Difference between LP & HP Steel Tank

[ScubaBill1955]

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.

 

[J.R.]

Ummm...

Faber 108... 108 cubic feet of air at 2650 psi... 8.02 in diameter... 26.77 in long... weighs 49.72 lbs full... 41 lbs. empty...

Faber 100 DVB (HP)... 3442 psi working pressure... 100 cu. feet of air... 7.24 in diameter... 34.30 inches long... 33.46 lbs full... 25.39 lbs empty...

The 108 will give you 2.98 lbs positive bouyacny empty and is 5.02 lb negative full
The 100 will give you .59 lbs neg. empty and is 8.41 neg. full...

(I.e., both tanks have about an 8 lb shift due to air)...

There are a lot of elements in analyzing a tank... how large... how heavy... what bouyancy characteristics...

Both will hold about 100 cu feet of air... (ok... the 108 holds 8 more cu. feet... assuming a compete fill)

Check out

http://www.huronscuba.com/equipment/scubaCylinderSpecification.html

They've done a great job at gathering together a bunch of info on tanks...

Personally... I use the Faber 100... I also have an Alum 80... my wife dives Steel 80's... (size issue)...

Hope this helps...

 

[JBRES1]

A lp tank is a tank that will be full when it has 2640 psig in it, and a high pressure tank will be full when it has 3442 to 3500 psig .
The lp tank will be larger than a hp tank when rated to the same size.
A lp108 is about the same size as a hp 130 tank.
A lp 95 is larger than a hp100 in both height and diameter, but both tanks hold about the same cubic feet of air.

A lp108 is one big tank, so be careful about going too big.
I dive both hp100's and hp117's and find I trim out much better with the 100's.
I also dive lp 95's and they trim out very much like the hp117's.

Jim Breslin

 

[DA Aquamaster]

It depends on the fill and on the tanks.

The LP 95 and the X8-130 make a good comparision.

Both have an internal volume of about 15 liters at 1 bar, or in easy to use imperial terms holds about 3.6 to 3.8 cu ft per 100 psi.

The service pressure on the LP 95 is 2400 psi and it holds the 95 cu ft with a 10% overfill to 2640 psi. LP tanks are certified under the 3AA steel tank engineering specification that has been around for several decades.

The service pressure on the HP 130 is 3442 psi so it holds 130 cu ft at that pressure. It is certified under a special permit to a different and less conservative engineering standard.

However in North Florida it is common to fill LP tanks to 3600 psi, which results in the LP 95 holding about the same 130 cu ft as an HP 130.

In general, an LP or HP tank of similar size, weight and intenral volume will hold the same amount of gas at the same fill pressure. In areas of the country where you cannot get an overfill to 3600 psi, the HP tank makes more sense as you can actually get the volume for the size and weight. If you live in North Florida, there is a strong argument for LP tanks as you can get the overfill and get the same volume as the hp tank with the higher rated capacity.

The LP 108 actually holds 112 cu ft at 2640 psi and if overfilled to 3600 psi will hold 152 cu ft - which is where your LDS operator is correct in saying it is what you want if you want a lot of gas - assuming you live where you can get an overfill to 3600 psi.

There is however a lot more to consider. How the tanks effect your trim, their size and weight, your gas requirements and what your buddies use.

 

[sambolino44]

...The service pressure on the LP 95 is 2400 psi... with a 10% overfill to 2640 psi...However in North Florida it is common to fill LP tanks to 3600 psi...

Really? That seems imprudent. I wonder if you guys have tanks failing hydroscopic testing at a higher rate.

 

[Nemrod]

Why would it matter if the tank failed a hydro after many overfills? A tank is a tool, if the overfill allowed you to do a dive that would have been otherwise impossible or unsafe then the tool has fulfilled it's mission. They make more tanks, buy a new one if it fails. Cave fills are common practice and are usually done by people who know what they are doing specific to their equipment.

N

 

[DA Aquamaster]

The whether its prudent argument is one you can find in one of several past threads if you search.

The test pressure on a 2400 psi tank is 4000 psi, so a 3600 psi fill is under the test pressure, the question ends up being whether it is a good idea over the long term to do fills to that pressure, rather than just brief hydro tests. There are basically two views:

1. There is a remote risk that over time a tank on the weak end of the quality control scale will fail after years of overfills. One or more people will be killed or maimed, people will be sued and the practice will scream to a halt.

2. 3600 psi is still withion the safety margin and consequenlty #1 has not happened yet after at least a couple decades of operational history, therefore it is a safe practice.

The counter argument from the # 1 crowd to #2 is that fill pressures still seem to be creeping upward and eventually someone is going to get bit.

The counter to that is the feeling that bad tanks will be culled at hydro test before they fail.

Which is of course countered by those who say that argument is negated to an increasing degree as overfills approach the test pressure

That is in turn countered by the argument that the 3AA engineering spec is very conservative and there is still a large margin of safety left.

It just sort of continues from there....

I have not heard of any data on failure rates on cave filled tanks one way or the other, but I have also not heard of any higher failure rates on them either. So it is affirming of the practice in a no news is good news kind of way which does not really settle the argument about what may eventually happen in the future.

Personally I think that if, for example, you want 130 cu ft it makes more sense to buy a high pressure 130 rather that you can legally get filled anywhere than to buy a new LP 95 that you can only get fileld to 130 cu ft capacity in North Florida. The weight and bulk are pretty similar either way.

Of course...since I go to North Florida often to cave dive, that does not stop me from buying sets of LP 95's if I find them at a great price (ie. half what I'd pay for new HP 130's.)

The practical sides of the argument follow those lines as well:

1. You don't see many LP 95, LP 104 or LP 108 owners saying "no" to 3600 psi fills and once you try it, it gets addictive in N FL with the greater penetration afforded by the increased gas. After all it turns your double 95's into double 130's.

2. There are some higher capacity HP tanks that have been made (PST 149, etc) but with PST's little meltdown and relocation they are almost impossible to find, so if you want more than 130 cu ft, you end up needing to rely on an over filled set of LP 104's or 108's - or rely on stages.

3. LP tanks tend to be slightly cheaper new, and they tend to remain very good used buys and are still very popular in N FL.

4. The drive to N Fl is still a lot more dangerous than standing by the tank during the cave fill.

 

[ucfdiver]

Really? That seems imprudent. I wonder if you guys have tanks failing hydroscopic testing at a higher rate.

My LP tanks know what 4200psi feels like, and they passed hydro earlier this year (now 12 years old, and I don't think they've ever been filled to less than 3600). They were owned by a cave instructor who teaches about 2 classes a week, so I'm guessing these tanks have been overfilled 2-3x a week for 12 years now, and I've been doing it about 4-5x a month.

Here's a copy/paste of a post I made last time this subject came up-

This debate goes on way too much. It's really simple to make the decision once you understand some of the basic math, but far too often I read "hp are lighter", or "lp hold more air", and while both of those statements are true, there's a little more to it than that.

If you're filling in cave country, LP tanks are the best value. Here's basically the the LP/HP equivalency if you're in an area that overfills, and then I'll try to explain it so you don't have to just take my word on it.

LP85's = 100's
LP95's = 120's
LP104/108/112's=130's
LP120's=HP149's

Once you take a basic cave/cavern class, you'll learn about "tank factors" which basically says for every 100psi, the tank has ___ cuft of air. Here's how that's calculated- (tank volume(in cuft)/rated pressure*100)

For example, a LP104 has a tank factor of 104/2640*100 = 3.9. This means for every 100psi, you get 3.9 cuft of gas.

A 133 has a tank factor of 133/3442*100=3.9. This means for every 100psi, you get 3.9 cuft of gas.....see a pattern?

So at 3400psi (34*3.9) you get basically 132 cu/ft in either tank.

Now, one argument for LP tanks is that on dive boats, often times you can't get a "full fill" on a HP tank. Let's say that you can only get a 2640psi fill. Well then, the 133's and 104's both hold 104 cuft at that pressure. Basically figure out the size tanks you want in regards to volume, and if you can't get overfills where you're diving, get HP tanks, if you can, you might can save money by getting LP tanks.

You'll hear HP tanks are lighter than LP tanks. While this is true, remember what we just learned. Saying that a LP120 is heavier than a HP120 is true, but the LP120 can hold 149cuft when filled to 3500ish, while the HP120 can only hold 120cuft. Compare the LP95's, and you'll see that the weight is nearly identical, because they're the same size tanks.

Many shops consider overfilling LP tanks dangerous despite the fact that it's been going on for nearly 2 decades without a single known incident. I've had my personal tanks filled to 3600 every single time without complaint.

At this point, if buying new, I'd buy HP tanks, the price difference just isn't enough to make it worth ALWAYS getting tanks filled in cave country....sometimes it would be nice if I could get my tanks filled in Orlando before heading to Ginnie for a midnight dive.

Also, if you know you want to cave dive, I'd start out in double tanks ASAP. No reason to make more of a "learning curve" there than there needs to be....."Keep the end in mind".

I hope I was clear, let me know if I wasn't.

 

[SparticleBrane]

Just to make a quick note about tank 'equivalency' and size between the LP series and the newer exemption series -- 95s = 119s, not 130s or 120s.

Also, LP108s actually do hold 108cf of gas. The "112" version from OMS was really just a 108 from Faber.

 

[DA Aquamaster]

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.