Comparing tanks of different sizes and pressures

[wheeledgoat]

I initially wanted to know exactly how much of an upgrade a HP100 (@3442psi) represented over an AL80 (@3000psi), i.e. how much more air was I going to have. This has sent me down an rabbit hole from which I'd like to request some help to escape.

This post explains "Tank Factors" which at first blush seemed to give me what I needed. But as I'm trying to understand it, I can't get my head around it. Why does the rated pressure of a tank matter?

Even this Tank Air Volume calculator over at DiveBuddy uses the rated pressure as part of the calculation.

Why? I just cannot seem to grasp why a rated pressure (some number an engineer decided was the max safe pressure for that vessel) has anything to do with what's in the tank now. Shouldn't it simply be a matter of volume and pressure? (...and nRT which are all constants in this case...)

 

[Lorenzoid]

This post explains "Tank Factors" which at first blush seemed to give me what I needed. But as I'm trying to understand it, I can't get my head around it. Why does the rated pressure of a tank matter?

Even this Tank Air Volume calculator over at DiveBuddy uses the rated pressure as part of the calculation.

Why? I just cannot seem to grasp why a rated pressure (some number an engineer decided was the max safe pressure for that vessel) has anything to do with what's in the tank now. Shouldn't it simply be a matter of volume and pressure? (...and nRT which are all constants in this case...)

The rated pressure just yields the tank factor--how many psi per cubic foot--because it's based on a "full" tank. That is, the tank is presumed "full" when filled to its rated pressure. Once you know the tank factor, then yes, all that matters is volume and pressure "now."

 

[kelemvor]

I don't really get "tank factor" either. I guess maybe before everyone had a calculator on their smartphone and PC they needed a way to make the math slightly easier.. maybe? The "tank factor" concept needlessly complicates something that is incredibly simple in my opinion.

I do it this way: (rated cuft/rated pressure)*actual pressure

For example, I've got some worthington LP108 tanks. They're rated for 108cuft at 2640psi. They're filled right now to 3600psi. So: (108/2640)*3600=144.53cuft. Worthington says I should only put 108cuft of gas in the tank but my fill station went ahead and put 144cuft into it.

Roughly.. I guess there's some factors like ideal gas laws or some such that make this slightly off. I think it's a good rough estimate though.


If you're not filling tanks to more (or less) than their rated pressures then you don't need to calculate anything at all to know how much more gas you get in an HP100 vs an al80. It's about 20cuft.

 

[bamafan]

This might help

Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan

 

[BRT]

The answer to your original question is 30% more at rated pressures.

 

[Magnus Lundstedt]

That is why i prefer the metric system.

a 12l tank at 200bar is ~2400l
a 10l tank at 230bar is ~2300l
a 11l tank (AL80) at 200bar is ~2200l
a 15l tank at 200bar is ~3000l

Just take the tank volume and multiply with the pressure in bars (up to about 260bar)

 

[kelemvor]

That is why i prefer the metric system.

a 12l tank at 200bar is ~2400l
a 10l tank at 230bar is ~2300l
a 11l tank (AL80) at 200bar is ~2200l
a 15l tank at 200bar is ~3000l

Just take the tank volume and multiply with the pressure in bars (up to about 260bar)

With the imperial system, the tank is already rated by how much it holds. You don't do any math. 80cuft tank holds 80cuft. You only need to do math if you're doing something like filling to more or less than the rated pressure.

 

[NAM001]

Your original question is about how much gas you have 100 vs 77you have about 30% more gas in it. The tank factor is giving you a method to determine how much time you have left on the tank. The tank size has no meaning, when looking at all the configurations. IE What gives you more air??? al80's doubled or a single lp120. That is really asking the difference of 160 vs 120. the tank factor comes in when yo watch your spg and monintor how fast you re using air. 100psi on an al80 is not the same as 100 psi on an lp120. You can say that 100 psi / min on a 3000 psi tank has 30 min till bone dry. and the 100 psi on an lp120 is about 27 ,untill dry. HOwevewr those 100 psi segments represent different quantities of cu ft. You dont consume psi you consume cu ft. There is where the tank factor comes in. with the al 80 if a true 80. 100 psi is 1/30 of the 80 cuft or about 2.5 cuft. If you are using 100 psi you are burning a lot of air unless you are doing that at 100 ft. with the lp120 using 100 psi is using 1/26 of 120 or amost 5 cuft a min. and then there is the 3400 psi tanks and you are using 1/34 of the tank capacity a minute. Using tank factors is some regards makes all tanks on a level playing field. If you burn .5 cu ft a min sac (ok may be wrong term) and you go to 100 ft that should be about 2 cuft a min the al80 should have about 40 m,in till dry 40 min into 3000 psi (rated presure at 80 cu ft. says that 2 cu ft a min is = to about 75psi a min. or 37 psi per cu ft. or what?? about 10 psi per liter.

Lets look at that same tank that has 37 psi per cu ft. double those tanks and it is now 19 psi per cu ft.

 

[Magnus Lundstedt]

With the imperial system, the tank is already rated by how much it holds. You don't do any math. 80cuft tank holds 80cuft. You only need to do math if you're doing something like filling to more or less than the rated pressure.

But thats depends on how much the tank is filled at the dive center or on the boat when i get it, i then have to figure at what pressure it is "full" and then recalculate to see how much i actualy have.

IMHE on the boat or at the center, they fill all tank to about the same pressure, regardless how much it is rated.

And the formula Volume X Pressure is a very simple thing to remember.

It is more simple than that, they fill all tanks to the same pressure, so an 15l tank is 50% more than a 10l tank. ..

 

[wheeledgoat]

So I'll get my 100cuft out of my HP100 when filled at 3442psi, but only about 88cuft if filled at 3000psi. hmmmmm.

thanks for the replies. I can see where I went wrong now; my brain was jumping to the assumption that the "80" in Al80 and the "100" in HP100 were the inside volume of the tank, completely overlooking how ridiculous that would be (that 80 and 100cuft are the sizes of rooms, not tanks). I abstractly thought the HP100 was physically bigger inside, and it kinda made sense since steel is stronger and can be thinner, therefore a similarly [external] sized cylinder had 20% larger [internal] capacity...

I didn't put together that the numbers are already taking into account the rated pressure and doing the math for you, as @kelemvor pointed out.

Feel kinda dumb now... thanks scuba buddies!