pressure vs. volume

[fnfalman]

Thanks a lot everybody for detailed explanations! I beleive I've got the idea. My physics teacher could have learned a lot from you!

I'm glad we're of help.

Follow this link if you were to want to learn how to figure out what your volume gas consumption is and how to plan for your bottom time based on gas consumption. I'd say air consumption, but that doesn't take into account of Nitrox and Trimix.

Rock Bottom and Gas Management for Recreational Divers

 

[peterbj7]

A couple of points, probably just to confuse. The European way of measuring tank volume is to measure how much water the tank will hold, whereas the American way is to measure how much air at standard conditions can be compressed into it. If you know this "volume" measurement for a European tank you know all you need to know to use that tank, but with the American system you also need to know at what pressure it was deemed to hold its stated volume of gas. The units used (litres, cu.ft., gallons, bar, psi etc) have nothing to do with it. What matters is just WHAT you are measuring.

I find the American system a constant nightmare when I have to know exactly how much air a given tank holds, essential for any form of technical diving. And it's not as simple as has been stated above - I have owned some 300 tanks and I can tell you that even amongst aluminum 80's the rated pressure varies amongst manufacturers - you can't say that a nominal 80 actually holds 77.something.

 

[cloudflint]

good lord, I can do calculations for air available, gas consumption e.t.c in my head using bars and liters but looking at the calculations with psi and cubic feet makes my head hurt

Heres some formula using litres and bar:

Air required for dive = time * surface breathing rate (litres/minute) * depth (bar)

Air in tank = tank volume (litres) * pressure (bar)

Breathing rate = pressure used (bar) * tank volume (litres) / time

How long will tank last me at depth = air in tank (litres) / Surface breathing rate (litres per minute) / depth (bar)

The only good thing about working with cubic feet i found was that I use 30 liters/minute as an average surface breathing rate which works out at about 1 cubic foot per minute so for working out how long a tank will last or how much air is required its dead easy