HP or LP- which and why?

[JPENDERGRAST]

I have been blessed with a wonderful wife who lets me dive as often as I want. With that said, I've gotten quite a few dives under my belt for being certified for less than a year and now looking to invest in some additional tanks. I have several lined up and already have two steel HP 80s which I love. I'm about to purchase a third one, but I'm also looking at a couple 119 and 120 cu ft. tanks.

So, my question is, does a HP tank hold more air than a LP tank? I feel like that's a dumb question, but after talking to a fellow diver who is very knowledgeable, I am now confused.

From what he was explaining, the HP and LP 120 will hold the same amount of air, simply at a different pressure rating. Is that true? That goes against what I thought I knew about tanks and I think he is confused, but trust his knowledge. If that is true, what is the benefit to having a HP or LP tank?

P.S. I've searched the forums for this but the closest answer I could find were a bunch of posts about over filling.

Confused,

John

 

[DiveMonkeyEZ]

Your friend is correct. You go off the total cf of air the tank is rated for to determine it's max volume. So a 120 is a 120, regardless of what design (native volume x pressure) is used to arrive at the total.
There are some pros and cons. Most LP tanks are older and therefore may have less life-cycle left to them. But just about any dive op can fill an LP tank.
The HP tanks are more compact for the volume, generally newer, but some dive ops here and there may not have compressors rated to fill the 3442psi or 3500psi tanks. There are professional tanks that are rated at even higher pressure, but be aware that filling will be an issue, and that they will require first stage regulators that are rated for the extra pressure.

 

[LeoS]

To put it in simpler terms...

The HP 'version' of a tank with the same amount of rated capacity will always be physically smaller, because the gas inside is packed denser (in higher pressure). Please CMIIW.

 

[Indian Valley Scuba]

Here's a little reading you can peruse on tank selection. The bottom line is to figure out what the ideal amount of gas you might need to carry, including reserves, when you dive with that tank. Of course your dives may vary so there is no one answer here, but there is not glory in carrying around excessive amounts of gas ( plus the tank they come in) when you don't need that much.

Enjoy the read: Tank Selection 101

And here is a summary of tank specifications including dimensions, weights and capacities.

 

[drdaddy]

I have been blessed with a wonderful wife who lets me dive as often as I want. With that said, I've gotten quite a few dives under my belt for being certified for less than a year and now looking to invest in some additional tanks. I have several lined up and already have two steel HP 80s which I love. I'm about to purchase a third one, but I'm also looking at a couple 119 and 120 cu ft. tanks.

So, my question is, does a HP tank hold more air than a LP tank? I feel like that's a dumb question, but after talking to a fellow diver who is very knowledgeable, I am now confused.

From what he was explaining, the HP and LP 120 will hold the same amount of air, simply at a different pressure rating. Is that true? That goes against what I thought I knew about tanks and I think he is confused, but trust his knowledge. If that is true, what is the benefit to having a HP or LP tank?

P.S. I've searched the forums for this but the closest answer I could find were a bunch of posts about over filling.

Confused,

John

As others have said, the cubit foot is the volume held, so a low(er) pressure tank would have have to be bigger to hold the same volume as a high(er) pressure tank. That is why your steel 80 is smaller than your aluminum 80. And the aluminum tank also has thicker walls because steel is stronger than aluminum.

Some would recommend a HP 3442 119 over a 120, because the 119 is shorter (less likely to hit you in the butt), but it is wider.

I like high pressure steel tanks because for size and weight I can get more air. However, this does have a limit. For me, the 119 and 120 Hp tanks are heavier than I'd like, especially since I like to have some weight that's available to dump.

All my regulators are yokes. I have 3 aluminum 80s, 1 low pressure steel 72, 3 HP 3442 steel 80s, and 3 HP 3442 steel 100s. I have 2 13cf and 1 19cf pony bottles as well (2 sling, 1 backmount).

My favorite tanks are my HP 3442 100s, and I dive almost exclusively with them. There can be a problem at some places getting my 3442 HP steels filled all the way, but even a short fill of 3000 psi will give me more air than a fully filled 80.

 

[rakpix]

i am curious about something. there are a number of pony bottles in the 6-19 cu ft range that are sold as "300 bar - 4350 psi - pony bottles." why is it that you can cram so much air into such small bottles, but no where does anybody think to fill a big "HP cyclinder" to such a preassure and get MORE air in the SAME package. what's the difference between these bottles? thanks.

 

[DA Aquamaster]

The larger the diameter of a tank and to a lesser extent the longer the tank, the higher the wall stresses. What that means is that if two tanks have the same service pressure, the smaller tank can have thinner walls. So a pony bottle can operate at 4350 psi and still have comparatively thin walls and light weight.

In larger tanks, there is a point of diminishing returns that is quickly reached and 3500 psi is about the upper limit before weight begins to become excessive.

You will note that some LP tanks are similar in size and weight to HP tanks. For exmapel a Faber LP 95 weighs 37 pounds and the Worthington LP 95 weighs a bit more at 43 pounds while the Worthington HP130 also weighs 43 pounds and is very similar in size. All the tanks hold about the same volume at similar pressures with the HP130 holding 130 cu ft at 3442psi and the LP 95s holding 129 cubic foot when overfilled to 3600 psi.

BUt while the opackages are similar the steel alloys and/or heat treatment are different. The HP tanks will use a different alloy or heat treating process to gain more tensile strength, but the trade off is that the steel is less ductile and that may have an impact on the service life of the tank. However given that a 3AA steel tank will last at least 50 years (and many have bene in service for a 100 years now) a 3442 psi special permit tank that only lasts 1/2 or even 1/3 that long would still be a very good deal and the service life would still probably equal or exceed that of an aluminum tank.

The other issue with 300 bar tanks is that pressures about 3600 psi tend to increase wear and tear on compressors and/or require a booster, so those really high pressures are less popular with many dive shops.

Huron scuba has a good tank comparison chart as does XS scuba, where they have specs for both LP and HP worthington tanks that lets you get a good feel for size, weight and volume.

In general, the HP 100 is a great tank that trims well for most people is only pounds heavier than an AL 80 yet holds 23 more cubic feet and is actually lighter when you consider the 5 pounds more weight required to get neutral with a near empty AL 80.

The X8-130 is probably the best overall bet for a really large capacity steel tank. The X7-120 is hard to trim unless you are 6' tall. The Faber LP 95 is great choice if you can get 3600 psi overfills, but the worthington 95 is about the same weight as the X8-130 so if you can't get a faber 95, go with the 130.

 

[CaveMD]

it may be more simply put mathematically if you understand equations

a tank that has an internal volume of 10 L will hold 10 L of air @ 14.5psi( atmospheric pressure)

that same tank with internal volume of 10 L will hold approx 100L atmospheric equivalents of air @ 145psi

1000L @ 1450 psi
2000L @ 2900 psi
3000L @ 4350 psi

so basically if you have a HP 119 cu tank (assuming 3500psi) it will be approximately the same physical dimension as a LP ~88 cu tank(2600psi)

 

[RJP]

So, my question is, does a HP tank hold more air than a LP tank?

Which is longer, string or rope?

 

[TGreene]

Which is longer, string or rope?

Why, the twine of coarse!