba.com/services page/Tank Inspection/information/CYLINDER SPECIFICATIONS.pdf

 

[swimlikethefish]

Every 35 to 40 bar is a roughly, pending tank size and volume, about 0.4 to 0.5 kilos. So 70 to 80 bar is 1 kilo. Most tanks swing 2.5 to 3 kilos from full to all the way empty. It really depends on tank manufacturer, Faber, PST, XS Scuba.... If you had more specifics it would be easier to answer. Also, steel or aluminum.

As far as the weight goes, I'm not a huge proponent of ankle weights. Try adjusting the tank on your bc instead. Hope this helps.

 

[Marie13]

OP is in the UK, so he needs a non-imperial chart.

 

[rongoodman]

Most tanks swing 2.5 to 3 kilos from full to all the way empty. It really depends on tank manufacturer, Faber, PST, XS Scuba.....

The only thing it depends on is the size of the tank. The weight of the gas contained doesn't vary with who the manufacturer is.

 

[swimlikethefish]

The only thing it depends on is the size of the tank. The weight of the gas contained doesn't vary with who the manufacturer is.

That was a guesstimate, I use imperial so that was a rough approximation. In my world every 500lbs is approx 1 lb on a steel 100 or Al80.

If you want to be technical 100 cuft of air is exactly 8.0713 lbs. 80 cuft weighs 6.457 lbs. Those weights would be exact at the rated tank pressure and exact volume. But an Al80 isn't 80 cuft it's 77.6 so the air in that tank from full to empty would be 6.2633 lbs.

 

[60plus]

Thanks for your replies. I did say it was a steel cylinder. Also many of us in the UK can and do work in both imperial and metric measurement. I just hire my cylinders when on holiday so I don't know the maker. What your answers have told me is all I need. I think adding 1 kg to my belt and moving 1 kg from the BCD to the belt then adding a touch of air to my BCD should improve both my excess + buoyancy and trim. I normally have my BCD near fully deflated.
Cheers