Newbie Question on Tank Volume

[DiverNewb]

What's it mean to you have an 120CuFt tank?

Does that mean that if it were filled to its pressure capacity (like 2200 psi) and emptied at atmospheric pressure the air inside would expand to fill 120CuFt?

I guess what I'm asking is this: Is there more, less or the same amount of breathable air in a high pressure 120 versus a low pressure 120?

 

[ranger979]

You definition is correct. It is the amount of space the air would fill at atmosheric pressure. A low pressure 120 holds the same amount as a high pressure 120 when they are both filled to their working pressure.

 

[The Kraken]

You are correct.
What it means is that at 1 Atmoshpere Absolute (1 atmospheric pressure) the tank contains 120 cubic feet of breathing gas.

The quantity of air is equal in the two different tanks. It's just that the HP tank is smaller than the LP so therefore it requires a higher pressure to fill the smaller HP tank to 120 cubic feet than it does the low pressure tank.

the K

 

[DA Aquamaster]

Most companies that sell steel tanks market their tanks by stating the capacity with the tank overfilled by 10%. Without the 10% overfill, you have less actual capacity.

A 10% overfill is allowed for the first 5 years. Tanks can be recertified for a 10% overfill but not all hydro testers do this, and most only do it when requested. If the rank has an REE number stamped on it, it is a lot easier to get it plus rated again as a lot less math is required by the tester. Without an REE number on the tank, many hydro test facilities will not plus rate a tank.

 

[ChrisA]

I guess what I'm asking is this: Is there more, less or the same amount of breathable air in a high pressure 120 versus a low pressure 120?

Practically they are the same except if you want to be "nit picky". Because you never actually bearth a tank down to zero the low pressure tank will have more cubic feet of air in it at 500 PSI So there is very slightly less "breathable" air in the HP tank if you concider the air below 500 PSI non-breathable. but like I said, that nit picking

Some older boats around here can't fill HP tanks as thier compressors only go to 3000 or less So if you can only get a 3000 PSI fill the LP tank holds more air

 

[spectrum]

Practically they are the same except if you want to be "nit picky". Because you never actually bearth a tank down to zero the low pressure tank will have more cubic feet of air in it at 500 PSI So there is very slightly less "breathable" air in the HP tank if you concider the air below 500 PSI non-breathable. but like I said, that nit picking

Some older boats around here can't fill HP tanks as thier compressors only go to 3000 or less So if you can only get a 3000 PSI fill the LP tank holds more air

If HP and LP are compared for "off limits" air content at 500 PSI then there smaller tank (HP) will have less "off limits" air. The bigger (LP) tank will have more "off limits" air .

Some quick math tells me that with two 80CF cylinders with one LP at 2640 and one HP at 3442 there will be 3.3 CF more in the LP cyclinder at 500 PSI. Something between 1-5 minutes depending on depth SAC and other conditions.

Good trivia but not a decison making issue.

Pete

 

[D_B]

I would add that DA Aquamaster is referring to LP tanks when he is talking about the 10% overfill rating , have not seen any HP tanks with that rating and he is correct about rated capacity in a LP tank only comes at that 10% overfill , on those tanks so rated
Edit: Faber makes a Mid Pressure Series , 3180 PSI (3498 PSI with +)
DB

PS: I am still undecided on the LP / HP tank as to which one to get ... I like the smallness of the HP (new 3450psi) tank.
How is the boat compressor / fill pressure issue , is a low HP fill still a problem in the US , or is it better now

 

[Mark Vlahos]

I am still undecided on the LP / HP tank as to which one to get ... I like the smallness of the HP (new 3450psi) tank.

This information is from Pressed Steel, but generally it should come pretty close to applying to cylinders from other manufacturers.

The High Pressure E8-130 cylinder has the same internal volume as the Low Pressure 104. What this means is, if you were to fill the E8-130 to 3442 PSI you would have a tank with approximately 130 cubic feet of air in it, but if you filled the exact same tank to 2640 PSI (2400 PSI with the allowable 10% overfill) you would have 104 cubic feet of air in the tank. Following this same comparison the tanks below have the following capacities.

E8-130 at 3442 PSI = 130 cu. ft., at 3000 PSI = 117 cu. ft., at 2640 = 104 cu. ft.
E8-119 at 3442 PSI = 119 cu. ft., at 3000 PSI = 107 cu. ft., at 2640 = 95 cu. ft.
E7-120 at 3442 PSI = 120 cu. ft., at 3000 PSI = 107 cu. ft., at 2640 = 96 cu. ft.
E7-100 at 3442 PSI = 100 cu. ft., at 3000 PSI = 89 cu. ft., at 2640 = 80 cu. ft.
E7-80 at 3442 PSI = 80 cu. ft., at 3000 PSI = 72 cu. ft., at 2640 = 64 cu. ft.
E7-65 at 3442 PSI = 65 cu. ft., at 3000 PSI = 58 cu. ft., at 2640 = 52 cu. ft.

If you follow what I am showing, you can buy a high pressure tank and fill it to high pressure when you need the extra volume, but when you do not need the extra air or if a high pressure fill is not available you can always use it as a mid pressure tank, or even a low pressure tank.

If you own a low pressure tank you are limited to the low pressure fills and the volumes associated with those fills.

I own both Faber 95's (low pressure) and PST E8-130's (high pressure). At my next annual inspection I will switch things around a bit and have the Fabers as single tanks and the PSTs as doubles. Low pressure steel tanks can be found at fairly reasonable prices while high pressure steel tanks are still pretty expensive.

Mark Vlahos

 

[mark99]

PS: I am still undecided on the LP / HP tank as to which one to get ... I like the smallness of the HP (new 3450psi) tank.
How is the boat compressor / fill pressure issue , is a low HP fill still a problem in the US , or is it better now

Trust me, I've been doing boat dives in So. Cal all summer, and my HP tank has never been filled over 3050 psi on a boat. Sometimes it's as low as 2850. And don't think you can just "complain" and get topped off, usually you can, but with 25 tanks to fill you have to wait your turn. My LDS can give me a full HP fill if they are patient and keep cooling/topping it up. I'm sure they don't feel comfortable sending it out hot with an overfill, as the HP-PST tanks are not rated + (10% overfill allowed).

You should take a close look at the chart above showing the available CF at HP and at 3000 psi. The E7-100 is a popular size and gives you 9 CF more air at 3000 psi than the AL-80 at a size 2 in shorter than the AL-80 (7.25X24.12" vs. 7.25"X26.10"). If you are tall enough consider the 120- 7.25"X28.25".