Internal Volume

[landlocked]

In an earlier thread, FredT stated: "The amount of air a tank holds is related to it's INTERNAL volume, not the external size of the tank." So, when we talk about an AL80 tank, are we talking about the internal volume or the external displacement? Next step, how do I compare the internal volume of an aluminum 80 tank to a Low Pressure steel 104?

thanks

 

[VTWarrenG]

When we talk about an AL80, we're talking about neither the internal nor external displacement. We're saying the tank will hold 80 cf of gas at STP when filled to its rated pressure (usually 3000 psi). The actual volume is unspecified.

Other countries use units that make much better sense... they refer to tanks in liters of internal volume, and pressure in atmospheres (bar).

The only real way to compare the internal volumes of two tanks is to measure them. If you're looking for the ratio of an AL80 to an LP104, the ratio is just 80/104.

- Warren

 

[The Chairman]

an AL80 tank holds 80 cf at 3000 psi (3014.7 psia)

80 cf/3014.7psi = 0.0265 cf/psi

0.0265 cf/psi x 14.7 psi/ata = 0.390 cf/ata

So your al80 tank has almost 4 tenths of a cubic foot as it's internal volume. You can do the math for your lp tank in precisely the same way.

 

[kkoski]

This is a good question.

An Al80 tank holds 80 cubic feet of air at 2640 psi. If you submerged an Al80 in a bathtub it would displace much less than 80 cubic feet of water, maybe 1.5 cubic feet (maybe you ment "how much space the tank takes up" by "external displacement").

A LP104 holds 104 cubic feet at 2640 psi.

Here's a handy list:

HP80 80 cf @ 3500 psi
HP100 100 cf @ 3500 psi
HP120 120 cf @ 3500 psi
LP80 80 cf @ 2640 psi
LP95 95 cf @ 2640 psi
LP104 104 cf @ 2640 psi
LP121 121 cf @ 2640 psi

(In other words, high pressure tanks are rated at 3500 psi, low pressure at 2640 psi.)

So, how much air does a HP100 have in it at 3100 psi? Just take (3100/3500)*80 = 88.4 cf.

88.4 cubic feet is roughly the space inside a 3.5 x 3.5 x 7 foot closet (maybe you ment "how much space the air would take up outside the tank" by "external displacement"?). All the air in that closet is stuffed in that small tank. Pretty cool.

Hope this helps.

-Katrina

 

[landlocked]

Originally posted by kkoski
.... So, how much air does a HP100 have in it at 3100 psi? Just take (3100/3500)*80 = 88.4 cf.....

I'm sorry, but I am being a little slow today. (Too much golf and not enough diving?) Where did the 80 come from? Thanks one and all for the answers. They cleared some things up for me.

 

[kkoski]

No, I'm the slow one, it's suppose to be 100, not 80! Arrrr. Sorry.

 

[Rick Murchison]

Originally posted by NetDoc
an AL80 tank holds 80 cf at 3000 psi (3014.7 psia)

80 cf/3014.7psi = 0.0265 cf/psi

0.0265 cf/psi x 14.7 psi/ata = 0.390 cf/ata

So your al80 tank has almost 4 tenths of a cubic foot as it's internal volume. You can do the math for your lp tank in precisely the same way.

Well, almost... the standard AL80 holds 80CF at 3100 psi(g) (3114.7 actual) and 77.4CF at 3000 psi(g).
The "super 80" holds 80 at 3000.
---------------
If you want to win a bet, it's nice to know that a steel 72 is bigger on the inside than an AL 80 - especially if you can get 'em together with the boots off. The AL80 is considerably larger externally.
Rick

 

[The Chairman]

The working pressure of the former is 3000 psi and the latter is 2640 psi. Don't forget the ambient pressure of 14.7 psi (sea level) when doing your calculations.

lp80... 80 cf/2654.7 psi = 0.0301 cf/psi
0.0301 cf/psi x 14.7 psi/ata = .442 cf/ata

consequently the lp80 has an internal volume 0.052 cf greater than an Al80.

hp80... 80 cf/3514.7 psi = 0.0228 cf/psi
0.0228 cf/psi x 14.74 psi/ata = 0.335 cf/ata

Obviously, not all 80 cf tanks have the same internal volume.

The lp104 would have an internal volume of 0.576 cf.

Katrina's formulae for figuring how much volume /pressure is valid.

 

[Walter]

Fred is correct. "The amount of air a tank holds is related to it's INTERNAL volume, not the external size of the tank." It is not the only factor. Two tanks with the exact internal volume will hold the same amount of air if filled to the same pressure.

"Other countries use units that make much better sense... they refer to tanks in liters of internal volume, and pressure in atmospheres (bar)."

In what way does it make more sense? Which tank holds more air an 11.1 liter tank or a 10.2 liter tank? Without knowing the working pressure and doing some math we won't have the answer. A high pressure tank can have a lower internal volume yet still hold more air than a low pressure tank. While internal volume (and pressure) does determine the amount of air a tank can hold it makes more sense to me to compare apples to apples and compare tanks using the amount of air they'll hold at their working pressure and not their internal volumes. Incidentally, bar is close to the same as atm, but it is not exact.

 

[Goldminer]

Originally posted by kkoski
This is a good question.

An Al80 tank holds 80 cubic feet of air at 2640 psi.

The working pressure of most Al 80's is 3000 psig and they hold about 77.4 cu ft at this pressure.

So, how much air does a HP100 have in it at 3100 psi? Just take (3100/3500)*80 = 88.4 cf.

{snip}

Hope this helps.

-Katrina

I believe that you are not quite correct here as you have not allowed for Van der Waal's (sp?) forces in your calculation. Your equation works at low pressure where air's behaviour approximates an ideal gas. But air is not an ideal gas.

The pressure in a tank is caused by two phenomenon i) the number of molecules of gas pumped into the tank and ii) the number of gas molecule collisions inside the tank. The latter constitute Van der Waal's forces. At high pressures these collisions become frequent and contribute significantly to the pressure read on the guage. The ideal gas laws ignore Van der Waals forces.

Expressed another way it takes more cubic feet of air to put the first 100 psi into a 3500 psi tank than the last 100 psi. Therefore breathing a tank down from 3500 to 3100 (400 psi) as in your example takes fewer gas molecules out of the tank than breathing it down from 1400 to 1000 (also 400 psi).

At 3500 psi working pressures the difference between real and ideal gas behaviour becomes significant and noticeable.

A little bit of thermodynamics fun early (as in near noon) on a holiday Monday morning. Happy Canada Day everyone.