LP vs HP

[RonzoTheGreat]

I FEEL IGNORANT?
please explain a benefit of lps over hps other than cost?
sorry if this question is dumb ...

im trying to expand my "arsenal of tanks" and my doubles are 3442 hp faber 120s ... why would i want lower pressure?

gosh i feel dumb

 

[Mark Michaud SELAUSAR]

Pressure and volume are apples and oranges. Higher pressure tanks do not equate to more gas. A 120cu ft tanks has to be at 3442 psi to have 120 cu ft of gas. A lp 120 needs 2400 psi to obtain the same volume. Overfill by a third, to 3200 psi, and you 40 cu ft more gas or 160 cu ft. Mark

 

[fppf]

Yes but we all know we should not over fill tanks.

LP tanks are easier to PP blend with because standard industry pressure is 2600 PSI.

That said, have you seen the size of a LP 120? I use HP and just work with the higher pressures.

 

[BKP]

I enjoy my 3442 HP100 (it's lighter, smaller, etc.), however, the main advantage I see with an LP is I often get short fills with the 3442 (even if you tell the shop to watch the pressure). Everyone seems to give the LPs at least a 10% overfill (and I haven't heard of any exploding).

That said, I still prefer the HPs. I typically only fill to 3000, anyway (it's easier on the reg). Even at at 3000psi, you're still getting a solid 89cu ft. (as opposed to 77cf with a standard al 80), with no positive buoyancy.

 

[RonzoTheGreat]

ok ... all of you made sense so far but am i in the right train of though to think if i had two tanks at 80 for example and one was LP and one HP ... i would actually need less pressure for the same amount of gas in the LP ... so it actually means when they go and overfill the LP, ill have more than if they were just pushing the HP at and overfilled Lps limit?

wow ...

 

[BKP]

ok ... all of you made sense so far but am i in the right train of though to think if i had two tanks at 80 for example and one was LP and one HP ... i would actually need less pressure for the same amount of gas in the LP ... so it actually means when they go and overfill the LP, ill have more than if they were just pushing the HP at and overfilled Lps limit?

wow ...

Yes.

However, an HP100 is 33lbs. empty. an LP95 is 42 lbs. empty. (Which was my 2nd most important criteria, when I was asking the same questions).

 

[texdiveguy]

A lp 120 needs 2400 psi to obtain the same volume. Mark

Don't you mean 2640psi/120cf.

 

[texdiveguy]

I use a HP st130 as a recreational single and just love it for size and the fact I don't have to wear any additional wt. wet suit diving....plus no swapping out of tanks between typical dives. I use lp st. doubles and love them also, again no additional wt. wet or dry/with undies. Its been a long time since I had an Al. tank on my back...

 

[Mark Michaud SELAUSAR]

2400.........2640 sure, you are probably right. Kinda more about the concept than actual numbers. As to overfilling, normal cave fill for LP's is 3600 psi. At that pressure the volume is far greater than 85 cu ft (my tanks). The Faber tanks I dive in the US have a DOT rating that is far less than the European rating on the same tank outside of the US. Yoke v DIN and perceived liability has a lot to do with that lower rating here in the states. On the same note, I only do this with LP steel stanks. Never aluminum. Mark

 

[DA Aquamaster]

There is a similar thread running where the issue has just been beaten to death.

In short, you need to look at the size and weight of the tank as well.

1. An LP tank of similar volume at it's rated pressure (plus the 10% overfill) will always be larger and heavier than a 3442 psi tank of the same gas volume at it's rated pressure.

2. In most cases if you take a 3442 psi tank and a low pressure tank of the same external dimensions and similar weight and fill them to the same tank pressure, you get nearly identical gas volumes.

3. To get equal gas volume out of both a 3442 psi and a 2400/2640 psi tank of similar external dimensions and weight, you have to overfill the 2400 psi service pressure tank to 169% of it's service pressure (69% overfill). In comparison if you get a "short fill" of only 3000 psi in your 3442 psi tank you still have the same volume of gas than you would with an LP tank of the same external size and weight overfilled to the same 3000 psi pressure (25% overfill).

4. Unless you live in North Florida, you are unlikely to get much more than a 10% overfill. So for a given size and weigth of tank, you are probably going to get more volume with an "average" fill pressure than with an LP tank.

What this all means is that you can get a higher volume 3442 psi tank of the same extrernal size and weight as an LP tank of much lower rated volume and will get the same gas volume at the same pressure. But you can do it legally with a 3442 psi tank where you would need a massive overfill to get the same volume with the LP tank.

In really simple terms, a 95 cu ft LP tank is a lot larger and heavier than a 100 cu ft 3442 psi tank and for the same size and weight of the LP 95, you could carry a 3442 psi tank of 120 cu ft and not have to depend on getting a huge overfill to get the same 120 cu ft of gas. Or in other words, you'd still have a lot more gas with an underfilled 3442 psi 120 than with a 10% overfilled LP 95 and would be carrying the same weight on your back either way.