More Air.....Once More

[jwlast]

One more novice diver equipment question. Does a 120 cu ft tanks that holds air at 2600 psi hold the same amount of air as a 120 cu ft tank at 3500 psi? Per the previous thread, looking to increase bottom time, but if lower pressure = less air, that's not a good trade off. Been diving up til now with a 80 cu ft tanks at approx 3100 psi.

Thanks......

Jeff

 

[Scubaroo]

They both hold 120 cubic feet of air, if 2600psi and 3500psi are their respective working pressures.

 

[jwlast]

Thanks for the reply. Reason I ask is I saw this in another post "The HP100, for example, is 100 cubes at 3500 PSI. At 3000 PSI it is about 85 cubes."

I'm no engineer, but it seems to me higher pressure equals more cubes. So a 120 at approx 2600 psi doesn't offer the same amount of cubes as the same tank at 3500 psi. Hence' less bottm time.....or am I miising something?

Confused........

Jeff

 

[Northeastwrecks]

The tanks are different sizes. An HP120 is smaller than an LP120. The smaller tank needs to be at a higher pressure in order to hold the same volume of gas.

At least, that's how I understand it.

 

[pescador775]

JW, in the US tanks are rated by air capacity at full pressure, eg a 120 cf tank holds 120 cf of air at the rated pressure whether 2400 or 3500 psi. As Northeast said, the disparity in working pressure while having identical capacity is due to the different internal volumes of the tanks. In Europe, tanks are actually rated by internal volume. For example, a 100 cf/3500 tank is a 12 liter. A 80 cf/3000 is an 11 liter. The giant low pressure/120cf/2400+ (2640 psi) is 19 liter. You are correct in saying that capacity and pressure are directly related. Some divers overfill their tanks. The LP 120 is sometimes filled to 3000 psi or more. Can you tell us how much capacity the tank will have at that pressure?

 

[rickyd]

Look at this site - it may help.

http://www.spearfishing.org/bruces_tips/tankv.html

 

[MikeS]

Well Omar revoked my PADI science guy certification but I’m going to try this anyway.

An aluminum 80 does not hold 80 cubic feet of air (or anything for that matter), that would be enormous. What it does hold, when filled to it’s working pressure of 3,000 psi, is the equivalent of 80 cubic feet of air if the pressure was reduced to 14.7 psi, roughly the pressure of air at sea level not taking into account temperature and local changes in the barometric pressure.

Additionally the 80 cubic feet of an aluminum 80 is rounded up. To illustrate this check out http://www.sportdiverhq.com/tkcht.htm. Scroll down the table to Luxfer Aluminum 77.4 which is normally referred to as an aluminum 80 yet in reality it only holds the equivalent of 77.4 cubic feet.

Let’s put aside the difference between Real and Ideal gas laws, but be careful here, this is were I lost me science guy certification. For the sake of this discussion I’m going to ignore the difference since it’s minimal; but if you’re interested check out Vance Harlow’s Oxygen Hacker’s Companion which has a good description.

Using a little algebra here, if the tank holds the equivalent of 77.4 cubic feet at 14.7 psi it holds X cubic feet at the operating pressure of 3000 psi.
77.4 cf * 14.7 psi / 3000 psi = X cf = approximately 0.38 cubic feet

If you scroll down the table to Pressed Steel 100.1 and apply the same formula you’ll get:
100.1 cf *14.7 psi / 3500 psi = X cf = approximately 0.42 cubic feet

So comparing the two you can see that the actual volume of the tank is not much different, 0.38 versus 0.42, but since the HP100 has a higher working pressure it holds more air if both are filled to their respective working pressure.

To take it a step further; if you filled the HP100 to 3000 psi what would the volume be? Trick question, the volume would still be 0.42, but at a higher pressure. O.K. the real question; if you filled the HP100 to 3000 psi what would the volume of air be if the pressure were reduced to 14.7 psi?
0.42 cubic feet * 3000 psi / 14.7 psi = x cubic feet = approximately 85.7 cubic feet

Hope this helps,
Mike

 

[raxafarian]

MikeS once bubbled...
Well Omar revoked my PADI science guy certification but I’m going to try this anyway.

An aluminum 80 does not hold 80 cubic feet of air (or anything for that matter), that would be enormous. What it does hold, when filled to it’s working pressure of 3,000 psi, is the equivalent of 80 cubic feet of air if the pressure was reduced to 14.7 psi, roughly the pressure of air at sea level not taking into account temperature and local changes in the barometric pressure.

umm.... the tank does hold 80 cubic ft. of air. (or 77 cubic ft. for an AL80) at 3000psi. I'm not sure what you mean by "the equivalent of 80 cubic ft.

I think you have your formulas a little mixed up. The tank does not hold 77.4 cubic ft. at 14.7psi. It holds 77.4 cubic ft. at 3000psi.

for your example of how much air in a hp100 @ 3000psi:
100.1 / 3500 = .0286 cubic ft per psi
.0286 * 3000 = 85.8 cubic ft. at 3000 psi

same answer as you... you do not need to account for atmospheric pressure.... your pressure gauge reads tank pressure above 14.7

 

[MikeS]

raxafarian once bubbled...
umm.... the tank does hold 80 cubic ft. of air. (or 77 cubic ft. for an AL80) at 3000psi. I'm not sure what you mean by "the equivalent of 80 cubic ft.

The volume of the tank remains constant it does not change with pressure. A volume of 64 cubic feet would require a 4 foot cube. As you fill the tank and the pressure increases the mass of air increase but the volume remains the same ala Boyle’s Law.

So let’s take the aluminum 80 as an example. The volume is approximately 0.38 cubic feet. If we open the valve to the to the atmosphere the volume will be 0.38 and the pressure will be approximately 14.7 psi. If we double the pressure to 29.4 psi the volume will still be 0.38 cubic feet but the mass of air is double; per Boyle’s law … the density will very directly with the absolute pressure.

So let’s say we fill the aluminum 77.4 to rated pressure of 3,000 psi and get a big plastic bag. If we release the air at sea level the plastic bag will expand to hold 77.4 cubic feet. If we went under water to 33 feet and release the pressure into the bid plastic bag it would expand to hold 38.7 cubic feet. At 100 feet 19.4 cubic feet, etc., etc.

When tank volume is expressed in liters it refers to the actual volume. Check out http://www.paradisedive.com/tspecs.htm#alum. Go down the chart to the 80, see that the size in liters is 11.1. Convert that to cubic feet 11.1 * 0.035314667 = approximately 0.39 cubic feet. 0.39 cubic feet is the volume and the air contained in that volume at 3000 psi is equal to approximately 77.7 cubic feet at 14.7 psi.

Mike

 

[raxafarian]

I withdraw my previous post and now agree with what you are saying.