Jerry's Green Tank...

[surflung]

Jerry's Green Tank...
- A couple of weeks ago, SeaHunt Jerry called to say he had a line on "one of those surplus oxygen tanks you like... You know, the ones with the round bottoms like your twin 38s." I asked how much the guy wanted for it and he said "20 Bucks". So Jerry hauls this tank all the way up to Fortune Pond to give it to me.
- It has "Aviators Breathing Oxygen" stenciled on the side and it has a military anti-shatter wrap on the body. The date of manufacture/hydrotest is Feb 1968 and it hasn't had a hydrotest since. When I got it home, I had a heck of a time getting the valve off... That original plumbers clay type sealant. I don't think it had ever been opened since it was made. Inside was beautiful and clean.
- I took it up to Minnesota School of Diving in Brainerd, MN for hydrotest. Bill let me watch the test and it passed well within the margin of acceptance. So, how do I get this thing converted for Scuba diving?
- Well first, I already had a high pressure bushing/reducer. So I put 6 wraps* of teflon tape on the threads and screwed it in as tight and far as it would go... I believe it has engaged at least 5 threads deep. Then I put 6 wraps of teflon on a 1/2" tapered thread USD J-Valve and screwed that in as far as it would go.
* 6 Wraps is Antique Diver's trick to assure no leaks on tapered threads.


- I started filling it and checked for leaks at 500, 1000, 1500, and 1800 psi... No leaks.
- How Many Cubic Feet? This is the funny part. It's stenciled right on the side of the tank: "646 Cu In". Divide by 12 and its a 53.8 cf tank at 1800 psi. With a ~10% overfill to 2000 psi it'll be 59.7 cf... So it's pretty much a 60 cf tank.
- Physically it is shorter (and lighter) than a standard "72" and longer than a "Twin 38".
- I dove it without a wetsuit and required 4 lbs of lead to get neutral... So it has nice buoyancy characteristics. I stuck one of those green "Sea Hunt" tank decals on it... I'm thinking it's interesting to leave it in the original green "oxygen tank" paint and stencil scheme and dive it with a green hose regulator!

[scubadiverbob]

Make sure to go deep enough people wonder ...... (O2?)

[duckbill]

You need to get that wire wrap off of the cylinder.

6 wraps of tape might be too much, depending on the thickness of the tape. Too much tape, and only the tip of the thread gets the full load. Not good. The tape is just supposed to act as a lubricant while turning. The thread taper is supposed to provide the seal, not the tape.

[SeaRat]

The wire wrap was on our PJ cylinders (former CO2 20-man life raft cylinders) too. We removed it prior to use of scuba cylinders. It is there for use on combat aircraft, where if it took a round it would not shatter, but simply vent. I think Duckbill is correct that it should be off before hydrostatic testing, but if the tester did not insist on it, I'm not sure--look up the regs on that. I always use just enough Teflon tape to cover the threads (probably about two wraps as it was overlapped), and made sure I wrap it in such a way so that when I screw the valve in, it is not against the wrapping. That means, I think, wrapping clockwise from the bottom up (but I'll have to check that with a valve sometime).

John

[surflung]

- Teflon tape functions both as a lubricant AND a sealant. In the firearms industry, we use it to lubricate and seal breech plugs on muzzle loading rifles. Steel threads will cut thru it like butter. We gauge how far up the threads to wrap it by first screwing in the plug to see how far it goes in. With 6 wraps it goes in just as far or farther and never fails to seal. The same is true with 1/2" tapered scuba valves. Try it and you'll see. The idea of using 6 wraps comes from a working scuba compressor service man who uses 6 wraps for all of his tapered thread compressor connections as well tapered thread cylinder valves(AntiqueDiver).
- With regard to the anti-shatter wrap on this tank, it's not there to boost the tank strength and is not part of the pressure containment vessel. I choose to leave it on for authenticity. But if you're concerned for safety, remember this tank has just undergone a completely successful hydrotest to 5/3rds of its working pressure. That means it was run up to 3000 psi... That's 1200 psi over its marked working pressure of 1800 psi... And the residual stretch was well within the acceptable margin. It has also been visually inspected by an expert and is rust and pit-free. If all of that is not safe enough, what is?
- One reason I can see for removing the anti-shatter wrap is because it adds weight and for the old days of Scuba when they wanted tanks to be more floaty, that was probably a common practice. Another reason for removing it might have been surface prep for galvanizing. Neither of these has to do with the strength of the vessel.

[surflung]

Re-Calculating Capacity...
- A friend who wishes to remain anonymous called to say I can't just divide the 646 cu. in. by 12 to get the cubic feet... And after he explained it, and I imagined 12" x 12" x 12" as a cubic foot, I understand it, I think:
- The tank is marked 646 cu in. One cubic foot is 12 x 12 x 12 = 1728 cu inches. 646 is less than that so it can't be at 1800 psi. If we figure it at ambient pressure of 14.7 psi, here's my math:

646 in3/14.7psi = X/1800 psi, X solves to 79,102 in3

1 ft3/1728 in3 = X ft3/79,102 in3, X solves to 45.7 ft3 (at 1800 psi)

Filled to ~10% over at 2,000 psi: 45.7 ft3/1800 psi = X/2000 psi, X = 50.7 cu ft.

- So, the Green tank is 45.7 cu ft at 1800 psi and 50.7 cu ft. at 2000 psi.

(For comparison, a standard "72" is only 57.5 cu ft. at 2,000 psi.)

[surflung]

Luis Heros Sent Me Another Way to Calculate..

The 646 in cu is the actual volume inside the cylinder, also known as the water volume.

To convert into cubic feet you divide by 12 cubed or 12 x 12 x 12 = 1728 cu in / cu ft.

646 cu in/ 1728 cu in / cu ft. = 0.3738 cu ft. Again, this is the actual volume inside the cylinder.

At 1800 psi you are filling the cylinder with 122.4 atm (atmospheres)

1800 psi = 122.4 atm

Therefore the volume of air that is being compressed inside the cylinder is:

(0.3738 cu ft.) x (122.4 atm) = 45.8 cu ft. (at 1800 psi)

Or approximately a 46 cu ft. cylinder.

With the 10% overfill; you have approximately a 50 cu ft cylinder.


You are welcome to post this if you like and you can mention my name. This is very straight forward calculation.

[surflung]

Thoughts on Removing the Wire Anti-Shatter Wrap...
- I haven't got anything official on this but I've been told the reason the wire wrap was removed from this type of surplus tank when they were converted to Scuba diving was to avoid undetected corrosion from forming under the wrap. Kind of like the corrosion that occurred on vinyl coated tanks when moisture got in thru a pin hole and spread between the vinyl and the steel. I think they call this "Crevice" corrosion... Which is enhanced or accelerated in comparison to everyday surface corrosion. Especially for tanks intended for use in salt water, its definitely a factor to be considered... And probably also pertains to my tank even though I dive it in fresh water.
- In the time since I first posted this thread, my son gave me an old set of twin tanks made from surplus tanks that are identical to the one above but that had the wire wrap removed and the tanks re-painted. The re-paint has long since weathered and I can distinctly see where the steel that was under the wire wrap must have been just bare, non-galvanized steel.
- So as much as I prefer to leave this tank in it's original wrap, I'll probably have to eventually do as you guys are suggesting and remove the wire anti-shatter wrap to avoid potential corrosion.

[Deleted]

So as much as I prefer to leave this tank in it's original wrap, I'll probably have to eventually do as you guys are suggesting and remove the wire anti-shatter wrap to avoid potential corrosion.

Eben, what kind of wire is it? If it is copper, try to save it when you remove and offer it to a 4H boys club or some organization such as that...........copper wire makes a dandy crystal radio set tuning bar......Lord knows how many tuning bars that much wire would make.

[surflung]

Jerry's Green Tank Update...
- With regard to the Wire Wrap on this Shatter Proof surplus tank, I was wrong and everyone else was right! I have removed the wire wrap from this tank and discovered the beginnings of crevice corrosion forming under the bottom end of the wire wrap... Nothing bad but enough to prove the wrap should be removed.

- It's a steel wire. And after it was wrapped, I think it must have been squeezed or pressed into place. Unwinding it came off as square rather than round wire.
- The photo above is after...
1. The wire was removed.
2. The tank was re-qualified by Hydro-Testing and Visual Inspection.
3. The outside surface was bead blasted to remove surface rust.
4. The outside was cleaned by wiping down with Acetone.
5. The outside was roller painted, two coats, with Galvalite 96% Zinc Paint.

I'm going to let the paint cure for a couple weeks before installing a bushing and a really nice USD J-Valve. After that, I think I'll fit it with the Aqua Lung Bac Pac and stick an AquaLung decal on it.