Why do LP tanks weigh MORE than HP?

[Black-Gorrilla]

i find it odd that an LP tank weights more than the closest size HP tank...
for example...
an
-LP121 (xs sxuba) weighs 49.9# empty...

-HP120(Worthington) weighs 38# empty...

WHY? if a LP tank has to hold only 2640psi to get the volume... and the HP has to hold 3442 to get the volume...
wouldn't that mean that the HP tank has to be thicker to hold the pressure? so that (logically...) should mean that an LP does not HAVE to be as thick to hold the pressure?

for some reason i keep getting errors while triying to search... but im sorry if this is out there somewhere.


THANKS FOR ANY INFO IN ADVANCED (you know i love you guys right?)

[Jack Hammer]

If you look into the physical dimensions of the tanks I think you'll find your answer. IIRC, a HP120 is ~the same physical dimensions as a LP95. If you fill a HP120 to 2600# you get about 95cf of air. Hence the similarites. Off the top of my head, the LP121 in your example is comparable in physical size to a HP150.

I really don't know what the difference in metals used are. I've seen it mentioned before, but I don't recall. Could be slightly thicker or different stamping process. Could even be a variation of the steel alloy. Bottom line is the HP tanks are physically smaller and rated to safely fill to a higher pressure.

Jack

[navyhmc]

If you look at the sizes of the two tanks in question you'll note that the LP is taller than the HP and it need to have more internal volume as the psi per cu ft is less so more volume per cu ft is needed in the LP tank.

For comparison:
HP-120: 120.6 cu ft, 7.25" dia, 28" length
LP-121(+): 120.0 cu ft* 8" dia, 29.2" length.
* 120 cu ft @ 2640 psi

I note that both have very similar water weights though.

Other than that, I have no idea why the LP weighs more. I will say that they are real fun to carry up and down the path to the water at Oronogo.

[mike_s]

I'm betting also that if you look at tank wall thickness, a HP tank has a thicker, stronger tank wall than a LP steel tank does because it has to hold about 1000psi more pressure.

[cogrwy]

Don't know about the wall thickness, but my HP100s are physically smaller outside than my LP95 (7.5" vs 8" and slightly shorter).

[in_cavediver]

First, realize you have to talk apples to apples. A LP104 is not the same as an HP100. Compare the LP104 to the HP130 or the LP85 to the HP100. In that, the PST LP104 is 3lbs heavier than the PST HP130. The LP85 is 2lbs lighter than the HP100. All in all, pretty close matches. Of course, the LP104 @ 46lbs is a lot heavier than the HP100 @ 33lbs. Its also 3" taller and 0.75" larger in diameter.

Now for the metallurgy. There is a difference in the materials used in LP and HP tanks. Its the tensile strength range and specific alloy used. Generally speaking, the LP tanks have a lower tensile strength than the HP tanks. This isn't a bad thing, for either. It's just a different optimization. The LP tanks have a HUGE data set for life cycle and it shows if you take care of it, it will outlast you. (industrial gas bottles are around from WWI and WWII and going strong). The HP tanks cram more gas in (like a cave fill). Their life cycle information is still forming.

[cogrwy]

[quote=in_cavediver;176964] Its also 3" taller and 0.75" larger in diameter.

Excuse my previous post. I meant to say 7.25" vs 8", but my fumble fingers missed the 2.

[RoyN]

The faber high pressure models (the non FX version) is quite heavy. I'm using the HP100 and it is so heavy that I don't even use any weights at all.

[in_cavediver]

Originally Posted by RoyN

The faber high pressure models (the non FX version) is quite heavy. I'm using the HP100 and it is so heavy that I don't even use any weights at all.

Ah, but this is a different measure. We have a few different weights to watch. We have its empty weight which affects us carrying it out of the water and then we have its buoyant force wieght which impacts us in the water. While they are related closely in the physics of it, we can treat them as independent values in the use of them.

What we care about diving is what is the net buoyant force of the tank when empty. An Al 80 is 4lbs positive so it needs 4lbs of lead to be neutral. An HP 100 is I beleive 2lbs negative when empty so you have to drop 2lbs of lead to be neutral. This information is great if you change tanks as you'll know how much wieght to change. In the above example, its 6lbs of lead to drop from the AL80 to the steel HP100.

Now, for the 2nd bit of information which doubles divers really care about. How much does the rig weigh out of the water. If we take our example above, the Al 80 weights 31lbs and takes 4lbs of lead to be neutral so it adds and totals 35lbs of weight to be carried. The HP100 weighs 33lbs but drops 2lbs of weight so its net out of water weight is 31lbs. The net difference is 6lbs less lead on the belt and a net drop of 4lbs you have to carry.

[RoyN]

My version faber tank is 14lbs negative full and 7lbs negative empty.