Did I get double 50's?

[John C. Ratliff]

I was going to suggest what Duckbill suggested, but I would do it with a single 80 AL tank, only because I would not have to figure the cubic feet of a "72." I believe that at 2250 psig the "72" (actually 71.2 cubic feet at the 10% overfill of 2475 psig) is actually 64.7 cubic feet. So I would use the 80 cubic foot tank at 3000 psig to make the calculation. Then, by simply knowing the 80 cubic feet, the original 3000 psig, and the final psig of the 80 cubic foot tank, you could calculate fairly easily the cubic feet of air in the twins at that pressure. Then you would simply compare that pressure/volume relationship to the cubic feet at the 2250 psig pressure.

SeaRat

 

[Akimbo]

It looks like Voit switched from tapered to O-ring sealed valve connections in 1966, about the same time they were trying to figure out if they wanted to be Voit or Swimaster.

 

[oldmossback]

It looks like Voit switched from tapered to O-ring sealed valve connections in 1966, about the same time they were trying to figure out if they wanted to be Voit or Swimaster.[/QUOT

My tapered valved voits are teflon tape sealed, not o ring..........I've never seen a tapered oring valve.....not that they could not exist.......I've just never heard of or seen one......

 

[duckbill]

I'm pretty sure he means they switched from 1/2" tapered with teflon tape seal to, I assume, 3/4" straight threads sealed by an o-ring.

 

[Akimbo]

I'm pretty sure he means they switched from 1/2" tapered with teflon tape seal to, I assume, 3/4" straight threads sealed by an o-ring.

True. Specifically that is ½" CGA, which is a tapered thread very similar to ½" NPT and both frequently used with Teflon tape, versus the ¾" O-ring sealed straight pipe thread introduced by US Divers in 1960. USD applied for a patent on the design but I don’t know if it was ever granted since the industry had pretty well switched by 1966. If a patent was granted, perhaps they licensed it at a low cost of for free to get the standard established.

I never understood the reason a straight pipe thread was chosen over a UNC or UNF thread. All standard O-ring sealed connections like on second stage regulators or industrial fittings, that I am aware of that are used in the US are UNF or UNC based. Anybody know?

 

[bjhawthorne]

To those who want to do the calculations, here's the math:
Let Vx= unknown volume of double cylinders.
Vinit= initial volume, 80 cu ft if using a std "aluminum 80".
Pinit= initial pressure, 3000 psig using the al 80, or whatever your gage reads depending on how good a fill you got.
Pfinal= measured final pressure after the tanks have equalized.

Eqn #1 Vfinal= Vinit + Vx or Vx= Vfinal - Vinit

Eqn #2 Pinit * Vinit = Pfinal * Vfinal (from gas law)

Let's say you had an al 80 with a pressure at 3000 psig and after connecting to the twins the final pressure is about 1300 psig. Then: Vfinal = (3000)(80)/1300 = 185 cu ft (approximately)
and so Vx= 185 - 80 = 105 cu ft, but we have two, so each cylinder is about 52 cu ft.

 

[duckbill]

To those who want to do the calculations, here's the math:

That's really nice of you to post that. I've been too lazy lately to go to the trouble.
That formula will work fine if all are 3000 psi cylinders. However, kramynot2000's are 2250 psi, and Mr. Fudd's are 2015 psi.

So, take the answer as per yours calculations at 3000 psi, and find the equivalent volume at the ratio of the two pressure ratings.

eg. carrying on your example:
105CF x 2250psi(or 2015psi as the case may be) / 3000psi = Volume of the unknown cylinder(s) at the rated pressure

Also, keep in mind that most Al80s are actually 77.4CF@3000psi
Steel 72s are 71.2CF@2475psi, or 64.7CF@2250psi

 

[oldmossback]

Duckbill and others,

How does one know a steel 71.2 is 71.2 at 2475psi and not at 2250psi? Where does one get this information?......the gas exchange technique noted above showing the set of twin tanks to be 52 cuft each at 1300psi.yet they are rated a 2250 without a over fill and called 50 cuft tanks, gives me suspious pause.....I wonder this because most LDS's all my life have filled my tanks to 2250 approx and I and my diving friends have assumed we had 71.2 cuft approx.......it was not until 15years ago that I understood this was not so.........Thank God for the information highway called the internet.......as there is not much coverage in the dive manuals back then concerning this issue. I do not remember any instructor expressing the difference between 2250 and the 2475 plus rating since the fills were always around 2250. We assumed, wrongly I suppose, that these fills were appox 71.2 cuft.

So, have I and others been subject to false advertizing? Are the LDS's still subjecting divers to false advertizing?

And finally, not using any formula, has there been, and I am sure there has, any physical testing, other than a actual gas transfrer as noted above, that shows what the actual volumn is at 2250 and 2475, and where do I find this information......

 

[captain]

Lets call it non disclosure not false advertising . The 71.2 is with a 10% overfill which is what the tank was capable of when advertised and sold, if a dive shop decided to fill to only 2250 then that is a separate issue. What wasn't said was that there was no way to be sure the tank would continue to qualify after the first 5 years for the 10% overfill or the testers would even do the necessary calculation or have the necessary information to do the calculation to determine if the tank qualifies.
Even today the capacity of new spec 3AA tanks are advertised with the 10% overfill so a modern 3AA2400 needs 2600 psi to reach advertised capacity.

I have some so called 90 cu/ft aluminum Navy tanks. All the Navy manuals state itis 90 cu/ft but the internal volumn in cu/in's is clearly stamped on the tanks and if you do the math they hold 88 cu/ft, close enough for government work I guess.
It's all in the details.

 

[duckbill]

Mossback,

Yes, that information showing the actual capacity of steel 72s is available. I had it in a specifications chart that I lost when my old computer crashed.

Sure, one could have the hydro guy fill the cylinder with water and pour the water into a measure to get the capacity, from which you could calculate the capacity at pressure.

The air equalization method will work for any cylinder of unknown capacity. You are equalizing the pressure based on the combined capacities and you can mathematically plug in any pressure value for the unknown cylinder you want using the formulae given, be it the marked pressure or not. In other words, you could plug in 20,000psi if you wanted to see what the unknown cylinder would hold at that pressure (LOL!). If your twins are rated at 2250+, then you can calculate for 2250 and 2475, or simply calculate for 2250 and add 10% to find the capacity for 2475.