What do the numbers on scuba tanks mean?

[Scuba_Nick27]

galvanized refers to the coating, all steel tanks are made from chromoly steel, the older vintage tanks have a slightly different alloy than newer ones.

Yeah, I basically meant that.

 

[2airishuman]

That makes some sense ... except for where does 3442 come from? I always wondered that. Seems so random.

HP cylinders were originally designed around a test pressure of 5250 and a working pressure of 3500 PSI. I'm uncertain of the exact details but yoke valve connections have a maximum permitted pressure that is around 3300 PSI or 230 bar, so the original HP cylinders had DIN valves and 7/8" necks so they wouldn't be compatible with the yoke valves for safety. Somewhere along the line someone found a way to get an approval for 230 bar service based on a liberal conversion ratio between bar and PSI that resulted in a 3442 PSI service pressure. The loss of the "last 58 PSI" was a small price to pay for being able to use yoke (or convertible) valves and so that is what the industry did.

The test pressure (5250) on these cylinders, and the metallurgy, and the construction techniques, are all the same as for 3500 PSI cylinders, but by derating them by 58 PSI they can use a (Western hemisphere) standard neck and valve.

This is the sort of thing that I am referring to when I say that the packaged gas industry's procedures are based more on tradition than engineering.

 

[broncobowsher]

the test pressure is 3/2 working pressure per the special permit. this is different than the 5/3 used for 3AA vessels...

That is why I was giving a warning about who to let hydro it. If the knuckle head just goes off the old school 5/3 pressure it will fail. If they know what the tank is and follow the correct 3/2 pressure test, they generally pass.

 

[runsongas]

HP cylinders were originally designed around a test pressure of 5250 and a working pressure of 3500 PSI. I'm uncertain of the exact details but yoke valve connections have a maximum permitted pressure that is around 3300 PSI or 230 bar, so the original HP cylinders had DIN valves and 7/8" necks so they wouldn't be compatible with the yoke valves for safety. Somewhere along the line someone found a way to get an approval for 230 bar service based on a liberal conversion ratio between bar and PSI that resulted in a 3442 PSI service pressure. The loss of the "last 58 PSI" was a small price to pay for being able to use yoke (or convertible) valves and so that is what the industry did.

The test pressure (5250) on these cylinders, and the metallurgy, and the construction techniques, are all the same as for 3500 PSI cylinders, but by derating them by 58 PSI they can use a (Western hemisphere) standard neck and valve.

This is the sort of thing that I am referring to when I say that the packaged gas industry's procedures are based more on tradition than engineering.

ISO standard max 230 bar @ 15C for yoke converted to psi and Fahrenheit. can't blame tradition for the max yoke pressure, it is a standards and regulation issue. some of the heavy yokes are actually stamped for 4000 psi.

 

[BRT]

That makes some sense ... except for where does 3442 come from? I always wondered that. Seems so random.

Can't be 3500 or it would have to have a DIN valve.

 

[Divin'Papaw]

Another reason to hate Yoke. Always thought they were evil.

 

[Still Kicking]

After testing tanks for +/- 5 years, I don't see many steel tanks that fail hydro. But of those that do fail, a disproportionate number belong to operators with access to their own fill stations (commercial operators). I surmise that these tank may have been overfilled on a regular basis. I have zero evidence to back this up and YMMV....

 

[2airishuman]

That is why I was giving a warning about who to let hydro it. If the knuckle head just goes off the old school 5/3 pressure it will fail. If they know what the tank is and follow the correct 3/2 pressure test, they generally pass.

It's more complicated than that. Hydro places all use the test pressure of 5250 stamped on the cylinder.

True hot-dipped galvanized HP cylinders, for reasons that are poorly understood, go out of round by a tiny amount (thousandths of an inch). At hydro, the PST cylinders are subject to a maximum inelastic expansion limit, and the test for this produces false positives because the cylinders go out of round. The fix is to bring the cylinders up to test pressure before running the actual test, but many hydro shops refuse to do that -- and then fail perfectly good cylinders.

Worthington cylinders are exempt from the maximum inelastic expansion test. Faber cylinders aren't actually hot-dipped and don't go out of round.

 

[runsongas]

it is due to the differing expansion coefficient for the zinc layer. all hot dip galvanized cylinders should be rounded out at 90% of test pressure (including the new fabers) to prevent false failures. if you don't round out a worthington, you could also fail the REE limit incorrectly.

https://static1.squarespace.com/sta...19fa49fc6/1434147341305/retest_procedures.pdf

 

[Scuba_Nick27]

Another reason to hate Yoke. Always thought they were evil.

Haha yeah. DIN forevaa!