Detmining cubic footage

[rushrhees]

Hello, I am talking with someone online about buying their tank, however they are not a diver and do not know the cubic footage, this is what is know, it is an LP steel tank with 2250 psi stamped on it and a plus sign (by the way is the plus sign have significance?). The tank is 27' high(no valve) and 7' diameter. If anyone can find the cubic footage from this data please let me know, or if there is another marking on the tank which tells the volume of it.

 

[teknitroxdiver]

If you can write down all the markings that are on the neck, the capacity will be somewhere in there. There should also be a manufacturer on it. Post the numbers on here and someone will be along to sort it out for you...

On a side note, what color is it?

 

[Tom Winters]

Danged if that doesn't sound like an old steel 72. What's the original hydro date on that thing? Hey listen, on something like a used tank, if you really don't know what you're getting, pass on it, pay a little more, and maybe buy it from a dive shop you're comfortable with or someone who knows the history of their tank and can answer every question that you have. The life you save might be your own.

 

[teknitroxdiver]

Forgot to answer your question about the +:

The + rating comes with all new LP steel tanks, and basically means that the tank is certified for a 10% overfill, 10% on top of the rated 2250 psi. So, that means you could 'politically correctly' fill the tank to 2475 psi.

Here's a chart of tanks, your's doesn't seem to be on it. But that neck information will tell all. http://www.saudidiving.com/tanks.htm

 

[bedmund]

The tank's capacity is not necessarily marked on the bottle. It's a pain finding out because you'd need to know the water capacity by filling it up and then measuring the amount it takes. One gallon = .1337 cu. ft. And that's at 1 ATM (14.7psi). Convert that to 2250 psi and that tells you how much gas you have at that pressure fill.

There's also a way to figure it by measuring the tank's dimensions and accounting for the thickness of the walls. It's in the Oxygen Hacker's Companion, so I'll need to go look it up.

Or you can buy a tank from someone who knows what they have.

 

[teknitroxdiver]

It's in the Oxygen Hacker's Companion, so I'll need to go look it up.

Hmm another reason to buy that book. Yeah it would be great if you would look it up because I'm about to injure myself Googling for tank dimensions. Not a soul on the web knows how long a 72 is....

 

[bedmund]

Yeah it would be great if you would look it up because I'm about to injure myself Googling for tank dimensions.

Ok here goes. I'm referencing the OHC directly.

Wall thickness for AL is about 1/2" and 3/16 for Steel. Get the outer diameter and subtract 2x the wall thickness (for both sides). You can take the circumference and divide by pi (3.14) to get the diameter. Divide that by 2 to get the radius. The area of that cross section is pi x r squared. Measure the length but don't include the end domes. Multiply the length x area and you get the volume at 14.7 psi. The you convert that to volume at fill pressure psi. 2250/14.7=153.06 ATM. Multiply the 1 ATM volume by the fill ATM.

I hope that's confusing enough.

 

[Bob C]

I see in the OMS catalog that they make a steel tank that is 26' X 7" dia.
That is close to the one you are talking about. The listed working pressure is 2640 psi and show the volume is 85 cu ft. This shoule be close.

Bob C.

 

[teknitroxdiver]

Well, I was in the process of mathing that out, but seeing as good ol Bob C has saved my brain much pain, I can use those numbers a lot easier. For the OMS 85 you have 0.032197 CF/psi. That number times 2250 gives us the volume of 72.443182.

So, it's an old steel 72.

 

[bedmund]

Oh sure. Do it the easy way.