HIGH PRESSURE TANK vs LOW PRESSURE TANKS...?

[Superlyte27]

Aside from price, the only reason I can see to go with LP tanks over HP tanks is if you plan on overfilling an LP120.

For nearly every other LP tank there is an HP tank made with nearly the exact same weight and dimensions. The HP tank is just rated to a higher pressure and will hold more air at that higher pressure. So if your local shop won't fill to 3500 you can always use it at lower pressure and you'll have just as much air on your back as you would with a similarly sized LP tank. If you have some old regs with low pressure yokes, just don't get your HP tank filled all the way.

Basically, an HP tank works just as good as a similarly sized LP tank at low pressure. It just has the option to hold more air.

Except that the LP will always hold more air than the HP equivalent.
For example... AN LP85 holds more than an HP100. An LP95 holds more air than an HP120 and an LP108 holds more air than an HP130 at the same pressure. Oh, and LP tanks are typically cheaper and for many people have better buoyancy characteristics.

 

[elmer fudd]

Except that the LP will always hold more air than the HP equivalent.
For example... AN LP85 holds more than an HP100. An LP95 holds more air than an HP120 and an LP108 holds more air than an HP130 at the same pressure. Oh, and LP tanks are typically cheaper and for many people have better buoyancy characteristics.

For the most part you're talking trivial differences. Take the LP108 and HP130 for example, (Worthington). They're almost identical in size and weight. The HP130 is just .5" shorter. Weight is identical. So yes, there will be about a 2% difference in capacity at the same fill pressure. Chances are though that they won't be filled to the same pressure. Odds are the shop will fill the HP tank to around 3400 and the LP tank to around 2600.

There really isn't much extra metal at all on the HP tanks. The LP tanks are built to government safety specs, but modern HP tanks are built under an exception to the standards. That's why the old HP tanks used to be so heavy.

 

[elan]

For the most part you're talking trivial differences. Take the LP108 and HP130 for example, (Worthington). They're almost identical in size and weight. The HP130 is just .5" shorter. Weight is identical. So yes, there will be about a 2% difference in capacity at the same fill pressure. Chances are though that they won't be filled to the same pressure. Odds are the shop will fill the HP tank to around 3400 and the LP tank to around 2600.

There really isn't much extra metal at all on the HP tanks. The LP tanks are built to government safety specs, but modern HP tanks are built under an exception to the standards. That's why the old HP tanks used to be so heavy.

The worth 108 is noticible longer IIRC correctly more than 0.5" and is 3 lb heavier - 46 vs 43lb HP100 and the sibling - LP85 are about 1" different.

Although not that huge but it makes a difference in trim when paired ip.

 

[kaerius]

False, at least so far as the common 3442psi/3500psi HP tanks go. Get up to the 4000psi+ HP tanks (I think there's a Faber in that category), and you'd be correct...but A-clamp/yoke connections are fine with 3500psi connections.

From AA's specs page:

Max-Pressure Rating (all)

3500 psi (241 bar) with yoke connection 4350 psi (300 bar) with DIN connection

Let me rephrase: To fully utilize the higher pressure rating of a HP tank you need to have a DIN reg, period. If you're filling a 300BAR/4350 PSI tank with 241BAR/3500 PSI, because that's all your yoke can handle, you're only using 80% of its capacity.

 

[elmer fudd]

The worth 108 is noticible longer IIRC correctly more than 0.5" and is 3 lb heavier - 46 vs 43lb HP100 and the sibling - LP85 are about 1" different.

Although not that huge but it makes a difference in trim when paired ip.

I was taking my data from this site:
Scuba Cylinder Specifications from Tech Diving Limited - 928-855-9400
It may be wrong, but the point I was trying to make was that for each LP tank they generally make an HP tank with very similar dimensions and weight.

 

[HIGHwing]

To your point an LP85 and HP100 are almost the same size. But, say you pull into the local shop and ask for a fill of your HP versus my LP. The tank monkey might fill the tank to 3500, you'll wander out to the boat, connect your regulator and have 3,000-3,100psi as the tank cooled down. My LP will get filled to 3,000 and it will cool to about 2,600. I got a full fill and my tanks were able to be filled at the same time as the aluminums.

Ask anyone with HP steels about getting solid fills (outside of cave country) and you'll find it takes patience else Charles Law steals your gas. I actually stopped diving my HP100s for this reason; dive shops are not patient enough to achieve working pressure. Some shops don't always have 4,000psi available in the bank with enough volume behind it to fill them properly anyway and you're forced to accept a lesser fill. Some shops even refuse to allow tanks to cool and top them again based on some premise I don't understand fully.

For me the best all around steel is the LP85. Nice size and weight, good buoyancy, proper fill gets me more than an AL80, it can hold 115-120 cu/ft of gas when I desire, it's a good citizen on the fill whips when shops can't reliably fill beyond 3,000psi, it's excellent for back-mounted doubles and is a favorite of side mounters alike.

Ymmv

 

[triplecove]

Basically, an HP tank works just as good as a similarly sized LP tank at low pressure. It just has the option to hold more air.

I think you have this statement backwards You'll have a hard time filling a high pressure tank past its capacity but Lp's get fill past their capacity all the time.

 

[elmer fudd]

I think you have this statement backwards You'll have a hard time filling a high pressure tank past its capacity but Lp's get fill past their capacity all the time.

That's because they have a low capacity to begin with. It's a, "these go to 11", philosophy. Just imagine the overfills you could get with an 1800 psi tank.

The big difference between LP and HP tanks is that one is rated to 2400 psi and the other 3442 psi.

Chances are I'll have a MUCH easier time getting my HP tanks filled to 3500 and above than I would with LP tanks. I've even gotten fills over 4000 before. I doubt you'll see much of that with LP tanks, considering that it's approaching their hydro test pressure.

 

[Steve_C]

HP 100s steel

I have a pair or AL 80s which I got when I started diving and do not use much. They are longer and tend to hit me in the rear. I have dove AL 80s, LP 95, HP 100, AL 100, 120s, and a couple others. The HP steel 100 give me the best fit. The less added weight with steel is a bonus.

 

[kaerius]

The less added weight with steel is a bonus.

Depends what your usual weight requirements are due to body fat/muscle ratio, dive experience, water temperature and thermal protection.
When I was working in Cyprus the dive shop had steel tanks of various sizes. I used two kilo with the LP56s(8 liter for us using metric), and was at least 2-4 kg overweighted without a weightbelt with the LP80s(10L)/LP100s(12L), this with 4mm shortie, even when adding 4mm longjohns under the shortie, I was still at least 2kg overweighted with the LP100, no weightbelt.

On the other hand, diving in sweden with a HP80(10L, 300BAR/4350 PSI), 7mm semi-dry with a 5mm shortie on top, I needed 6kg.

Last divetrip I needed 3kg with AL100(12L) and 5mm shortie. If I skipped the suit I could probably get away with no weightbelt there too. Steel would have overweighted me.