Tank Volume

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125 lover

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I would like to know the volume of gas for two tanks at their max working pressure.

AL 80 @ 3000 PSI holds 80 Cubic feet or is the volume somthing different like 72 Cubic feet ??


LP125 @ 2400 PSI holds 125 Cubic feet or do I run it up to 2640 PSI to get my 125 Cubie feet ??

:confused:

Thanks
 
Standard AL 80 holds 80 @ 3100 - at 3000 it holds 77.4. There are "Super 80's" that do hold 80 at 3000.
Steels are rated at their 10% overfill - 2640 in this case.
Rick
 
Originally posted by 125 lover
I would like to know the volume of gas for two tanks at their max working pressure.

This website can provide you with the rated capacity of most popular tanks at their working pressure.

Tank Specs

If you want to know the volume of any given tank for any given pressure you can find out using the following formula:

Actual volume = (Rated volume/working pressure) x actual PSI

Example: 112 cft low pressure steel tank with working pressure of 2640 (2400+ 10% overfill) @ 2000 psi would yield

112/2640 = .042

.042 x 2000 psi = 84.84 cft.

So our LP112 @ 2000 psi has 84 cft of air in it.


Dive often, but dive safe

John
 
Originally posted by Cave Diver

If you want to know the volume of any given tank for any given pressure you can find out using the following formula:

Actual volume = (Rated volume/working pressure) x actual PSI

Example: 112 cft low pressure steel tank with working pressure of 2640 (2400+ 10% overfill) @ 2000 psi would yield

112/2640 = .042

.042 x 2000 psi = 84.84 cft.

So our LP112 @ 2000 psi has 84 cft of air in it.


This formula is only an approximation because it assumes that air is an ideal gas. It isn't, rather it is a "real gas" and, as pressure increases or temperature drops, air's behaviour deviates further and further from that of an ideal gas. The net effect of this is that it takes a different amout of air to put the first 100 psi of gas into a tank than it does to put the last 100 psi in (say at 3,000 psi).

As gases approach the pressure at which they liquify the ideal gas law does a very poor job of describng their behaviour. Therefore the relationship between pressure and volume is not linear as described in your formula.

Your formula also does not take into account temperature. This is okay in a static situation where temp is a constant but while we are breathing down a tank the air is expanding and the temp therefore dropping. Your calculation performed during a dive could give misleading results.

If you read OMS's website you will see that their claimed tank volumes are adjusted for Van der Waals forces (the inter-molecular forces which cause real gas behaviour).

See

http://members.aol.com/profchm/realgas.html and

http://www.omsdive.com/cyl_spec.html scroll to bottom
 
Originally posted by Goldminer
Your formula also does not take into account temperature. This is okay in a static situation where temp is a constant but while we are breathing down a tank the air is expanding and the temp therefore dropping. Your calculation performed during a dive could give misleading results.

Yes, I am familiar with the fact that temperature changes pressure, tho it will not change the actual volume. The example I gave was meant to be used as a guideline and the volumes are approximate. If you were to double check my numbers you would see that if you were to take the original formula (112/2640 = .042) then multiply that back by the original working pressure (.042 x 2640) you come up with 110 cft of air. Did we lose 2 cft? No. It's just a matter of the fact that when round the numbers off, it throws a slight bit of error into the equation.

I would hope that any certified diver was taught in their basic open water class that leaving a tank out in a hot car causes the pressure to increase. I would also hope (and give the readers of this board credit for having more sense) that no one would assume that just because their tank is now 110 degrees and has significantly higher pressure, that doesnt mean that the volume of the tank changed and they can get more bottom time from it.

While you do make a good point about the behavior of gases in relation to temperature, it is my belief that anyone who is needing to know to that an exact degree what the volume of air is in a tank should be taking additional training, where such laws and rules will be taught to them as well as the reasons why they are important.

Again, my reply was just to give an approximate answer and illustrate a point, not to provide a starting point for gas blending or to have them start caculating deco stop points or turn points. Since the original question didn't ask *at a given temperature* I didn't feel the need to elaborate on the effect of temperature.

Thank you however for posting the follow up information, as that may be enlightening to others who never gave much consideration to their tank temperature.

BTW, since we are on the subject, does anyone know the ACTUAL temperature that an aluminum 80 filled to 3000 psi would have to be at to be truly 'full'? Or do we just accept the standard of 3000 psi @ room temperature = a complete fill?

Food for thought...

Dive often, but dive safe

John
 
Originally posted by Cave Diver

I would also hope (and give the readers of this board credit for having more sense) that no one would assume that just because their tank is now 110 degrees and has significantly higher pressure, that doesnt mean that the volume of the tank changed and they can get more bottom time from it.
John

This does bring up an interesting question though.

Assume that you have two identical tanks, filled to exact same pressure, at the same temperature, and therefore have the same mass of air. Two divers dive go to different locations at sea level and dive to the exact same depth in seawater. The first diver dives in water that is 90 degrees F. The second diver dives in water that is 40 degrees F. Would the diver in the warmer water get more “air bottom time,” and if so would the difference be significant?
 
Originally posted by Cave Diver

BTW, since we are on the subject, does anyone know the ACTUAL temperature that an aluminum 80 filled to 3000 psi would have to be at to be truly 'full'? Or do we just accept the standard of 3000 psi @ room temperature = a complete fill?
John

Catalina and Luxfer don't give this info on their web sites but OMS does (Faber steel tanks). Their capacities are calc'd at 70° F. This may not be standard though as I notice that other firms selling Faber tanks quote different volumes for the same tank. I.E. the LP C98 I think becomes a 95 when others sell them.
 
Originally posted by MikeS


Two divers dive go to different locations at sea level and dive to the exact same depth in seawater. The first diver dives in water that is 90 degrees F. The second diver dives in water that is 40 degrees F. Would the diver in the warmer water get more “air bottom time,” and if so would the difference be significant?

Let's see, the 90° diver would see an increase in his tank pressure and the 40° diver would see a decrease from say a room temperature fill. So the warm water diver could provide the pressure to his reg (assuming balanced regs) at any given depth by delivering less mass of air. I guess then that the warm water diver would see more bottom time. Calculating how much is actually a complicated thermodynamic problem.
 
Would the diver in the warmer water get more “air bottom time,” and if so would the difference be significant?

Yes, but not for this reason. The gas is warmed to the same temperature as its transported to the lungs. So they each need the same mass of air to fill there lungs.

The diver in the cold water just has to work harder to fill his lungs and warm the incomming air. That would cause him to breath harder and give less bottom time.
 
Originally posted by JimC


Yes, but not for this reason. The gas is warmed to the same temperature as its transported to the lungs. So they each need the same mass of air to fill there lungs.

I'm sorry but I didn't follow this. How does air delivered to a divers mouth by a reg in 40° water get heated to the same temperature as that by a reg in 90° water? I don't think that it matters what happens once the gas is out of the reg and in the divers lungs. It is out of the sealed system of the tank and regulator by that time. Or am I missing something here?
 
https://www.shearwater.com/products/teric/

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