"Cave Fills"

[DA Aquamaster]

Ok, but what is the test pressure used to maintain a "+" rating?

Tobin

The test pressure is the same, the difference is in the amount of elastic expansion that is allowed and an additional calculation used to determine it's within the lower limit.

This additional calculation, and the need to have a specified/documented REE limit to perform it, are the two major reasons some RINs won't do plus ratings. They either can't find or won't look up the required documentation, or they won't (or in some cases just don't know how to) perform the additional calculation required.

 

[DA Aquamaster]

Are tank manufacturers really making different tank for the US and EU? I would think the best business model would be to make one size tank and change the stamp to satisfy the regulations for where ever they are going, rather than make two very similar tanks. Build one tank that will satisfy both governments and stamp appropriately.

I think there are a few cases where this may occur, but I suspect it's a case of an existing tank also meeting another certification standard and being able to also be approved by that agency with no redesign required.

Faber has a data base (it's protected and you have to apply for permission to access it) and if you search by internal tank volume you'll find a few tanks that have the same size, weight, service pressure, etc, but are listed as being certified to different standards.

Much more commonly however, you'll find numerous tanks in the data base that have the same internal volume but are slightly different in external diameter and different in weight, suggesting differences in wall thickness needed to meet different certification/approval standards and criteria.

All in all, you'll find a lot more variation in similar looking tanks than you'd expect, and that suggests that tanks are most often specifically designed to meet specific approval standards.

 

[cool_hardware52]

Are tank manufacturers really making different tank for the US and EU? I would think the best business model would be to make one size tank and change the stamp to satisfy the regulations for where ever they are going, rather than make two very similar tanks. Build one tank that will satisfy both governments and stamp appropriately.

Tanks are made in fairly large batches. The regulations require samples to be subjected to destructive testing. In short making tanks is a big deal. While the basic process is the same, the parameters can be adjusted for specific designs. Changing the specifics from batch to batch is trivial if you are using the same presses, heat treat ovens etc. I have no doubt that the cylinders for the Euro market are different from the tanks sold in North America.

Tobin

 

[KenGordon]

I have both 300 bar and 232 bar 12.2l Fabers. They are quite different. The 300 bar one is slightly taller and much heavier.

There also 232 bar cylinders known as euro cyclinders. Those are the favourite for twinsets as they are heavier than Fabers and available with concave bottoms so can stand up without boots. They are heavier than a 232 bar Faber but lighter than a 300 bar one.

Do any metalogy experts know the impact of the temperature on the robustness of the cylinder? A hydro test is more or less ambient temperature whereas a fill is usually somewhat warmer.

 

[Germie]

Are tank manufacturers really making different tank for the US and EU? I would think the best business model would be to make one size tank and change the stamp to satisfy the regulations for where ever they are going, rather than make two very similar tanks. Build one tank that will satisfy both governments and stamp appropriately.

In Europe we have on ali cylinders for the valve M25X2 thread. In America G3/4 is commonly used (lpt).
Because of some accidents where people tried to put an M25 valve in a G3/4 cylinder (yes it looks like to fit but does not fit), some fillingstations have problems with G3/4 cylinders and don't want to fill. In european cylinders there is 'ce' in it instead of 'dot'. In earlier years it was possible to get dot cylinders tested in the Netherlands. And once tested there is no problem with a retest. But if you now buy dot cylinders and want to get them in test that is expensive (almost impossible), and shops don't sell cylinders without 'CE' anymore. I have 2 older ali80's (1995) that have now the dutch lion stamped in the cylinder, so I can bring them for a retest if they are out of test. Last test they stamped a big 'G3/4' in it to assure that there is no monkey that put an M25 valve in it.
But if I buy a new ali cylinder I will get the M25 thread. In the Netherlands you have to do the hydrostatic test every 5 years. With new cylinders 5 years after production date. In Germany it is with new cylinders 5 years after buying (you will get a cert with the cylinder to proof it was not used before you bought it). In France the hydrostatic test is every 3 years and so all countries differ from each other. For the same cylinders. We have 1 Euro coin, but all countries have different rules.
There is another thread on old cylinders, but that is not used a lot anymore (small conic). M18 is a thread that can be used too, but I have only seen it on rebreather cylinders.

Most common are here 232 bar steel cylinders. With of course the M25X2 thead.
I have 12L Fabers, Worthinton and Eurocylinders, all 232 bar. All cylinders have a little bit different weighting.

Carbon cylinders have a limited lifetime of 15 years after production date.

We don't have burstdisks.

There are 300 bar steel tanks, but they are most times too heavy. And the 300 bar fills are not possible everywhere.

 

[Charles2]

Ok, but what is the test pressure used to maintain a "+" rating?

The test pressure is the same. The difference is that the "+" rating is based on a little closer evaluation of the cylinder and arithmetic is involved.

A DOT 3AA scuba cylinder may be filled with a compressed gas to a pressure 10 percent in excess of its marked service pressure, provided:

1. The cylinder is equipped with a frangible disc pressure relief device (without fusible metal backing) having a bursting pressure not exceeding the minimum prescribed test pressure.
2. The cylinder's elastic expansion was determined at the time of the last test or retest by the water jacket method.
3. Either the average wall stress does not exceed 67,000 psi or the maximum wall stress does not exceed 73,000 psi.

The average wall stress must be computed from the elastic expansion data using the following formula:
S = 1.7EE / KV−0.4P
Where:

• S = wall stress, pounds per square inch;
• EE = elastic expansion (total less permanent) in cubic centimeters;
• K = factor × 10 −7 experimentally determined for the particular type of cylinder being tested or derived in accordance with CGA C-5
• V = internal volume in cubic centimeter (1 cubic inch = 16.387 cubic centimeters);
• P = test pressure, pounds per square inch.

The maximum wall stress must be computed from the formula:
S = (P(1.3D2 + 0.4d2)) / (D2−d2)
Where:

• S = wall stress, pounds per square inch;
• P = test pressure, pounds per square inch;
• D = outside diameter, inches;
• d = D-2t, where t = minimum wall thickness determined by a suitable method.

I suspect that each manufacturer has a chart for each cylinder affected. Whether or not the tester has those charts available is another question.

All this makes my brain hurt.

 

[cool_hardware52]

[QUOTE="Charles2, post: 7608461, member: 409929"
The maximum wall stress must be computed from the formula:
S = (P(1.3D2 + 0.4d2)) / (D2−d2)
Where:

• S = wall stress, pounds per square inch;
• P = test pressure, pounds per square inch;
• D = outside diameter, inches;
• d = D-2t, where t = minimum wall thickness determined by a suitable method.

I suspect that each manufacturer has a chart for each cylinder affected. Whether or not the tester has those charts available is another question.

All this makes my brain hurt.[/QUOTE]

While the math is no doubt a hurdle for many retesters, it is the requirement to measure the actual wall thickness that makes getting a + rating on any cylinder lacking a marked REE nearly impossible. The only practical method is via ultransonics and the instruments have been expensive and problematic. I expect these Ultrasonic Thickness Gauges to continue to come down in price, but I don't expect retesters to buy them, or train their staff to use them in order to be able to test a dwindling population of tanks with out marked REE data.

Tobin

 

[packrat12]

Should have been around when Steamboat in Branford had chill fills. The vat was chilled to 40 degrees, and you would get big fill,but those tanks left in the sun would blow burst disks or extrude O-rings.

Wow that was a memory. I used Branford Dive Center. It was the only time I burst a disk... Cold fill, hot summer and pizza at the dinner just down the street. We also had to regularly bleed some air off as it was just too high. These were AL80s too.

I also remember the taste of the air. It was sort of oily and I believe it was because the Suwannee would flood regularly and he had to clean the compressors and tanks afterwards. I always could tell when my air was last filled there.

 

[DA Aquamaster]

Tanks are made in fairly large batches. The regulations require samples to be subjected to destructive testing.

The DOT requires 1 in every 200 tanks to be tested to destruction, including pressure tests, crush tests and a tensile strength test using a specifically sized and shaped coupon cut from the side wall.

 

[cool_hardware52]

Do any metalogy experts know the impact of the temperature on the robustness of the cylinder? A hydro test is more or less ambient temperature whereas a fill is usually somewhat warmer.

I don't qualify as an expert in metallurgy, but I routinely heat treat and temper steel, including P20 which is a 4130-4135 Chrome Moly often used for injection molds. Very similar to the steel alloys used for at least some scuba tanks.

In short to harden steel you need to heat it to the critical temperature, ~1250-1450F and then quench it or chill it rapidly. "As Quenched" will be as hard, and as brittle as that alloy can be made. After quenching the material is again heated to a temperature well below the critical temperature and cooled slowly. This is known as tempering or drawing. The higher the tempering temperature the "softer" the result will be. Tempering essentially trades hardness for toughness, meaning less brittle, and more able to withstand loads with out rupture. If the Tempering temp is high enough, i.e the critical temperature, you will anneal or return the material to the soft state.

Ok, what does this mean for scuba tanks and "hot fills"?

Almost nothing. The lower end of the tempering range starts about 400F Heating to temps below this has no effect. Pretty hard to get your tanks to 400F at the fill station without building a fire under them.

BTW various alloys do funny things. If you want to anneal steel, heat it up to the critical temperature, shut off the oven and let it cool over night. If you want to anneal copper heat it up and drop it in a bucket of water......

Tobin