Air volume at different PSI?

[KGNickl]

HP100 tank
3442 PSI it has 99.5 cu ft.
3000 PSI has ????? cu ft.
2640 PSI has ????? cu ft.

LP77 tank
2640 PSI (2400+) has 77.7 cu ft.

LP85 tank
2640 PSI (2400+) has 82.9 cu ft.

Trying to decide on which tanks to pick up. Want a good idea of how much air I would get for a HP tank at 3000psi (boat fill...) and on a LP tank PSI fill (2640). Going to pick up (2) tanks and double them up, but also have the plugs so I can break them up for singles.

So who can fill in the blanks where my question marks are?

 

[anzac65]

HP100 tank
3442 PSI it has 99.5 cu ft.
3000 PSI has ????? cu ft.
2640 PSI has ????? cu ft.

LP77 tank
2640 PSI (2400+) has 77.7 cu ft.

LP85 tank
2640 PSI (2400+) has 82.9 cu ft.

Trying to decide on which tanks to pick up. Want a good idea of how much air I would get for a HP tank at 3000psi (boat fill...) and on a LP tank PSI fill (2640). Going to pick up (2) tanks and double them up, but also have the plugs so I can break them up for singles.

So who can fill in the blanks where my question marks are?

(3000 / 3442 ) x 99.5 = 86.7 Cu ft etc........... which is why your SPG tells you how much air you have.

 

[texdiveguy]

Tank Volume

 

[Bismark]

Here is an interesting piece of information for you. They are called "Tank Factors".

1) Divide your tanks volume by your rated pressure eg: Al 80 at 77 cu ft at 3000 psi equals 0.025. This is the amount of cubic feet per PSI. Since your SPG cannot read in such small units, multiply this number by 100. That will give you the cubic feet per 100 psi, which can be read on your SPG. Your tank factor for your al 80 is thus 2.5. Hence, for every 100 psi, you will have 2.5 cubic feet. Then simply multiply the number of "100's of psi" by 2.5 and you will know how much volume of gas you have in your tank. 2500 psi in an Al 80 is 25 x 2.5 or 62.5 cu ft.

For your examples, 100 cu ft cylinder at a rated pressure of 3442 is 100/3442 or a Tank Factor of .029. Now multiply that by 100 and you will have 2.9 cu ft per 100 PSI. For 3000 psi that is 30 x 2.9 or 87 cu ft.

Each tank will have it's own Tank Factor. It is a good idea to memorize the more commonly used cylinders.

Tank factors become very useful when calculating gas between divers using different cylinders. You show up with 3000 psi in your single HP 100 and I show up with 3000 in my LP 95. Who has more gas? Or, can we do the same dive together. Your 100 has 87 cu ft while my overfilled LP 95 has 108 cu ft......... Kind of important to know........

 

[pickens_46929]

Go with LP tanks... generally you are assured of complete fills... with HP tanks you many not always end up with a full fill...

 

[willembad]

Go with LP tanks... generally you are assured of complete fills... with HP tanks you many not always end up with a full fill...

Aaaaarrrrrrrggggggg!!!!

Think about this:

You'll (maybe) pay more for the HP100's (FX100's) than the LP85's.
They are physically almost identical, close enough in dimension and weight to make both work just fine.
They practically contain the same amount of gas at the same pressure.

So...

The HP100 is basically an LP85 with a legal, get it anywhere you can, overfill working pressure. If you live in cave country and don't travel much with your own gear then the LP's are great. If not you'll be better served with the HP's.

Later,

Willem

 

[DA Aquamaster]

Right. That one drives me nuts too, but old habits and beliefs die hard.

An LP 95 is more or less the same size and weight as a 3442 psi E8-119 and an LP 104 is more or less the same size and weight as a 3442 psi E8-130. At the same 3442 psi an LP 95 would hold 123 cu ft and an LP 104 would hold 138 cu ft. At the smaller end of the scale an LP 80 and an E7-100 are almost exactly the same size and weight. The overfilled LP 80 holds 105 cu ft at 3442 psi compared to the 100 cu ft in the E7-100.

So...there is a slight advantage to a cave filled (3500-3600 psi) LP tank compared to a same sized and weight HP tank, but not that much. More importantly, outside of cave country you can actually get the HP tanks filled to their rated capacity giving you a lot more gas in the HP tank.

Also, it used to be true in the day when the 3000 psi AL 80 ruled, that getting a full 3500 psi fill was hard. But 3442 and 3500 psi tanks are now so common that getting a 3500-3600 psi fill is not the problem it used to be. More importantly, if you are at a shop where getting 3500 psi is a problem you are by definition at a shop that will only fill your LP 95 to 2400 psi, giving you only 86.4 cu ft compared to the 103.7 cu ft you'd get in an E8-119 underfilled to 3000 psi. And that is a big difference.

So unless you live in cave country where a 3600 psi overfill in your Lp tank is available, you are better off with an HP tank.

Personally I own 3 sets of double X7-100's as well as two sets of double steel 72's. The buoyancy and trim traits are very similar and they share the same tank factor (5.8) so switching between them is a non issue. The 100's work fine in caves and off shore and the lighter 72's are great on shallower offshore and quarry dives. With a still reasonable overfill to 2800 psi the steel 72's turn into steel 80.6's.

 

[Luis H]

The only possible advantage of a new LP tank is that the alloy in the 3AA may be just slightly better for corrosion resistance than the new high strength alloys. If this was even a factor it would only make a difference if you allow moisture inside the tank.

Therefore, if I was in the market for a new steel tank, it would always be an HP cylinder.

The slight price difference between a HP and LP is really insignificant for the life of a cylinder…unless you get a reasonable discount on a LP tank, etc. If the cost is significant then it is just a great buy and you like it.


The entire fear that if you own a HP tank you may get cheated by a possible short fill is way overblown. If you need more air get a bigger tank (physically bigger size).

In this respect I like the way the Europeans think. My steel 72s and DA Aqua Master HP 100 are both basically 12 Liter tanks (same tank factor of 2.9 cu ft/ 100psi). The only difference is that mine are rated at 170 bar (2475 psi) and his are rated at 240 bar (3442 psi). I am comfortably filling mine to 2800 psi on occasions and would like it if they were rated to 3442 psi, but they are just great vintage tanks that I didn’t pay much for at all (much less than any new tank).