Difference between LP & HP Steel Tank

[Catt99]

I was talking with a Scuba instructor who used to have a dive shop and I was asking him about steel tanks. He recommended a Faber 108 low pressure tank. He said that is what I want if I wanted "more air".

What's the difference between HP & LP? Does the LP hold more volume of air?
Thanks.

You've gotten some incredibly detailed responses -- including many addressing a discussion topic that gets a lot of attention ("prudent fill pressure on LP tanks?").

To answer your fundamental question: "LP" is shorthand for "low pressure" and generally means the tank was designed and manufactured for an expected fill pressure of 2400 psi (in many cases with a +10% rating for 2640 psi). "HP" is shorthand for "high pressure" and generally means the tank was designed for an expected fill pressure of 3442 psi or 3500 psi.

The rated fill pressure is used in determining how much air a tank holds, at least for its "name" -- tanks generally are referred to in the US by the cubic feet of air they hold at rated pressure, so an "LP 108" would be expected to holdf 108 cubic feet of air (compared to the widely rented aluminum 80s or AL80 that holds "80" cubic feet of air -- in fact closer to 77 cu ft but that's another topic). So, an HP tank with a certain physical size that is filled to its rated pressure would hold more air than an LP tank of the same physical size filled to its rated pressure -- the HP tank has more air in the same size breadbox because it was more highly compressed into the same space.

As you might sense from the discussion, "rated pressure" for a tank may have little relevance to the pressure it is usually filled to. Here in California, my limited experience suggests that LP steel tanks often get filled at either their rated pressures (+/- 2600 psi) or to 3000 psi, which is the rated pressure of the very popular aluminum tanks widely available for rental. In Florida, I'm lead to understand that there are many fill stations that will fill LP tanks well, well beyond their "rated pressure" (and this is true for a few places I know in California as well, an undoubtedly true in many places). An LP 108 (the "108" calculated at rated pressure of 2640 psi) will hold quite a bit more air than 108 cubic feet if it gets filled to 3600 psi.

Steel tanks differ from aluminum in several important respects - I'll not get into it here as there are many threads avaialble and many willing commentators. LP and HP steel tanks also differ, but that difference can depend on local fill conditions and your own views on filling, storing, diving, etc. There's a ton of info out there, and you've been exposed to some of it in this thread. An LP 108 would indeed hold "more air" as your instructor mentioned if you were otherwise normally renting AL80s or steel tanks with less than 108 cu ft at rated pressure.

 

[SparticleBrane]

Size and sort of weight wise an HP 119 compares with an LP 95, but if you do the math, a PST 95 holds 96.6 cu ft at 2640 psi and 131 cu ft at 3600 psi. A Faber 95 holds 95 cu ft at 2640 psi and 129.5 cu ft at 3600 psi. Both are basically 130's at 3600 psi. The HP 119 holds 124.5 cu ft at the same 3600 psi.

The Faber 95 is slightly shorter than the HP 119 23.8" versus 24" but the major difference is in wall thickness as the HP 119 is almost 4 lbs heavier and thicker walls mean less internal volume in the same size external package and that reduced internal volume costs the HP 119 5 cu feet in capacity at 3600 psi. But it is a stronger tank.

The Worthington LP8-95 is a very close copy of the Faber, not the PST and also hold 95 cu ft at 2640 psi.

The PST 95 is slightly longer at 24.75" and is also about 3 lbs heavier than the HP 119, and nearly 6 pounds heavier than a Faber 95. So it has thicker walls, but makes up for it with the extra lenght.

So 95's are essentially 130's at 3600 psi, but at "120's" at 3350 psi, and I suspect the idea that they are "120" is a hold over from the days where a cave fill was slightly less.

PST 104's hold 106.6 cu ft at 2640 psi and 145 cu ft at 3600 psi.

The OMS 112 really did hold 112 cu ft at 2640 psi and they were made by Faber, although faber marketed them as a 108. I never understood why.

I don't have the specs but the Worthington LP8-108 may really be an honest 108 at 2640 psi - that would appear to be the case as it is dimensionally identical to the Faber "112", but 2 lbs heavier, suggesting a bit more wall thickness and a bit less internal volume.

I have always been a little anal about actual rather than marketed tank volume, perhpas because I started with steel 72's that were really 65's at 2250 psi, but many divers are not. When diving thirds withmost tanks it really does not matter as you turn on pressure not volume. It does become a minor issue when gas matching different tanks, but the error is most often going to be lost in the noise of SPG variation and inaccuracy between the upper and lower third anyway.

But...the 10 cu ft misunderstanding on the 95 is potentially enough to make a difference.

I suppose we will have to agree to disagree. If you look on the XS Scuba website, their 119s and 95s are both 14.8L cylinders. Personally I just do this:

85 = 100 (although it depends on manufacturer and honestly, it's a bit of a stretch)
95 = 119
104/108 = 130


Although you might be getting "less" with a higher fill, I personally prefer the 3442 cylinders since they usually get filled to rated pressure around here. It can be difficult to get cave fills outside of cave country unless you fill your own cylinders. Then again for most of my dives I don't need that amount of gas anyway. :cool2:

There was a very pretty set of PST LP104s, "never wet" (photos showed they still had the original galvanized shine), on TDS awhile back but the seller wanted some outrageous price. Oh well...

 

[Anti-Hero]

This is interesting... I have a couple of brand new LP85's with a 2400PSI working pressure and a + rating. When I go have them filled again I'm going to call around and see if I can get them filled past 2640PSI.

 

[DA Aquamaster]

I suppose we will have to agree to disagree. If you look on the XS Scuba website, their 119s and 95s are both 14.8L cylinders. Personally I just do this:

85 = 100 (although it depends on manufacturer and honestly, it's a bit of a stretch)
95 = 119
104/108 = 130

Actually we completely agree regarding the Worthington tanks and their metric specs.

The difference is in the fill pressure and a sloppy definition of a bar.

1 bar = 14.5 psi, not 15 psi.

So, 3442 psi is 237 bar and 237 bar in a 14.8 liter tank is 3507 liters or 124 cu ft, so at the rated imperial pressure of 3442 psi the X8-119 is really a "124". This is very close to the 123 cu ft capacity cited on the imperial page for the 119. (I'll concede the 1 cu ft to them based on the slight error in the ideal gas law compared to what actualy happens to O2 an nitrogen above 3000 psi, but for math purposes, lets keep it simple.)

The 230 bar reported on the metric page is only 3335 psi and 230 bar times 14.8 is 3404 liters which is 120 cu ft which is obviously very close to the 119 cu ft implied in the name of the tank.

Now at 3600 psi, the overfill pressure we are talking about for the Faber and Worthington LP 95's, that is 248 bar which in a 14.8 liter tank is is 3670 liters or 130 cu ft, making the LP 95 a 130 when overfilled to 3600 psi. And obviously that same 14.8 liters at 248 bar in an HP 119 would make it a 130 as well.

The same thing is true with the X7-100's I also dive. They are a 12.2 liter tank and at the 230 bar service pressure cited on the metric page (again only 3335 psi), they would hold only 2806 liters or 99 cu ft - the rated capacity cited on the imperial page. However at the service pressure cited on the imperial page, 3442 psi (237 bar), they hold 2891 liters or 102 cu ft. And at a "normal" fill pressure of 3600 psi/248 bar they hold 3025 liters and 107 cu ft.

Most places I get them filled, they get hot filled to about 3800 psi and I end up with about 3600 psi. On my last couple trips to N Fl, I got hot filled to 4000 psi and had an honest 3800 psi when all was said and done - 262 bar, 3196 liters and 113 cu ft.

The noteable thing here is that an X7-100 Florida filled to 3800 psi cuts the difference between the nominal 100 cu ft capacity and the 130 cu ft capacity of a cave filled LP 95 nearly in half, meaning you are not giving up all that much in the lighter and smaller tank.

To make it all really confusing, for the LP tanks Worthington cites a fill pressure of 166 bar which is very close to the 165.51 you get dividing 2400 psi by 14.5 psi.

In that case, 166 bar times 14.8 is 2457 Liters, or 86.7 cu ft. At 2640 psi/182 bar, the capacity is 2694 liters or 95.1 cu ft. That is consistent with how Lp capacities are published in the US - at the 10% overfill pressure.

So...in the LP section they use a bar defined as 14.5 psi and in the HP section they use a bar defined as 15 psi which really confuses what is going on.

An LP 85 is a 12.8 liter tank and at 182 bar (2640 psi) it will hold 2330 liters or 82.3 cu ft, meaning the "85" in the title is a bit of an over statement. However, at 3600 psi / 248 bar it would hold 3174 liters or 112 cu ft. Very nice. If you compare the internal volume of the LP 85 (12.8 liters) with the internal volume of the HP 100 (12.2 lirers) it makes perfect sense that the LP 85 holds more at any given fill pressure. However it is also interesting that with the usual 3600 psi in the cave filled LP 85 and a becoming usual N FL 3800 psi in the HP 100, they hold almost exactly the same amount of gas.

So...in practice you give up some capacity with an HP 100 compared to a cave filled LP 95, but with the slight (350-400 psi) overfill you normally get in an HP tank in N FL, you are only giving up about half of what it appears initially and, a cave filled LP85 hangs right in there with the HP 100.

 

[NadMat]

There was also a time when getting a HP tank filled to its rated pressure was difficult due to limitations in the fill stations, often leaving you with short fills at some shops as the equipment just didn't pump to that high a pressure, so that if you wanted to be sure of getting the rated capacity of your tank you went with LP's as most everybody could fill to at least 2800-3000psi. Not really an issue these days, unless you are filling off of somebodys portable or boat compressor that doesn't pump up as high(or takes forever to do so).

And of course there are always trim issues, as different tanks trim out differently, but those can be solved by moving some of your weight around. I still prefer LP's myself, but that may just be my unwillingness to step up to latest greatest thing(change is sceery )

JMHO,YMMV

 

[ucfdiver]

There was also a time when getting a HP tank filled to its rated pressure was difficult due to limitations in the fill stations, often leaving you with short fills at some shops as the equipment just didn't pump to that high a pressure, so that if you wanted to be sure of getting the rated capacity of your tank you went with LP's as most everybody could fill to at least 2800-3000psi. Not really an issue these days, unless you are filling off of somebodys portable or boat compressor that doesn't pump up as high(or takes forever to do so).

And of course there are always trim issues, as different tanks trim out differently, but those can be solved by moving some of your weight around. I still prefer LP's myself, but that may just be my unwillingness to step up to latest greatest thing(change is sceery )

JMHO,YMMV

Just curious, what benefit does a LP104 have over a HP130?

 

[NadMat]

Not a lot these days, except perhaps different trim characteristics, but when HP was the 'latest greatest thing' you could purchase LP's for a lot less than HP's containing same volume of air at LP fill pressure. Haven't done price comparisons lately so don't know if there is still much of a price difference.

 

[SparticleBrane]

I have rarely, if ever, heard of LP cylinders failing hydro. Definitely heard of more 3500 and 3442 series cylinders failing. I can see getting a set of 104s from that angle.

 

[DA Aquamaster]

Just curious, what benefit does a LP104 have over a HP130?

The Hp 130 is a couple pounds lighter than a PST LP 104 but both are very similar in size. Of course the Worthington LP 108 is similar in size and weight to the LP104 so I'd go consider that one as well.

They are all 8" in diameter and about 25.5 to 26.5" tall, weigh 41-45 lbs and are -2 to -3 lbs buouyant when empty.

With a 3600 psi cave fill, an LP 108 holds 148 cu ft of gas. The Worthington HP 130 filled to 3442 holds 134 cu ft and at 3600 psi it holds 140. At 3800 it would hold 148, so in practical north florida terms there is basically no difference as both are 148 cu ft tanks. Everywhere else the HP 130 will get you an honest and legal 134 cu ft, so it would be my hands down first choice. The price difference tends to be about $50 - not as huge as it used to be.

 

[jmorriss]

What is the wall thickness on a worthington or equal LP tank? I found 0.19 in another thread and the numbers aren't working out.

Thanks!

Wrong thread...oops.