Service pressure of Steel 72

[WD8CDH]

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If all the production steel tanks were lined up and a steel 72 was included at a current price, I would be willing to bet that the vast majority of divers would line up to buy the HP 100.
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Richard

Ah, but what if you were given two choices of HP 100's, one was 7.25" by 24" and the other one was 6.9" x 25" (same size as an old steel 72), which one would you want.

 

[rstofer]

Ah, but what if you were given two choices of HP 100's, one was 7.25" by 24" and the other one was 6.9" x 25" (same size as an old steel 72), which one would you want.

I don't know... Such a tank would hold 110 cf at 3442 psi. I don't think I care about the extra inch of height but I don't know why I would care about the 0.35" of diameter either.

In the end, it would come down to tank weight (lighter is always better), buoyancy characteristics (I certainly wouldn't want anything more buoyant than an HP 100) and how it trims.

It's really hard to say whether such a tank would be a blessing or a curse. We'd have to try them, I guess.

Richard

 

[WD8CDH]

I don't know... Such a tank would hold 110 cf at 3442 psi. I don't think I care about the extra inch of height but I don't know why I would care about the 0.35" of diameter either.

In the end, it would come down to tank weight (lighter is always better), buoyancy characteristics (I certainly wouldn't want anything more buoyant than an HP 100) and how it trims.

It's really hard to say whether such a tank would be a blessing or a curse. We'd have to try them, I guess.

Richard

As a single, not much difference, but quite noticeable when properly set up as doubles. When combined with a close spaced manifold (184mm C-C from DiveRite or Thermo or similar) like usually used with the old LP72s and compared to a pair of 7.25" diameter tanks on a 8.5" spacing manifold, the apparent bulk is quite a bit less. Buoyancy and trim would be identical to a LP72.

 

[halocline]

The LP 85 is pretty close; 7" diameter, 25" or so long. The key attribute they share with LP72s is neutral buoyancy empty which makes them ideal for doubles if you want to use them in warm water. There are plenty of LP72s still around if you're willing to look for them. As long as you can get a reasonable fill (sometimes an unbelievable challenge around here due to the prevailing idiocy) they hold about as much gas as a similarly-badly-filled AL80 and they're easier to manage on land.

 

[Stu S.]

As long as you can get a reasonable fill (sometimes an unbelievable challenge around here due to the prevailing idiocy)

Hey, Matt. The ability to get overfills is dwindling. I would often take an LP tank along to Gainesville; visiting my college kids. I could get an overfill there.

After selling tanks and buying new, we are an HP tank family. That's not something I planned on doing.

Good luck with yours.

 

[DCBC]

You're right the first time 2250 psi. The + allows for a 10 percent overfill during the first 5 year period only. Subsequent hydros do not allow this pressure. The burst disc should be rated at 3000 or 3200 psi so I wouldn't want to fill it to 2800 on a hot day...

 

[captain]

You're right the first time 2250 psi. The + allows for a 10 percent overfill during the first 5 year period only. Subsequent hydros do not allow this pressure. The burst disc should be rated at 3000 or 3200 psi so I wouldn't want to fill it to 2800 on a hot day...

Sorry but you are wrong, the + can be granted at anytime in the tanks life if it passes the criteria for the + rating. A tank could have been hydroed 5 times previously with no + and be + rated on the sixth time.
The primary reason tanks do not receive the + after the original hydro is either the testing facility doesn't bother to do it or they do not have the REE specification for that particular make and size of tank necessary to calculate if the tank qualifies for the +.

 

[DA Aquamaster]

You're right the first time 2250 psi. The + allows for a 10 percent overfill during the first 5 year period only. Subsequent hydros do not allow this pressure. The burst disc should be rated at 3000 or 3200 psi so I wouldn't want to fill it to 2800 on a hot day...

+ 1 on the "+" rating can be done at any time. I have seen 50 plus year old welding tanks still receive a plus rating.

Also, the test pressure on a 3AA tank with a 2250 psi service pressure is 3750 psi and by DOT rgulation the burst disc is supposed to rupture at between 90% and 100% of that pressure. So the burst disc assembly should be rated somewhere between 3350 psi and 3750 psi, not 3000 psi to 3200 psi.

 

[Luis H]

You're right the first time 2250 psi. The + allows for a 10 percent overfill during the first 5 year period only. Subsequent hydros do not allow this pressure. The burst disc should be rated at 3000 or 3200 psi so I wouldn't want to fill it to 2800 on a hot day...

I don&#8217;t keep up with the regulations and codes in Canada, but id the US CFR 49 will tell you how to hydro test DOT pressure vessel and how to re-qualify them with the + stamp.

I have and have seen plenty of old tanks that have been re-qualified with the + stamp. And as hydro test operator I have re-qualified several.

The release pressure of the burst disk is supposed to be (per the codes) the same as the test pressure with an allowable tolerance of +0 or -10%

For a steel 72 the stamped pressure is 2250 psi. The test pressure (for a DOT 3AA is 5/3 of the stamped pressure) is 3750 psi. Therefore the burst disk is supposed to released somewhere between 3375 psi and 3750 psi.


Added: DA Aqua Master can type faster that I can.

 

[DCBC]

You guys are technically correct from a U.S. perspective. To the best of my knowledge, no steel 2250 psi cylinder in Canada can be given a "+" rating on a subsequent hydrostatic inspection. In the U.S. this was not usually done as most testing facilities would not test or be capable of testing to the standard required (see NOAA dive manual).

As far as burst discs are concerned, when 2250 psi tanks were common in Canada, the 5/3 rule wasn't applied. For that matter burst discs were often replaced with a solid device which would prevent gas venting (yes very dangerous, but I never heard of any negative incidents). This was not really any surprise, as octopus regs were unheard of (nor required by the certification agencies) and people thought you were a safety nut if you insisted on a "J" instead of a "K" valve. Diving has come a long way. When I worked in a dive shop (prior to owning my own) 2250 psi cylinders often required burst disc replacement; 3200 psi discs were usually used. When I read the question, I thought it odd that 2250 psi cylinders were being discussed as I haven't seen 71.2 cuft cylinders for some time...