Man, I'm glad I have my own compressor and stick. 
LP vs HP question
- Thread starter rds912
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Agent 47
Contributor
I don't know if anyone has posted it yet, since I didn't read this whole thread, but when it comes to comparing steel tanks, this is a really good read:
http://www.pstscuba.com/PST Scuba Product Specifications.pdf
Note the "tank model" and "old part #" columns. It'll make some of the conversations you'll inevitably have with uninformed lds salesmen nothing short of hilarious. Of course, it only gives info. for PST cylinders, but it's still a nice reference.
Want to know the exact capacity of an LP95 or LP104 "overfilled" to 3442 psi? It's on there. The changes in pressure ratings came about due to changes in DOT regulations for steel cylinders, which now allow an exemption for scuba tanks, btw.
http://www.pstscuba.com/PST Scuba Product Specifications.pdf
Note the "tank model" and "old part #" columns. It'll make some of the conversations you'll inevitably have with uninformed lds salesmen nothing short of hilarious. Of course, it only gives info. for PST cylinders, but it's still a nice reference.
Want to know the exact capacity of an LP95 or LP104 "overfilled" to 3442 psi?
It's on there. The changes in pressure ratings came about due to changes in DOT regulations for steel cylinders, which now allow an exemption for scuba tanks, btw.I don't know if anyone has posted it yet, since I didn't read this whole thread, but when it comes to comparing steel tanks, this is a really good read:
http://www.pstscuba.com/PST Scuba Product Specifications.pdf
Note the "tank model" and "old part #" columns. It'll make some of the conversations you'll inevitably have with uninformed lds salesmen nothing short of hilarious. Of course, it only gives info. for PST cylinders, but it's still a nice reference.
Want to know the exact capacity of an LP95 or LP104 "overfilled" to 3442 psi?
Changes in pressure ratings DID NOT came about due to changes in DOT regulations for steel cylinders, the exempt tanks are made to different requirements with stronger materials. LP tanks are made to a rather old generic standard while the the high pressure tanks are made to specific engineered DOT approved specs.
Agent 47
Contributor
Hi Henry,
The exact same tank may have a different pressure rating in europe than in the US because of different safety factors.
In the US, higher pressure tanks are made to different specs with DIFFERENT MATERIAL that is stronger than the material used in lower pressure tanks. The higher pressure tanks are not just LP tanks allowed to be run at higher pressure because of a change in DOT regulations. They are different tanks.
DOT has a generic spec for LP tanks (3A and 3AA) that most LP tanks are made to. Higher pressure tanks (or even LP tanks that are made with thinner than normal walls) are made to Special Permits or Exemptions that are agreed upon specs between the DOT and specific manufacturers for specific tank designs. The DOT has many hundreds of the Special Permits and Exemptions that define wall thickness, heat treating, material yield strength, testing, material stress, etc.
BTW, it's probably possible to build an HP tank to DOT 3AA standards but the wall thickness to meet the material stress part of the spec would make the tanks too negativly buoyant for safe scuba use.
The exact same tank may have a different pressure rating in europe than in the US because of different safety factors.
In the US, higher pressure tanks are made to different specs with DIFFERENT MATERIAL that is stronger than the material used in lower pressure tanks. The higher pressure tanks are not just LP tanks allowed to be run at higher pressure because of a change in DOT regulations. They are different tanks.
DOT has a generic spec for LP tanks (3A and 3AA) that most LP tanks are made to. Higher pressure tanks (or even LP tanks that are made with thinner than normal walls) are made to Special Permits or Exemptions that are agreed upon specs between the DOT and specific manufacturers for specific tank designs. The DOT has many hundreds of the Special Permits and Exemptions that define wall thickness, heat treating, material yield strength, testing, material stress, etc.
BTW, it's probably possible to build an HP tank to DOT 3AA standards but the wall thickness to meet the material stress part of the spec would make the tanks too negativly buoyant for safe scuba use.
Agent 47
Contributor
There was a time when exemptions didn't exist. They weren't allowed. I don't know how you can argue that the advent of exemptions wasn't a change in regulations. At any rate, the PST E8-119 is the same tank as the old LP95, and the E8-130 is the same tank as the old LP104. The only change in those particular tanks was a change in part (model) numbers when PST acquired an exemption. I have no clue about the situation with any of the other manufacturer's tanks, and I wouldn't be a bit surprised if some of them had to engineer new tanks to be able to qualify for an exemption. As far as scuba cylinder designs vs. usage, PST was way ahead of the game here in the U.S. But then, why would they go to the expense of running a separate line to produce lower quality tanks for the one country who only allowed 2000 psi fills for steel tanks for the longest time?
That time would have been well over 30 years ago. The current 3AL standard for aluminum tanks resulted from the incorporation of several exemption permitted tanks from several companies who made and sold them from the early 70's onward and exemptions were not a new thing then. Exemptions have been around and allowed for a LONG time - longer than all of the current LP 3AA tank designs.
The only recent regulatory change I am aware of is the change in terminology from "Exempt" to "Special Permit" when referring to non 3AA or 3AL certified tanks.
Let's compare them:
The PST E8-119 holds 119 cu ft at 3442 psi, is 24" tall and 8" in diameter and weights 41 lbs empty.
The PST 95 holds 96.6 cu ft at 2640 psi, is 24.75" tall and 8" in dimater and weighs 43 lbs.
More importantly, if you do the math you find the LP 95 holds 126 cu ft at 3442 psi, not 119.
Similarly:
The PST E8-130 holds 130 cu ft at 3442 PSI, is 26.12" tall, 8" in dimater and weighs 43 lbs.
The PST 104 holds 106.2 cu ft at 2640 psi is 26.88" tall, 8" in diamter and weighs 46.4lbs.
And again if you do the math, the LP 104 holds 138 cu ft at 3442 psi, not 130.
If you further crunch the numbers and compare lenght and volume of the respective tanks you find that the slightly longer lenght of both LP tanks only partially accounts for the greater volume the LP tansk have at 3442 psi - which indicates that the LP tanks have to have slightly thinner tank walls to provide the slightly greater internal volume.
I am willing to bet that if you filled a sample of 30 LP 104's with water that the same amount of water would not fit into a sample of 30 E8-130's and that the same results would be obtained with LP95's and E-8-119's.
In addition to tank wall thickness, the E series tanks use steel with a higher tensile stenght giving them the strenght needed to handle the wall stresses involved while operating at a 3442 psi service pressure with a much greater margin of safety than an LP 95 illegally overfilled to the same 3442 psi.
In short, while you would be hard pressed to tell the difference between the LP 95 and E8-119 or the LP 104 and E8-130 blind folded as they are very similar in size and weight, they are by no means the same tank and the LP 95 and LP 104 would not pass the certifcation requirements passed under the special permits used for the E series tanks.
The point that I was trying to make was while many people have said that an HP tank is identical to a LP tank but is rated and stamped for higher pressure because of an "exemption", that is not true, HP are different tanks made from stronger materials.
Since an HP tank of aproximatly the same physical size of a LP tank will hold about the same volume of air at the same pressure, if the price differential wasn't too much, I would always purchase the HP tank for the added versatility rather than overfilling the LPtank and increasing liability and decreasing safety.
Since an HP tank of aproximatly the same physical size of a LP tank will hold about the same volume of air at the same pressure, if the price differential wasn't too much, I would always purchase the HP tank for the added versatility rather than overfilling the LPtank and increasing liability and decreasing safety.
Agent 47
Contributor
Actually PST has said differently. I wish their old site was still up, with the big write-up about exemptions for steel cylinders and that whole bit. Talking about aluminum and steel is apples and oranges. Discussion of what's been allowed for aluminum tanks and when just has no relevency to what's going on with steel.
At any rate, they list exactly the same dimensions and capacities for those tanks I mentioned above, and indeed indicate they are the same tank. If you click on that link I posted, it shows a table of tanks with new and old part numbers. That's the last remaining scrap of real info I could find on their new, partly built website. I should be inclined to take PST's official numbers and explanation over those posted here, however, I know how marketing people are. (Not always all that brilliant, and usually more concerned with something sounding good than having any real truth to it.)
Still, as far as measuring different capacities, you have to take into account the variations in dimensions and wall thickness of two "identical" tanks. Anyone who's tried to put together a set of doubles with mismatched tanks of the same model can tell you, they're not all the same. Manufacturing tolerances are far from perfect.
Also, as with anything, blanket statements are dangerous, and there are almost always exceptions. I'd take it on a tank model by tank model basis. If you try to apply one hard fast rule or opinion to every tank out there across the boards, then 99% of the time you're going to be partly wrong. There are plenty of HP tanks out there, even most the PST ones, that have no relation to any LP tank whatsoever. But...there also happen to be a couple that do.
At any rate, they list exactly the same dimensions and capacities for those tanks I mentioned above, and indeed indicate they are the same tank. If you click on that link I posted, it shows a table of tanks with new and old part numbers. That's the last remaining scrap of real info I could find on their new, partly built website. I should be inclined to take PST's official numbers and explanation over those posted here, however, I know how marketing people are. (Not always all that brilliant, and usually more concerned with something sounding good than having any real truth to it.)
Still, as far as measuring different capacities, you have to take into account the variations in dimensions and wall thickness of two "identical" tanks. Anyone who's tried to put together a set of doubles with mismatched tanks of the same model can tell you, they're not all the same. Manufacturing tolerances are far from perfect.
Also, as with anything, blanket statements are dangerous, and there are almost always exceptions. I'd take it on a tank model by tank model basis. If you try to apply one hard fast rule or opinion to every tank out there across the boards, then 99% of the time you're going to be partly wrong. There are plenty of HP tanks out there, even most the PST ones, that have no relation to any LP tank whatsoever. But...there also happen to be a couple that do.
Agent 47
Contributor
One more thing, and I shouldn't bring it up, since I'm not really very well schooled in this area.....but when calculating tank capacities at higher pressure, make sure to use formulas based on the real gas laws. Some of the math most peeps use for lower pressures will give you erroneous results when you're working with higher pressures. As it was explained to me, the relationship of compression of gases to pressure isn't linear, and when you get into high pressure fills the resulting error in calculated gas volume can be substantial if the correct formula isn't used. At any rate, I haven't worked through the math, so I'm taking a friend of mine's word for it.
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