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?

 

[Angelo Farina]

Thanks for that. Iteratively evaluating using the natural gas Z curves, 10 liter cylinder, 250 bar initial pressure, and 21 C (70 F) initial temperature, I can confirm the temperature required to reach 400 bar is 83 C (181 F) -- so not totally impossible.

If a similar LP tank of air was cave-filled (250 bar/3600 psi) and heated to the same temperature, the pressure would rise to 329 bar/4770 psi (considering compressibility), which is about 8% over hydro pressure. Good thing I use galvanized LP85s and not black ones.
Angelo, thank you for persisting. The difference when using a real gas is more than I thought it would be.

I juts reported the results of an investigation conducted by a fellow professor, it was not my work...
He just gave me the document to review for avoiding gross errors, as this was a big legal dispute and he wanted to be sure that his conclusion could not be wrong.
The point is that the tank actually exploded. A manufacturing fault could not be entirely excluded, as the other three tanks at exactly the same pressure and temperature did not explode. But the electronic microscope analysis of the fracture surface did not reveal any evident fault in the metal structure.

 

[Superlyte27]

Thanks for that. Iteratively evaluating using the natural gas Z curves, 10 liter cylinder, 250 bar initial pressure, and 21 C (70 F) initial temperature, I can confirm the temperature required to reach 400 bar is 83 C (181 F) -- so not totally impossible.

If a similar LP tank of air was cave-filled (250 bar/3600 psi) and heated to the same temperature, the pressure would rise to 329 bar/4770 psi (considering compressibility), which is about 8% over hydro pressure. Good thing I use galvanized LP85s and not black ones.
Angelo, thank you for persisting. The difference when using a real gas is more than I thought it would be.

I can confirm that an lp tank will not blow anywhere remotely close to 329 bar.

 

[BlueTrin]

OP, never get a fill in north central Florida...........you'll have a panic attack.

Here it is quite hard to get a fill close to 232 bars

 

[Rose Robinson]

Hi Rose,

You are right, following those instructions are a nearly foolproof way of ensuring that the fill station operator is not hurt or killed. I would liken it to the food service regulations, where anything between 40F and 140F (5C and 60C) is required to be discarded after two hours. There are plenty of commercial kitchens where this rule is followed (more or less), and it's true that I would worry about eating in one that routinely flouted it.

However, when folks are knowledgeable about when it is safe to break the rules, I also think the marginal risk is small. Sous vide steaks are cooked to a temperature in the "danger zone" and held there, for instance. My guess is an actuary and metallurgist who sat down would find that the fill station operator would be better off overall if they overfilled the tanks but drove 1 mph less getting to work. In any case, I appreciate your sensible perspective. I bet you sleep well at night.

Hi wnissen,

I do not believe, you can use the words safe, and break the rules in the same sentence.

I'm a little reluctant to compare an improperly cooked steak, which may end up giving you ''Montezuma's Revenge'', to an overfill explosion, that could level the building, and kill everyone in it.

I am a Funeral Director, owner/operator, overseeing the day to day activities of four separate locations.
The health and safety of all my employees, including contractors is paramount, I cannot emphasize that strongly enough. My employees are my ''family''

Breaking of industry standards, Legislative requirements, manufacturers instructions is not allowed/tolerated, under any circumstances.

What's the name and address of the restaurant you cook in, where tainted food is served?

By the way, very well, snug as a bug in a rug!

LOL,

Rose

 

[Rose Robinson]

This is a perfectly acceptable approach. No issue whatsoever following this appraoch, apart from needing to instruct just about every fill station operator at every fill to not overfill. The rest of us with carry on doing what we have done for decades without issue.

Hi Divin'

Which does not make it right.

Rose

 

[Manatee Diver]

to an overfill explosion, that could level the building, and kill everyone in it.

 

[Boarderguy]

I'm diving after work today and checked my tanks before I loaded them. In my garage at 0415 the hp 119 cf was sitting right at 3750 and the hp 80 was 3700... THANK YOU for giving me a few extra minutes off each tank. I'd much rather have my tanks at 200-300 over pressure and still under max load than the same under working pressure. You aren't going to hurt a steel tank with these types of fills.

 

[Bearded Diver]

My only old-school 72 was manufactured in 1970. (I traded a still-new Al 80 for it ~2000.) It is my favorite cylinder for some types of diving. Each time I have had it hydroed, I have requested and received a "+" hydro. My most recent "+" hydro was a couple of months ago.

rx7diver

Do you know the manufacturer of the tank?

 

[rx7diver]

Do you know the manufacturer of the tank?

My 72 is stamped "USD" and "U.S. Divers" (the old name of Aqua Lung; google says the name change happened in 2003). I am almost certain (99.9% certain) that it was manufactured by Pressed Steel Tank Company (PST).

My university open water scuba course (NAUI/YMCA, in 1986) used these cylinders in class--except, some of those were already very old at that time. They were/are owned by the university. Last year, my daughter took the same course, which continues to use these very same cylinders!

rx7diver

 

[NDonahue]

Considering the risk of running out of gas

There is no safety in an overfill. Show me a diver who gets a 3k fill and still calculates the turn based on 2400. More likely, that extra gas is used for deeper penetrations using the same gas rules.

A diver who is conservative enough to preserve the excess gas as a safety margin is likely also the dive to instruct shop not to do overfill.