Steel tank volume question

[The Kraken]

you need a trig function to get the math right

Why does my Steel 104 @ 2640PSI contain more than your ALU80 @ 3000 PSI?

A "trig" function?
My mathematical education surely must be lacking!
Last time I checked, trigonometry dealt with the relationships of 3 angles and three sides of a triangle.

the K

 

[xiSkiGuy]

Where can i get those calcs for tank pressures/volume etc? or what is the equation?

You have to "know" the tank's volume at the rated pressure: ie 100cf at 3442psi or 77.4cf at 3000psi or 85cf at 2640psi, etc, etc.
Then you take the pressure you have (in the example it was 2900psi) and make a ratio.
Then it is simple math: 100cf/3442psi = Xcf/2900psi and solve for X:
100cf/3442psi=0.0291
0.0291*2900=84.39cf

 

[The Kraken]

Very easy Cody, the tank volume and service pressure is stated on the stampings on the tank. Now for a monkey wrench, an AL80 is, I think, 77.4 cu. ft. full.

Divide the capacity by the nominal pressure to get your cu ft/psi. Then just multiply the psi of the tank by that number.

the K

 

[Codyjp]

That was easier than i though. Thanks for the help.

 

[trigfunctions]

You guys have got me confused. I was correct, right? At high pressure, the steel 100 would hold 100 cf, but at 2900 psi I wind up with a little more air than an AL 80 but not much. Right? So I paid extra for a tiny bit more air, right?

 

[spectrum]

It's also probable that although the dive shop was renting steel 100's, their fill equipment is only rated to 3000 PSI. If that's the case, then all they're really renting you is the added benefit of a steel tank, not more air. Remember, steel tanks also don't have the buoyancy changes as you assume air that the aluminum cylinders have.

Not quite,

First the AL80 which is only good for 77.4 CF with a perfect 3000 psi fill would have 74.82 cubic feet with the same 2900 psi fill. Compared to the HP100 with 84.25 @2900 the steel diver is up by 9.42 cubic feet, not trivial.

As mentioned cylinder material or rating has nothing to do with buoyancy swing. That is a function of consumed air mass. Air si roughly .08 LB/Cf no mater what you pack it in. Consume 70 cubic feet you will be 5.6 pounds lighter.

Cylinder material has a lot to do with your weight walking and swimming and the amount of diver weight you need to carry.
Check this out:
http://www.scubaboard.com/showpost.php?p=1191538&postcount=16

Looks like the calculations have been covered



Pete

 

[trigfunctions]

the steel diver is up by 9.42 cubic feet, not trivial.
Pete

Right, but the dive shop specifically told me that I had the equivalant of 100 cf. Either they forgot to fill it to 3400 psi or they intentionally misled me.

 

[The Kraken]

Cut to the chase . . .
. . . the tank can hold 100 cu/ft at 3442, but can your LDS's compressor pump it?

the K

 

[spectrum]

I'm probably mincing words but first there is nothing equivalent about the 100 CF. You either had that air or not.

You would not be the first victim of a frazzled fill station attendant.

Second we have all gone with the HP100 assumption based on you saying the size was comparable to your GFs AL80. How precise is that comparison? You may have had an overfilled LP cylinder or a higher than 100 CF rated HP cylinder. Just thoughts. Not enough data to be conclusive.

Pete

 

[trigfunctions]

I don't know Kraken - I rented the tank from the LDS. I told them that I wanted it becuase it would hold more air, and they told me that it had about 100 cf in it.

No big deal - only about 5 dollars or so extra for 2 tanks, but still it peeves me that they either made a mistake or intentionally misled me - neither one is good.

At least now I know why my air didn't last any longer. I thought I had about 20% more air in the tank but used up even faster than normal