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+--- Thread: Measring cubic feet using psi (/showthread.php?tid=4997)
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Measring cubic feet using psi - Raiderswim - 04-20-2010
Not the best math guy, so I'm looking for a formula or method to show how many cubic feet I have in a tank larger than an 80. Example: AL80 @ 3000 psi is 80 cubic feet. If I had a steel 120 with 2600 how many cubic feet is that? How do you do the math? Thanks!
Re: Measring cubic feet using psi - ballfrog - 04-20-2010
Aluminum 80 at 3000 is 77 cu ft.
Re: Measring cubic feet using psi - mattd - 04-20-2010
It depends on the rating of the tank which should be stamped on it. From there it is just a ratio.
So, if your steel 120 is a low pressure steel rated for 2460, then it holds 120 cf at 2460psi. At 260, it is (2600/2460) * 120 = 127 cf.
Now, if that is a HP 120 rated for 3442, then it isn't full at 2600 and you've only got 90cf.
Re: Measring cubic feet using psi - arcFlash - 04-20-2010
If your using it to plan a dive you need to work the math after the tank adjusts to the cold water. If you just want the theory it don't matter.
3100 at the dive shop, 3000 out of the trunk, 2700 on the dive. Because the math assumes Standard Temp (part of STP rules) and your tank isn't at standard temp you overstate your real usable gas.
Re: Measring cubic feet using psi - DetectorGuy - 04-20-2010
Here is a link with the same info that ball frog had but the charts are more crisp.
Re: Measring cubic feet using psi - Hydro - 04-20-2010
couldnt you just use PV=nRT?
Re: Measring cubic feet using psi - deep_al - 04-20-2010
Right math, but you missed the fine print
An lp121 is 121 cuft at 2400+10% or 2640psi. lp steel tanks are allowed a 10% overfill, supposedly only while the hydro stamp still shows a + rating. That means if your lp121 doesnt come back from hydro with a plus after the date, your 121 is now technically a 110.
Re: Measring cubic feet using psi - ericmyers - 04-20-2010
Forget the math just Use Simple Tricks:
Example 1:
80AL is full at 3,000 right. So at 1500 you have 40cft. Or each 750 psi = 20cft
Example 2:
30 AL Pony with 40% 02 has only 2,000 psi left. How many cft is in there. Well my 30cft tank is 2/3rds full so = 20cft.
Example 3:
120AA is full at 3,500 psi. Half full will be at 1750 psi or 60 cft. There are (7) 500 psi's in 3,500. Each of those 500 psi's have how many cft? 120/7= about 17cft. So a tank that has 2,000 psi in it has 4 x 17 or 68cft. Which checks out because we know that 60cft occurs at 1750psi.
Example 4:
Double 120AA's are full at 3,500. I recently started my dive with only 2,000 in the set. How many cft did I have when I started the dive? Well let's see. 1750 is half full right? Half of 240cft is 120 cft so I started my dive with two tanks on my back but with barely just more than (1) tanks worth of air. To be exact 2000/3500 = .57 *240 = 137cft. But who needs to be exact as long as your approximation is accurate. Did it really matter that I had 17cft of gas than I estimated? Probably not.
My point is understanding the math is great but don't get bogged down in the numbers. Develop tricks that help you and double check your work.
Re: Measring cubic feet using psi - ericmyers - 04-20-2010
Regarding those Worthington type 2400's LP's
I guess you only get the cft with a +
So overfilled an LP 95 is 95 cft
Filled to 2400 an LP 95 is 9.5cft less or 85.5
So overfilled an LP 85 is 85cft
Filled to 2400 an LP 85 is 8.5cft less or 76.5
Re: Measring cubic feet using psi - bowtieman427 - 05-01-2010
For those that do not like math
for those that want to know:
V or air availiable in ft^3= (x psi of gauge) X (tank ft^3)/(psi of full tank)
for example 100HP tank actual 99.5 @ 3442 room temperature
Va = 3000psi X(99.5ft^3/3442psi) = 86.7ft^3
a normal 80 ft^3 @ 3000psi now down to 1500psi at room temperature logic says 40ft^3 1/2 full (should work)
Va = 1500psi X (80ft^3/3000psi) = 40ft^3
Hope this helps