It says right on my tanks - DO NOT OVERPRESSURIZE

[Ted Judah]

A year ago, I bought tanks for my wife and I. In that year we have used them on 16 dives and had them filled at 5 different dive shops. The pressures after each fill seem to vary wildly and is often overfilled.

Here are the markings on the tanks:

FABER MADE IN ITALY M8303 21/0154/ 073 02•21+
TC - 3AAM - 184/DOT - 3AA2400 DO NOT OVERPRESSURIZE REE67 BS85S


Below are the start fill pressures for each dive:

2400 lbs.
3400 lbs.
2600 lbs.
2500 lbs.
2500 lbs.
2500 lbs.
2200 lbs.
2800 lbs.
2800 lbs.
2800 lbs.
2950 lbs.
2400 lbs.
3000 lbs.
2750 lbs.
3250 lbs.
3525 lbs.

Am I missing something? should I be concerned? Should I request a certain pressure from dive shops?

 

[Boarderguy]

Hydro every 2 years?... that would get very expensive very quick it you have more than a pair of tanks. Though you still save money if you dive even semi often.

Jam those tanks to 3500 psi and enjoy a longer dive. If you don't like then request an expedited fill and remind the attendant about YOUR desired fill pressure.

Don't forget to never drive a Uhaul faster than 55. It says right there in the truck (and trailer) SPEED LIMIT 55.

 

[jborg]

I wonder if the overfill principle applies to all steel tanks? That is, if it's praxis to overfill a 2600 psi tank to 3525 psi, why don't we always overfill 232 bar tanks to 3525 / 2600 * 232 = 314 bar over here and skip the whole concept of 300 bar tanks? My 300 bar tanks then become 405 bar (5875 psi) tanks in the new scheme!

Is the thing here that US "LP" tanks are really just rebranded 232 bar tanks and that's why it works? Or is the thing that the safety margin mandated by the regulations is ridiculous and should be reduced? I mean, if the 2400 psi tank is really a 3600 psi tank, then obviously one could make an actual 2400 psi tank lighter and cheaper? (Though I don't understand why we'd want such a low pressure tank, but apparently it's a thing.)

 

[DogDiver]

 

[Jhnvan]

If you are in Canada, your service pressure is 2640. The + is irrelevant here. It gets tested to that pressure for its whole life. In the states is where the + matters in this case

 

[goldfishtornado]

mandated by the regulations

I think 49 CFR Subpart C covers it, §178.37 is where steel cylinders starts.

ridiculous and should be reduced?

I think the reg covers all DOT rated steel cylinders, it's possible an industrial cylinder really might hit the fatigue life limit, depending on applications. There might be a process to get a special waiver from the DOT, kind of like a supplemental type certificate for the FAA. If anyone is ever brave enough to try, please post with results.

 

[rob.mwpropane]

I am both an engineer and a firefighter. Heating up steel makes it significantly LESS resistant to stress.
"brittle" is a concept related to fragile fracture, not giving up due to excessive stress.
Tank explode due to fragile fracture if hit by a projectile, or falling on an hard surface from considerable height. The fracture in this case happens independently from the filling pressure: a tank hit by a projectile is cracking also if empty.
Instead they explode when overfilled due to giving up to excessive stress., which exceed the tensile strength of the material
Steel is a material with extensive resilience to being over-stressed, so overfilling a steel tank is not so dangerous as overfilling an Alu tank.
Here in Italy, where Faber tanks are manufactured, you cannot buy anything rated for less than 232 bars nowadays.
I do not really understand why one should buy a new steel tank rated for less than this (which means 3365 PSI). Overfilling such a tank to 3500-3600 PSI is absolutely safe, provided that it passed hydro at the prescribed intervals.
Here we give substantially no value to VIP.
I think that is something providing a false sense of safety, which means that in reality it is dangerous, exactly as the burst discs.
Here an hydro is due on a new tank after 4 years, and then at 2 years intervals.

I was saying the heat from filling... pressurizing a tank I don't think is what I would consider "hot".... that's all. Agreed on all the other points.

 

[grantctobin]

View attachment 742774

Except that ignores gas compressibility…

 

[rob.mwpropane]

Except that ignores gas compressibility…

Close enough for planning purposes though, yeah?

 

[grantctobin]

Close enough for planning purposes though, yeah?

Depends on the mix and pressure, but 5-15%. Whether that’s relevant to you for your planning is a you question.

 

[rob.mwpropane]

Depends on the mix and pressure, but 5-15%. Whether that’s relevant to you for your planning is a you question.

I understand the math isn't 100% accurate, but when I plan I use a SAC a good bit higher than average... in the grand scheme of things in the real world I'm not sure it matters.

So with the z factor what would an LP 85 filled to 3442 actually have?