Tank buoyancy numbers

[Rred]

How would they figure out what is "normal" for an oddball tank like my PST 96.2, which is not common and not referenced in any published tables?

 

[Charles2]

You might check out this video. Notice that they only indicate the cylinder size by diameter and length. They do not measure the volumetric capacity of the cylinder - only the elastic expansion and the permanent expansion.

 

[Rred]

Sounds like they ARE actually measuring the volume, when the guy says the residual expansion has to be less than 10%--of the original volume, in CC's.

I love the way he ignores REE and the fact that some math beyond "10%" might have to be calculated.(G) With modern tanks tending to 3440 for high pressure (in order to stay under the 3500psi point (where equipment requirements change and IIRC you have to go from yoke to DIN among other things) the whole idea of + ratings is going to be less of an issue. As soon as someone figures out how to get rid of the old tanks.

I heard back from DOT, who say it may take another two months to write up a formal reply, send it past legal and everyone else, and mail it to me. Basically, the big surprise of the conversation was that really, they want three tanks from the same manufacturing lot (because the metal in each lot may vary) to issue a standard. Otherwise it is individual REE testing if the manufacturer didn't publish or stamp them. (Shucks!)

But here's the nice surprise: They were astounding that the rules are SO old SO arcane SO hard to research and "perhaps it is time to reconsider them". Their comment, not mine. I asked how does that happen, and they said anyone, usually an industry body but ANYONE, is able to petition the DOT for a rule change and when the rules are that old...it becomes more likely to be considered.

Imagine if we asked for a simple rule change, i.e. "Because SCUBA tanks are used in recreational marine service and have different maintenance and use needs, DOT should require: 1) That every hydro shop performing a hydro test on a SCUBA tank first perform the initial VIP test to standards sufficient to the intended use, i.e. to the SCUBA industry. And that VIP test shall be binding upon the SCUBA industry. 2) That every hydro test performed upon a SCUBA tank which initially had a + rating, shall be attempted to be re-tested at that same + rating, not just at the lower base rating of the tank. The hydro test shall re-issue a + rating for all tanks that qualify for this higher pressure use."

Two strokes of the pen--and the SCUBA thieves could all go helix themselves.

After I get the formal written reply and instructions as to how to request the reformed rules, I think I'll have to start passing a petition around. Can't think of any reason that any diver wouldn't sign it and ask to get their tank inspected and filled AS THEY ARE ALREADY ENTITLED. And if some scuba shops go broke because they can't double-charge for hydro's every 5th year? TFB, capitalism can be cruel that way. It's a zero-sum game.

 

[guruboy]

Expansion values are relative to expansion during test. Not tank volume.

The REE number is an actual volume specific to the tank that is spec'd by the maker but still compared to expansion during test. Not the tank volume.

 

[Rred]

"Expansion values are relative to expansion during test. Not tank volume."
Yes, but we still measure expansion by the change in volume. A distinction without a difference.

 

[guruboy]

"Expansion values are relative to expansion during test. Not tank volume."
Yes, but we still measure expansion by the change in volume. A distinction without a difference.

Sorry, I guess I need to clarify-

Expansion values are relative to expansion during test. Not tank TOTAL RATED volume.

 

[Charles2]

Sounds like they ARE actually measuring the volume, when the guy says the residual expansion has to be less than 10%--of the original total expansion volume, in CC's.

FIFU - It's the total expansion volume, not the total tank volume that's important. The total tank volume is unimportant in this test.

 

[Rred]

The expansion volume will be a result of the pressure applied and the original actual volume. Actual, not rated. I think we've all heard how many tanks are rated with "sexy" numbers or their plus fill numbers, as opposed to their actual volumes.

 

[guruboy]

Expansion volume has nothing to do with the original tank rated total volume.

 

[JamesBon92007]

You may find the data on this table useful.

Notice that the actual volume on my steel 72 will range from about 69.6 to 72.2 cubic feet.

The cylinders in water weight were measure in fresh water. The weight of the air inside the cylinder was accounted for using actual cylinder volumes (not published data). I measured the internal tank volume by accurately measuring the water capacity during the hydro test process.

The density of air at STP (Standard Temperature and Pressure) is 0.075 Lbs / cu ft.

Or 80 cu ft of air will weigh 6 pounds

I understand using the weight of the compressed air to determine the capacity of the tanks but I'm not understanding how you are using the water capacity to determine air volume. What's the formula for doing that, and how do you convert ounces to cubic inches?

My main reason for asking is because I have two small steel tanks and I don't know the capacity. They are about 18.5" measuring to the bottom of the neck and a steel 72 measures about 25". That's roughly 74% of the size of a steel 72. They are 2250s and the same diameter so I'm coming up with approximately 53 cu/ft. Does that sound right? I cannot find any clue to the existence of either a "50" or a "55" or any such thing. They say "Voit" on them and were born February '62 (first hydro). I naturally found many references to "38s" but these seem to be a littler larger than that.

They are due for a hydro test so I'm planning on weighing them empty and once again filled with air and calculating the capacity that way but any clues you may have would be appreciated. I'm planning on selling them and would prefer to give an accurate description of their size.