CF air tank sizes

Hi folks,

A Joe B. Tiger Shark tank is listed as 72 Cu Ft. (http://www.airtanksforsale.com/) Can someone clarify how that translates into the the other tank sizes? What would that be compared to tank sizes given in minutes or liters (or any other volume measurement)? I thought this would be an easy thing to google and find a chart showing me these comparisons but I'm coming up short...

Thanks!

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Can you rephrase your questions? What do you mean by how that “would be compared to tank sizes given in minutes...” or “how that translates to other tank sizes?” I think 72 cubic feet would be roughly around 6.5L in volume (this is an uneducated guess). I have a 74 cf carbon fiber tank that is labeled 6.8L in volume. The amount of fills your get from it would depend upon the type of airgun(s) you’re filling. In this weather (if you’re affected by the polar vortex or cold temps) temperature is also a factor in your tank’s air pressure. 

If you have Windows 10 no need to Google. Just click on the built-in calculator and then click on volume. It will convert anything to anything, even teaspoons if you like.

Might be comparing apples to oranges. 6.8L may be physical size of tank which equals around .24 CF physical size. A 74 CF tank may be how much air it will hold under 300 plus bars of pressure.

A 74 CF physical size would be equal to like 553 gallon jugs. Or 10 55Gal drums????

By the way I have been known to get my math way off.

The problem with your system is that when you specify 72cft you also have to specify a pressure or else it's impossible to give a true tank volume!



72cft at 15.3psi (airpressure at ground level) equals 2039liters as stated above

72cft at 2300psi (old scuba) equals a 12.85liter tank

72cft at 3000psi (new scuba) equals a 9.85liter tank

72cft at 4500psi (CF tanks) equals a 6.6liter tank

See? Not that clear and easy to use.



Read here for all info: http://breatheair.com/2017/11/09/scba-cylinder-breathing-duration-calculation/

I went through the calcs a while back.

6.8L = water volume or roughly 415 in3 or 0.24 ft3

Ideal Gas Law

P1/V1 = P2/V2
And 
1 atm = 14.7 psi

14.7 psi / 0.24 ft3 = 4,500 psi / V2

V2 = 73.5 ft3

Compressibility factor (Z) for air is roughly 1.11 @ 300 Kelvin (80F) and 4,500 psi 

V2 / Compressibility factor = 73.5 ft3 / 1.11

V2 = 66 ft3

So we are really buying 66 ft3 tanks, and then the marketing department takes over from there to put their spin on the numbers. Reminds me of my high school days and power ratings for audio amplifiers.

You are correct there are no 72 or 75CF tanks they are 66. Also what they call 98 and 100 CF are actually 88 CF. It's marketing

My Scott SCBA tank 45 min. 4500psi says it is 66cf. On fire dept. Spec sheet. But they now have the same tank at 5500psi and 30 yrs. Rated.

Minutes (of breathing air) to liters = minutes x 40

Volume in liters to cft = volume x 0.03531467

Cft of air to tank volume at 4500psi = volume in liters / 300



So a 45min tank at 4500psi:

45 x 40 = 1800liters

1800liters = 63.6cft

1800liters at 300bar = 6 liter

So a 45min CF tank at 4500psi is 6 liters true tank volume or 366cui~0.211cft



Metric is easier ;-)

Minutes (of breathing air) to liters = minutes x 40

Volume in liters to cft = volume x 0.03531467

Cft of air to tank volume at 4500psi = volume in liters / 300



So a 45min tank at 4500psi:

45 x 40 = 1800liters

1800liters = 63.6cft

1800liters at 300bar = 6 liter

So a 45min CF tank at 4500psi is 6 liters true tank volume or 366cui~0.211cft



Metric is easier ;-)

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It is much easier

I dropped the ball!



30min tank = 4liter~244cui~0.141cft

45min tank = 6liter~366cui~0.212cft

60min tank = 8liter~488cui~0.282cft

All stated volumes are true tank volume.