tank capacity

[superhollando]

just ask here if anyone know it well....

for a S63 Aluminium scuba tank, working pressure is 3000 psi, water capacity is 9 litre, capacity is 63 cu. ft.

at the beginning this tank is empty. actually there is somewhat 9 litre air with 14.7 psi (absolute pressure)


Does it mean it needs additional 63 cu. ft. air to charge this tank to 3000 psi (or 3014.7 psi in absolute pressure) ??

So, since PV = constant, then...
3014.7 psi x 9 litre = constant << is that right ??

when we calculate the SAC rate, how much is the breathable air ?? 63 cu. ft. or (63 cu. ft. + 9 litre) ??

 

[DA Aquamaster]

It is 63 cu ft at 3000 psi.

Don't over think the rest of it. 3000 psi is in fact 3014.7 psi absolute, but the gas stops flowing out of the tank when the pressure equalizes at 14.7 psi, so you can't count the 9 liters that remains at 14.7 psi

If you want to over think it, go in a productive direction. For example most regs will stop delivering useful amounts of gas at a pressure above 14.7 psi absolute (which we will call zero to keep the math and concepts simple). If your particular reg stops delivering gas or gets very hard to breathe at 50 psi, 100 psi or perhaps 150 psi, then 50, 100 or 150 psi effectively become zero.

Using 100 psi, that represents 2.1 cu ft in your steel 63, so you'd do your gas planning based on 60.9 cu ft, not 63.

The only way to really tell when your reg will stop delivering gas is to breathe the tank down and observe the pressure where this occurs.

 

[Charlie99]

I'm not sure what you are asking.

There are 28.4 liters per cubic foot, so the the 9 liters water capacity is about 0.3 cu feet. You get that same answer by diving 63 cu feet by the 204atm (3000psi) service pressure.

The equation you wrote
"3014.7 psi x (63 cu ft + 9 litre) = constant" needs some revision.

It is actually 3014.7 psi x 9 liter = 63 cu feet ---- of course, this equation is a mish mash of mixed units. A better way of writing it would be

207 bar x 9 liters = 1860 liters or

(3000/14.7)atm x 0.3 cu ft = 63 cu ft

Whether the rated capacity counts or doesn't count the 1st 9 liters or 0.3cu feet that the tanks starts with isn't really material. The difference is less than 1/2 of 1%. Also, you can't really breathe a tank down to surface pressure due to pressure drops in the regulators.

I hope this helps answer your question, whatever it is.

Charlie Allen

p.s. "So, since PV = constant, then... " isn't quite what is going on. V, the volume of the tank is a constant. What varies are the pressure and the total free gas.

 

[superhollando]

The equation you wrote
"3014.7 psi x (63 cu ft + 9 litre) = constant" needs some revision.
yes, I got mess up today. you are correct


It is actually 3014.7 psi x 9 liter = 63 cu feet ---- of course, this equation is a mish mash of mixed units. A better way of writing it would be

207 bar x 9 liters = 1860 liters or

(3000/14.7)atm x 0.3 cu ft = 63 cu ft
if I calculate it in a reverse manner....
63 cu ft = atm x 0.3 cu ft
then, atm = 210 atm

since 1 atm = 1.013 bar
then, 210 atm = 207 bar

since 1 bar = 14.51 psi
then, 210 atm = 3004 psi, or say 3000 psi

Then I conclude that "63 cu ft" doesn't count those little 0.3 cu ft, because 63 cu ft is matched with 3000 psi. This is what I want to clarify....when the tank pressure is zero (gauge pressure), there is "no" air inside. when it reads 3000 psi, there is "additional' 63 cu ft air inside

 

[miketsp]

You're trying to get a precision that doesn't exist anyway as at 200 bar the basic gas law is already getting non-linear.
To improve your calculation try using Van der Waals instead.

See these curves:
Validity of the Ideal Gas Law