LP vs HP question

[Agent 47]

You know, it really doesn't matter one way or the other though. My thought was just that I hear so many arguments about how the LP95 is such a better tank than the HP tanks because you can use it at low pressure and have 95 cu ft of gas, or jam it and have more volume like an HP tank, which is just a friggin' silly argument to begin with. It's flawed in so many ways. Whether you decide to agree that it's the same as the hp119 as the chart says, or maintain that it's not, the difference in gas volume between an overfilled lp95 (to 3442 psi) and an hp119 (if there is any difference) or an hp120 isn't much. I've actually had an lds salesmen try to discourage me from buying hp tanks based on this argument, and have seen a couple of threads online where peeps have posted similar arguments that the lp tank is more versatile. It always blows my mind to hear that. I usually don't bother to pursue the discussion any further. I guess it probably comes down to slick salesmen wanting to get rid of old tanks?

 

[DA Aquamaster]

I agree with you that tanks vary - which is why I suggested you'd have to test water capacity with a sample of 30 tanks of each model to be sure of accurately accounting for normal tank variance.

I also agree with you that the "LP tank is a better way to go as you can always get a good fill" argument is sewriously flawed as tank for tank, size for size, weigth for weight an HP tank of the same size and weight as an LP tank will hold basically the same gas at the same pressure - it just thas the additional advantage of being able to be legally filled to 3442 psi when and where available.

 

[SparticleBrane]

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.)

The newer exemption/special-permit tanks and the LP tanks are NOT the same tank. They are made from a different, higher-strength alloy!! I have seen the PST thing you're talking about and it is showing that they have a new line of tanks that are essentially equivalent in size...it mentions nothing about alloys.


From an older post on this same exact topic (with almost the same thread name)...

I spoke with our engineers today and found that DOT 3AA cylinders have a typical tensile strength of 105,000-120,000 psi and Worthington's tensile strength on our X-Series (E-14157) is 135,000-155,000 psi with nominal at 145,000 psi which they tell me they hit "pretty well on the money" every time.

This demonstrates that a high pressure E cylinders is at least 30% higher tensile than 3AA cylinders.

Please pass this information on and do everything you can to stop the rumor that HP and LP cylinders are the same except for the markings.

Dive safe

 

[Luis H]

From an older post on this same exact topic (with almost the same thread name)...

Originally Posted by Leadking
I spoke with our engineers today and found that DOT 3AA cylinders have a typical tensile strength of 105,000-120,000 psi and Worthington's tensile strength on our X-Series (E-14157) is 135,000-155,000 psi with nominal at 145,000 psi which they tell me they hit "pretty well on the money" every time.

This demonstrates that a high pressure E cylinders is at least 30% higher tensile than 3AA cylinders.

Please pass this information on and do everything you can to stop the rumor that HP and LP cylinders are the same except for the markings.

Dive safe

The most common steel alloy used under the DOT 3AA Codes of Federal Regulation (CFR49 paragraph178.37) is ASTM 4130. The ultimate (and yield) tensile strength properties of ASTM 4130 can be intentionally changed with heat treatment, but the numbers you are quoting fall in a very reasonable range.

I don’t know what alloy is being used for the “special permit” steel tanks, but a few years ago I was talking to the head engineer at PST and I got the impression it was a proprietary alloy. At least he wasn’t sharing much details of the alloy, which I understand. I was mostly talking to him about 3AA cylinders, but we did talk a bit about their newest tanks at the time.

Could it be that the special permit alloy is chemically similar to ASTM 4130, but heavily heat treaded…it is possible, but IMO kind of unlikely. I am mechanical and a structural engineer, not a metallurgist, so I don’t feel fully qualified to explain the reason for my opinion, let’s just call it an educated gut feeling.

In any case even if the difference is mostly on the heat treatment process, that still means it has totally different properties.

My impression is that they would not go through the expense of getting a special permit unless they were manufacturing something different.

Many of the dimensions could very easily be the same (within tolerance) on some different tanks since they would reuse the same tooling for forming the cylinders.

 

[CompuDude]

I also agree with you that the "LP tank is a better way to go as you can always get a good fill" argument is sewriously flawed as tank for tank, size for size, weigth for weight an HP tank of the same size and weight as an LP tank will hold basically the same gas at the same pressure - it just thas the additional advantage of being able to be legally filled to 3442 psi when and where available.

I can't tell you how much I agree with you. Drives me nuts to hear people saying they want to buy an LP tank since they can get "full fills" and not "short fills" in an HP tank.

You get what you get. Unless you're in cave country, the vast majority of shops are going to stop filling your LP tank at 2640 (or even 2400 if they notice the plus is gone!), while an HP tank, even with a terrible fill, will still generally get 3200 psi or so.

When you get 3200 psi in an HP tank, you can legitimately complain. If you get 3200 psi in an LP tank, you had better slink away without saying a word, else you risk getting someone in trouble.

Why on earth is better to pray for an overfill in an LP tank than accept an underfill in an HP tank, when both give you same amount of gas but one is perfectly legal?

There IS a small cost consideration. LP tanks are less expensive than HP tanks. But I was somewhat surprised to note, recently, that the difference is a LOT smaller than it used to be: In the neighborhood of $40, comparing LP95 prices to HP119 prices at one major online vendor. In my mind, that $40 buys the peace of mind that some fill monkey isn't going to cut me off at 95cf (or less, if no plus rating), based on my tank's rating. I may not get a full 119, but I'm sure as heck going to get more than 95cf... every time.

 

[WD8CDH]

Luis,

You are correct, some different pressure tanks are are made on the same tooling. Also, some different capacities are made on the same tooling with just the lengths changed.

 

[mpgunner]

What about the impact on the 1st stage? Wouldn't using LP tanks be much easier on your reg?

 

[CompuDude]

What about the impact on the 1st stage? Wouldn't using LP tanks be much easier on your reg?

Did I miss a slew of reports about modern regs failing early from being used on HP tanks?

 

[DA Aquamaster]

The Scubapro Mk 10 was known to have some problems at pressures over 3000-3300 psi with pinching of the high pressure piston stem o-ring in cases where the tolerances between piston stem and regulator body were excessive due to either excessive wear or due to normal production tolerances in each part being at opposite extremes. But SP has since switched o-ring materials and I don't think it is as large a problem as it used to be - at least I have not seen a Mk 10 with indication of HP o-ring pinching in the last few years. Even if it occurred, the "failure" was just a small stream of bubbles leaking out the ambient chamber rather than catastrophic gas loss or a failure to deliver air.

Other "older" designs by other companies may or may not be well adapted to use at high pressure, but I think most recently produced regs have been designed to accommodate the reality that users may wish to put them on tanks with 3500 psi pressures in the US and on tanks up to 4350 psi (300 bar) in Europe.