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Discussion Starter #1
if i have a 80cft tank filled to 3000psi, running 20 psi out @ 1cfm, how long will the tank last
 

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The ideal gas equation is approximately:

P = Tk/V or V = Tk/P

where P is pressure, T is temperature, V is volume, and k is a gas constant

Allowing for losses due to uncontained temperature changes during expansion, your 3000 psi tank and 20 psi regulator should yield about a 100:1 ratio (3000/20 is 150, but let's call it 100). If everything is insulated, you might be able to do better than 100.

The tank to flow ratio is 80 cft: 1 cfm or about 80 minutes.

So, ideally your tank should last 80 x 100 = 8000 minutes or a little over 133 hours unless I've really screwed something up.

That's one hell of a tank, BTW. How many tires are you going to fill up? Don't forget that you'll likely get a lot of water in this process as the air will give up water as its pressure drops.

Who said physics isn't useful?
 

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RayOSV said:
The ideal gas equation is approximately:

P = Tk/V or V = Tk/P

where P is pressure, T is temperature, V is volume, and k is a gas constant

Allowing for losses due to uncontained temperature changes during expansion, your 3000 psi tank and 20 psi regulator should yield about a 100:1 ratio (3000/20 is 150, but let's call it 100). If everything is insulated, you might be able to do better than 100.

The tank to flow ratio is 80 cft: 1 cfm or about 80 minutes.

So, ideally your tank should last 80 x 100 or 8000 minutes or a little over 133 hours unless I've really screwed something up.

That's one hell of a tank, BTW. How many tires are you going to fill up?

Don't forget that you'll likely get a lot of water in this process as the air will give up water as its pressure drops.

Who said physics isn't useful?

Nice.......i was gonna just guess 7.
 

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Ca$h said:
Nice.......i was gonna just guess 7.
A good reason to stay in school. :lol
 

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Discussion Starter #5
RayOSV said:
The ideal gas equation is approximately:

P = Tk/V or V = Tk/P

where P is pressure, T is temperature, V is volume, and k is a gas constant

Allowing for losses due to uncontained temperature changes during expansion, your 3000 psi tank and 20 psi regulator should yield about a 100:1 ratio (3000/20 is 150, but let's call it 100). If everything is insulated, you might be able to do better than 100.

The tank to flow ratio is 80 cft: 1 cfm or about 80 minutes.

So, ideally your tank should last 80 x 100 = 8000 minutes or a little over 133 hours unless I've really screwed something up.

That's one hell of a tank, BTW. How many tires are you going to fill up? Don't forget that you'll likely get a lot of water in this process as the air will give up water as its pressure drops.

Who said physics isn't useful?
Thanks, I knew there had to be a smart person on SBN somewhere :). The tanks I am using are aluminum scuba tanks. I started a small little business gig doing temporary tattoos and face painting with airbrushes at festivals and events. I had a small 3hp 8gal compressor, but a lot of events dont have access to electricity and is noisy. So I got these 3 scuba tanks for free from my cousin who doesnt dive anymore, but they are still in the dive shop getting hyrod, check and filled so I havent tested them out yet.

I dont know if I need to add a moisture trap in the line because the dive shop said the air is really dry compared to normal compressed air.

Thanks again!
 

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RayOSV said:
The ideal gas equation is approximately:

P = Tk/V or V = Tk/P

where P is pressure, T is temperature, V is volume, and k is a gas constant

Allowing for losses due to uncontained temperature changes during expansion, your 3000 psi tank and 20 psi regulator should yield about a 100:1 ratio (3000/20 is 150, but let's call it 100). If everything is insulated, you might be able to do better than 100.

The tank to flow ratio is 80 cft: 1 cfm or about 80 minutes.

So, ideally your tank should last 80 x 100 = 8000 minutes or a little over 133 hours unless I've really screwed something up.

That's one hell of a tank, BTW. How many tires are you going to fill up? Don't forget that you'll likely get a lot of water in this process as the air will give up water as its pressure drops.

Who said physics isn't useful?
dude, you totally forgot to carry the 3.
 

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Discussion Starter #7 (Edited)
Now RayOSV: Say I'm using a larger spray gun to paint a car that runs 30 psi @ 15cfm. Meaning my 3000 psi tank and 30 psi regulated output to the spray guy should yield about a 100:1 ratio, and the flow being say 80 cft : 15cfm is about 5 minutes.

So 5 x 100 = 500 minutes about 8 hours of continous spray time? I asked in a car painting forum a while ago about using the scuba tank and they insisted the cfm draw would drain the tank out in a few minutes..
 

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To be able to spray you would need a minimum pressure to be maintained. As the air drains out of the tanks the pressure will drop so even though air is still coming out of the tanks it is not enough pressure for spraying.

Does the maths used above take this into account or is it saying that it will take X amount of hours for a volume of air to pss through the tank at a constant pressure?
 

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codenoid said:
snip... The tanks I am using are aluminum scuba tanks.
I dont know if I need to add a moisture trap in the line because the dive shop said the air is really dry compared to normal compressed air. ...snip
Whoa there. Is the capacity of these scuba tanks really 80 cu ft? 1 cu ft = about 7 gallons (average of wet and dry since I don't know which "gallon" to use :confused: ), so 80 cu ft = 560 gallons (HUGE tank). My air compressor has a 30 gallon tank which is MUCH b***** than 3 standard scuba tanks that go on your back.

I think what the dive shop is telling you is that they compress 80 cu ft of air at atmospheric pressures into the tanks at 3000 psi. The volume in the three tanks is probably about 15 gallons total or about 2 cu ft at 3000 psi. In your case, you won't get the pressure difference advantage as it's already taken into account. So I think in your first case of a flow of 1 cfm, you would only get about 80/1 = 80 minutes of air out of those tanks (less some losses).

In your second example of 15 cfm, you would get about 80/15 = 5 minutes of flow out of your scuba tanks. Sounds like the car painting forum was correct.

If I've missed something about the size of your tank, let us know. I was wondering where you got an 80 cu ft tank that would take 3000 psi?!?!?!!?

Plus, if you do use these tanks for face painting with an air brush, make sure that your regualtor works and is set to the right pressure!!!! Wouldn't want that kind of accident on some kid's face. :eek:

And, yes, Lawnmowerman, the volume of air would decrease as the pressure in the tank fell below the pressure that he wanted to deliver. That would happen near the end of the air supply. All of these numbers are estimates.
 

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RayOSV said:
Whoa there. Is the capacity of these scuba tanks really 80 cu ft? 1 cu ft = about 7 gallons (average of wet and dry since I don't know which "gallon" to use :confused: ), so 80 cu ft = 560 gallons (HUGE tank). My air compressor has a 30 gallon tank which is MUCH b***** than 3 standard scuba tanks that go on your back.

I think what the dive shop is telling you is that they compress 80 cu ft of air at atmospheric pressures into the tanks at 3000 psi. The volume in the three tanks is probably about 15 gallons total or about 2 cu ft at 3000 psi. In your case, you won't get the pressure difference advantage as it's already taken into account. So I think in your first case of a flow of 1 cfm, you would only get about 80/1 = 80 minutes of air out of those tanks (less some losses).

In your second example of 15 cfm, you would get about 80/15 = 5 minutes of flow out of your scuba tanks. Sounds like the car painting forum was correct.

If I've missed something about the size of your tank, let us know. I was wondering where you got an 80 cu ft tank that would take 3000 psi?!?!?!!?

Plus, if you do use these tanks for face painting with an air brush, make sure that your regualtor works and is set to the right pressure!!!! Wouldn't want that kind of accident on some kid's face. :eek:

And, yes, Lawnmowerman, the volume of air would decrease as the pressure in the tank fell below the pressure that he wanted to deliver. That would happen near the end of the air supply. All of these numbers are estimates.
yes the tank has an 80 cuft capacity, it will hold 80 cuft of air at 3000#. its capacity is figured at its max normal pressure so it has to be at 3000 # to have 80 cuft capacity, less air pressure means less air in the tank . the reason the tanks are so much smaller than your 30 gal compressor tank is that you tank is only running 150 # or so where the scuba tanks are much more. the tank i dive with is rated at 121 cuft at 2400#.
i just found this. it says the above in a better way:
Volume is defined by the internal dimensions of the tank, and normally measured in cubic inches of water. Capacity is defined by the amount of breathing gas the scuba tank will hold at its maximum rated service pressure
 

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kman458 said:
yes the tank has an 80 cuft capacity, it will hold 80 cuft of air at 3000#. its capacity is figured at its max normal pressure so it has to be at 3000 # to have 80 cuft capacity, less air pressure means less air in the tank . the reason the tanks are so much smaller than your 30 gal compressor tank is that you tank is only running 150 # or so where the scuba tanks are much more. the tank i dive with is rated at 121 cuft at 2400#.
i just found this. it says the above in a better way:
Volume is defined by the internal dimensions of the tank, and normally measured in cubic inches of water. Capacity is defined by the amount of breathing gas the scuba tank will hold at its maximum rated service pressure
I'm no expert in diving, but I think that this is the point: "amount of breathing gas" the tank will hold. This has to be at the lower pressure. The gas equations assume that if you say 80 cu ft at 3000 psi, it really is 80 cu ft of volume at the 3000 psi pressure. When you decrease the pressure through a regulator, the volume expands according to the ratio of the pressure difference. You get 80 or 121 cu ft of breating air ONLY at the lower pressure by starting with a much lower volume at the higher pressure.
 

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Discussion Starter #12
Um, so are at 80 minutes for 1cfm now?

The tanks are 80cft and hold 3000psi. Imagine sitting there with a little hose regulated at 20psi, I think it will take more than 80 minutes.

Soon I will have the tanks back and we'll have the real test. :)
 

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codenoid said:
Um, so are at 80 minutes for 1cfm now?

The tanks are 80cft and hold 3000psi. Imagine sitting there with a little hose regulated at 20psi, I think it will take more than 80 minutes.

Soon I will have the tanks back and we'll have the real test. :)
I think the point is that the volume of the tanks isn't 80 cu ft. The tanks will hold a much smaller volume of air that results from compressing 80 cu ft of air measured at standard atmosphere (about 15 psi) all the way up to 3000 psi and drastically reducing the volume down to a few cu ft. When you allow this compressed air to expand to the lower pressure, you end up with the orginal volume of 80 cu ft (approximately).

Scroll down toward the bottom third of this link
http://www.diveriteexpress.com/gas/steel.shtml

There's a table that shows the "capacity" of a bunch of tanks filled to various pressures. For the same size tank, the "capacity" increases with pressure. Again, I think diving tank "capacity" is how much air is delivered at breathing pressure after expansion through the regulator.

Try this link as well:
http://www.diveriteexpress.com/library/tanks.shtml#vollie
Scroll down to "The big lie about capacity" and it describe the difference between volume of the tank (measured in cubic inches) and the capacity (measured in cubic feet).

The gas equation that relates the expansion and contraction of gas with pressure only deals with volume.
 

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You guys are using two different terminologies and are confusing the crap out of me. I'm with what's his name when he said 80 cubic feet and 1 cubic foot per minute gives you 80 minutes.

Have you thought about getting a small electric compressor? they have small compressors especially for air brushes and the like.
 

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Discussion Starter #15
slo_tom said:
You guys are using two different terminologies and are confusing the crap out of me. I'm with what's his name when he said 80 cubic feet and 1 cubic foot per minute gives you 80 minutes.

Have you thought about getting a small electric compressor? they have small compressors especially for air brushes and the like.

Mentioned before.

codenoid said:
I had a small 3hp 8gal compressor, but a lot of events dont have access to electricity and is noisy.
 

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80 cubic feet scuba tanks, when filled (to your 3000 psi) will contain enough air to provide 80 cubic feet of 14.7 psi air (at standard atmospheric pressure).

If you are taking 1 cubic foot of 20 psi air out every minute you will need about
(14.7 psi) / (20 psi) * 80 minutes to empty them.
or about an hour.
 
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