LP 95 vs. HP 100

[PR_SMITH]

You do realize that the LP series tanks are in fact 3AA tanks, right? And the high pressure tanks that everyone references are exemption tanks and not "up to" 3AA standards, right? Look at the hydro pressures on the exemption tanks and the fact that none of them are "+" rated, but almost all of the LP tanks are.


Oh, and by the way, destructive testing is a very valid way to test mass produced products, including scuba tanks. Random sampling is used in damn near every line of manufacturing and "one of one" is in fact enough data for limited production runs to validate the engineering and materials on various products.

So you are saying that if a manufacturer produces a batch of say, 4.000 tanks, then testing one of them makes them all OK


Sure, I am aware that LP tanks are 3AA. I own 2x LP72's, and 2x LP98's. Not to mention, Oxy-Acetelyne, C02 and argon bottles.


Having previously been employed in the quality control field, I would agree that destructive testing is a valid way to test mass-produced products. However, I would argue that acceptance sampling,(used when a decision must be made to accept or reject a group of parts or items based on the quality found in a sample) which is a 1 0f 1 sample, is OK for Frisbee's but not for pressure vessels. Statistical quality control, is the preferred method for determining failure/quality levels.

Statistical quality control refers to the use of statistical methods in the monitoring and maintaining of the quality of products and services. Statistical process control, uses sample sizes based on the number of items in a batch, to determine the quality of that batch(not of all items of type X) ....... for the full story see Statistical process control - Wikipedia, the free encyclopedia

 

[spectrum]

However, if I'm doing a bigger dive, I'll ask for the max, which at most shops is 3900.

Only in my dreams, you folks dive in another world of fill station behavior, wow.

 

[marchand]

Thanks for the interesting link.

However, there are some caveats here:

1. The tank was of the DOT 3AA style, but was NOT a scuba tank.

2. The valve was not a scuba type valve, and therefore not suitable for direct comparison.

3. By virtue of not being a scuba tank, it therefore has different dimensions, and as such, may exhibit different burst limits.

4. This test was one of one. Insufficient data to obtain any "norm" from.

5. This test result could be seen as "permission" to fill to extreme pressures, say above 4000psi.


This is just the opinion of this amateur engineer.

1. The tank was a 3AA tank meaning it is made to the same specifications as all other 3AA tanks. SCUBA or not it doesn't matter.

2. The valve is the strongest part of the tank, it doesn't matter if it is a SCUBA valve or not. I believe SCUBA valves are good to something like 10,000psi.

3. Again, a 3AA tank is a 3AA tank; same specifications.

4. Yes, only one tank was tested, and because the CFR's say that the manufacturers have to test one out of every 200 tanks it is pretty safe to say that this particular tank is pretty darn similar to all the other ones like it. Therefore I think it safe to say that since the tank failed at 3.4 times its working pressure all of the other tanks like it wont fail at 2 times their working pressure.

5. I take it as an example that the 3AA tanks are a lot sturdier than people think.

 

[WD8CDH]

You do realize that the LP series tanks are in fact 3AA tanks, right? And the high pressure tanks that everyone references are exemption tanks and not "up to" 3AA standards, right? Look at the hydro pressures on the exemption tanks and the fact that none of them are "+" rated, but almost all of the LP tanks are.

Oh, and by the way, destructive testing is a very valid way to test mass produced products, including scuba tanks. Random sampling is used in damn near every line of manufacturing and "one of one" is in fact enough data for limited production runs to validate the engineering and materials on various products.

Most exemptions are to HIGHER standards than 3AA, not lower. Also, they are usually required to be hydro'd to about the same test pressure ratio as 3AA tanks at their +10% fill pressure so they have about the same safety factor. They basically include the 10% in their stamped service pressure. Usually, several tanks from each batch must be tested by several methods to distruction. Often there are 1 or more semi-distructive cycling tests required on a few tanks from each batch too. The batch size is usually limited to 200 tanks and must be from the same batch of metal which is also tested. And finally, 100% of the remaining tanks must be tested by specified non-destructive tests.

Also, exemption tanks have to be made with specific defined by the exemption material which is usually to a much higher spec than 3AA tanks.

So, basically HP exemption (or Special Permit) tanks are made from better materials to a higher spec and the exemption allows those specific higher quality tanks to be stamped and used at higher service pressures.

By the way, there are a few 3AA HP tanks but they are far heavier than Exemption HP tanks of the same capacity and pressure because they are made from inferior steel and therefore have to have thicker walls.

Some Beuchat and Heiser HP tanks were the heavy 3AA design.

 

[ucfdiver]

3. Cave divers are exceptionally safe and many will have a fit over a thing like a 2" fastex quick release buckle. But in stark contrast, the community as a whole seems to take a total departure from common sense on the issue of tank fills in the quest for a little more gas to allow a bit more penetration. As a result they invite a whole cascade of potential failure issues (tank, burst disc, regulator, HP hoses, etc) that are 100% preventable as they have the option of just using a larger set of doubles and/or stage bottles.

I can't quite decide if number 3 is due to a collective ego thing, huberis, ignorance, or the fact that it has not yet shown up in accident analysis as a leading cause of death in cave diving - although I'd love to do statistical research on reg and burst disc failures to see if their is a correlation between fill pressure and risk of failure.

If anyone can enlighten me with a solid argument, other than the seriously flawed and rather ignorant "it has not happened - yet" argument, why any cave diver should be ok with a fill to a tank's test limit, despite the potential risks and consequences, I am all ears.

1- HP hoses are rated to ( I believe) 5000psi, 25% over even a 4k psi fill.
2- There's no more stress on a regulator with a LP tank at 3500/3600psi than a HP tank.
3- Most burst disk these days are available in the 5k psi range, or just plug them.
4- Didn't OMS state their tanks could withstand 10,000 fills to 4000psi? I know it was something around those lines....
5- "It hasn't happened yet" is a pretty solid argument when you have almost 20 years of overfilling without incident.

But.............if you don't feel comfortable with all that, don't dive them by any means!

 

[dannobee]

Most exemptions are to HIGHER standards than 3AA, not lower.

I was referring to the 5/3 hydroing of 3AA tanks vs. the 1.5 x WP of Exemption tanks. And if you need an example, remember the PST tanks with the hocus pocus that the hydro operator had to perform in order for the tanks to pass. That wouldn't fly if it were a 3AA tank.

 

[D_B]

3900psi in a 2600psi tank?

cool! .... I'm going to my LDS to see if I can get 1300psi overfill on my HP100

 

[Luis H]

I was referring to the 5/3 hydroing of 3AA tanks vs. the 1.5 x WP of Exemption tanks. And if you need an example, remember the PST tanks with the hocus pocus that the hydro operator had to perform in order for the tanks to pass. That wouldn't fly if it were a 3AA tank.

The pre test procedure applies to all PST tanks including all 3AA and it is also recommended for all steel tanks.

You may look at the numbers the Ron presented for testing.
For example:
A 2400 tank is tested at 5/3 or 4000 psi.

The working pressure of that tank is 2640 (2400 + 10%).

If you multiply 2640 times 1.5 you get 3940 psi. Which is fairly close to testing pressure. Similar testing pressure as a special permit tank.


Now I do have a real issue to calling a higher strength material as higher quality or lower strength inferior quality. That is incorrect…you don’t get any material properties without giving up something. It looks like rust resistance is much better on the 3AA cylinders, but time will tell.
Added: Also higher strength steels tend not to be as ductile.

Ron
Do you have web links or documents that talk about the special permit tanks?
Is there anything about them on CFR49?

 

[PR_SMITH]

1. The tank was a 3AA tank meaning it is made to the same specifications as all other 3AA tanks. SCUBA or not it doesn't matter.

2. The valve is the strongest part of the tank, it doesn't matter if it is a SCUBA valve or not. I believe SCUBA valves are good to something like 10,000psi.

OK

3. Again, a 3AA tank is a 3AA tank; same specifications.

So despite different dimensions, it's still structurally the same?

4. Yes, only one tank was tested, and because the CFR's say that the manufacturers have to test one out of every 200 tanks it is pretty safe to say that this particular tank is pretty darn similar to all the other ones like it. Therefore I think it safe to say that since the tank failed at 3.4 times its working pressure all of the other tanks like it wont fail at 2 times their working pressure.

Fair enough.


5. I take it as an example that the 3AA tanks are a lot sturdier than people think.

5. I agree with you on this one; my intention was to point out that just because the tank withstood
>7000psi, that one shouldn't go anywhere near that limit when filling. I wasn't saying that you didn't know enough not to approach the burst pressure when filling.

 

[dannobee]

Now I do have a real issue to calling a higher strength material as higher quality or lower strength inferior quality. That is incorrect…you don’t get any material properties without giving up something. It looks like rust resistance is much better on the 3AA cylinders, but time will tell.

Well Said, Luis.