HP or LP- which and why?

[giles45shop]

Just to add a little more math:

To calculate the tank factor of any scuba tank, take the rated volume and divide by the rated pressure. For example, a LP95 is rated at 95 ft3 at 2640 psi (2400 psi + 240 psi (10%+) rating). Doing the math, you get a tank factor of 0.036 ft3/psi. So for every 1 psi of pressure, you have a volume of 0.036ft3.

For simplicity in doing in-your-head calculations, multiply that by 100. So for every 100 psi of presure, you have 3.6 ft3 of gas.

So for my LP 95 example, you get the following:

2400 psi = 86 ft3 (24 X 3.6)
2640 psi = 95 ft3
3000 psi = 108 ft3
3500 psi = 126 ft3

Now you can get into the whole HP/LP overfilling debate and understand it a little better. A LP tank will ALWAYS be physically larger in size than the equivalent volume HP tank.

The trick with LP tanks is that if you can get them overfilled, you can put more gas in a smaller package. For example, my LP 95's pumped to 3600 psi hold 129.6 ft3 of gas in a significantly smaller and lighter package than a HP130 @ 3442 psi.

I would talk to the places where you get fills at before I purchased a tank. Find out what pressure they can fill to (can they fill a HP tank to capacity?) and what pressure they will fill to (will they fill my LP tank to more than 2400 or 2640 psi?). If you buy a HP tank and they only fill to 3000-3100, you won't get the rated volume of gas on your HP tank. If you buy a LP tank and they won't overfill, you are stuck at the rated volume in a physically larger tank.

Around here, getting LP tanks filled to 3000 psi is no problem, and a few places will fill to 3600. In cave country I can get 3600+ fills everywhere.

Hope this helps,

John

 

[cbrich]

Talk with your LDS and FSO (fill station operator) before you make a choice. Ask them about filling a HP and LP cylinder. They probably can fill both, but only to 3,500 psig for HP and 2,640 psig for LP. If you travel a lot (by auto), get a LP tank because like others have said you might not can get a full fill on HP cylinders, and when you go down to Florida you get a very good fill at 3,600 psig. Again talk to your LDS before buying, and it's their call on what pressure they will fill too.

 

[DA Aquamaster]

5 years later it mnay also make a difference. Many test facilities don't seem to want to do the extra step for the "+" rating on LP tanks. It is not hard as long as there is a REE number stamped on the tank. What that means is that a low pressure tank that holds 95.1 cu ft at 2640 psi will only hold 86.4 cu ft at the 2400 psi it can eb filled to without the "+" rating. And some shops are really anal about only filing to 2400 psi anyway.

On the other hand, special permit tanks, due to the heat treatment they recieve are often not perfectly round. So in a normal hydro test, they will go from a slightly oval cross section to a round cross section as the pressure comes up. That will increase the volume slightly and that can be mis-interpreted as excessive expansion. The proper procedure involves a round out process where the tank is brough up to 90% of the test pressure briefly, then you start over and re-zero before you do the actual test. When someone reports a hydro facility failed 4 out of 5 HP tanks they sent in, they most likely screwed up the test.

 

[JPENDERGRAST]

Thanks everyone, this has been very helpful!

 

[CaveMD]

Why, the twine of coarse!

thats an easy question....rope is usually made out of more strings braided together

you basically get

string =x

twine is usually 3-5 strands so

twine = 5x

rope is often up to 10 + strings

rope = 10x

so for all possible lengths greater than 0

1x < 5x < 10x

string < twine < rope

 

[David_57]

HP has better buoyancy properties than a LP having said that you can jack up the pressure in a LP 95 so it would end up with well over 100 cubic feet.

 

[idocsteve]

Why, the twine of coarse!

I'm a frayed knot.

 

[rakpix]

oh steve, thats bad X_X

 

[no excuses]

HP has better buoyancy properties than a LP having said that you can jack up the pressure in a LP 95 so it would end up with well over 100 cubic feet.

if you have a lds in youre area that will do that.