3000psi 80cf AL Tank vs. 2400psi 80cf Steele

[mike_s]

I have a question about tank PSI and cubic feet. (another thread actually made me want to ask this....)


If you havea 3000psi AL 80 and a 2400psi Steel 80, are they going to have the "same amount" of air in them since they are different PSI even though they say the same number (80) cubic feet?

Part of me wants to say yes because it's measured in Cubic Feet, but I also know that volume and pressure are related?


Can anyone give any insight exactly on this? Just curious.


-Mike

[matt_unique]

I have a question about tank PSI and cubic feet. (another thread actually made me want to ask this....)


If you havea 3000psi AL 80 and a 2400psi Steel 80, are they going to have the "same amount" of air in them since they are different PSI even though they say the same number (80) cubic feet?

Part of me wants to say yes because it's measured in Cubic Feet, but I also know that volume and pressure are related?


Can anyone give any insight exactly on this? Just curious.


-Mike

For the "yes" part: A ton of bricks weighs the same as a ton of feathers. 80 CF is 80 CF whether it's at 3000psi or 2400 psi.

For the "no" part: An 80 CF alum tank actually holds 77CF of gas. (This 3CF deduction could be addressed with more pressure but the tank is not rated for that).

For a fixed container, higher pressure will ram more gas into it. If you fill a LP steel 80 tank to 3000psi it will have more than 80cf of gas. It is labeled an 80 because at it's rated pressure (2400 psi) it holds 80CF of gas.

--Matt

[String]

.. its times like this im glad i use water capacity and metric

[Don Burke]

I have a question about tank PSI and cubic feet. (another thread actually made me want to ask this....)


If you havea 3000psi AL 80 and a 2400psi Steel 80, are they going to have the "same amount" of air in them since they are different PSI even though they say the same number (80) cubic feet?

Part of me wants to say yes because it's measured in Cubic Feet, but I also know that volume and pressure are related?


Can anyone give any insight exactly on this? Just curious.


-Mike

80 standard cubic feet is 80 standard cubic feet, no matter what the pressure.

In this case, the steel tank would hold more cubic inches of water. Due to the higher pressure in the AL tank, both tanks hold the same amount of air.

Note that an "80 cubic foot" tank may actually hold 77.1 cubic feet or so of air.

Also note that a steel tank's stated capacity may be at 110% of the marked pressure.

[rab]

Except for the "round-off" error, you are correct. Both cylinders contain approximately the same amount of gas. The internal volume of the LP-steel will be larger in order for the pressure to be lower for that same volume.

Usually, cylinder capacity is either Cu.Ft.@WorkingPressure and the Cu.Ft. represents the volume at normal atmospheric pressure. When a cylinder's capacity is given in liters (as in a "12L" or "15L" cylinder), this refers to the "water volume" (how much water would it take to fill the inside) and the pressure is not used. Most times, a cylinder that is measured in liters will also specify its working pressure in "bar" rather than psi.

-Rob

[Xman]

Except for the "round-off" error, you are correct. Both cylinders contain approximately the same amount of gas. The internal volume of the LP-steel will be larger in order for the pressure to be lower for that same volume.

Usually, cylinder capacity is either Cu.Ft.@WorkingPressure and the Cu.Ft. represents the volume at normal atmospheric pressure. When a cylinder's capacity is given in liters (as in a "12L" or "15L" cylinder), this refers to the "water volume" (how much water would it take to fill the inside) and the pressure is not used. Most times, a cylinder that is measured in liters will also specify its working pressure in "bar" rather than psi.

-Rob

I agree, except that a lot of metric rated cylinders that state water capacity in liters will actually give working pressure in megapascals (which for ease of use can be divided by 10 to convert to bar).

At the end of the day, which is the way to go depends on the fills you tend to get from your LDS. Whilst in an ideal world, your cylinder should be filled to a temperature corrected level so that it settles to the working pressure, this is often not the case. If you use a HP cylinder (at least in some places) you will not get a fill that will settle back to the working pressure (you will get a light fill).
If this is not an issue where you get your fills, great. If it is, then take that into acount when you are figuring out how much gas will actually be available in your cylinder.
It also pays to do the calculations yourself when possible on water capacity and working pressure (water capacity (in liters) x working pressure (in bar) will give you available liters of gas). As has been stated above, sometimes the description of a cylinder as 80cf or 100cf is an approximation and not entirely accurate.

[Hockeynut]

When I read the title of this thread, 30000 psi , I thought, wow a new kind of tank you can really crank up the pressure.

How much smaller would a 80cf tank with a rated pressure of 30000 be? Maybe I should invent this.

[JustinW]

all is correct as stated before. If you are considering the two tanks, remember that a steel tank is going to be more negative, depending upon the make, always negative. When comparing a steel 80 to an Aluminum 80, with the same setup you will require 5-8 more pounds of lead on you for the aluminum versus the steel that will allow you to carry less lead. ideally you can dive a HP steel 80 and shed even more lead off your setup. I dropped 6 pounds going from a 2400 psi steel 72 to a 3000 psi steel 74

[michaelp68]

all is correct as stated before. If you are considering the two tanks, remember that a steel tank is going to be more negative, depending upon the make, always negative. When comparing a steel 80 to an Aluminum 80, with the same setup you will require 5-8 more pounds of lead on you for the aluminum versus the steel that will allow you to carry less lead. ideally you can dive a HP steel 80 and shed even more lead off your setup. I dropped 6 pounds going from a 2400 psi steel 72 to a 3000 psi steel 74

I'm trying to understand the benefit of that, and perhaps I'm being dimwitted.

But, whether you carry the weight as part of your steel tank or as extra lead if it's an aluminum tank, you're still carrying the weight, right? So, what's the real difference or benefit?

Michael

[Mick_O]

How much smaller would a 80cf tank with a rated pressure of 30000 be? Maybe I should invent this.

15L of air at 3,000psi would take up 1.6qt (US, liquid) at 30,000psi... I think. Someone feel free to check the math. PV=nRT right?