high pressure vs. low pressure

[gracefulc]

No question is a stupid question right?

I am still befuddled on the whole lp vs hp debate... I have used hp80's since that is what my lds rents and have been considering buying a tank or two.

Here is what I don't get; using the rule of thirds (1/3 out, 1/3 back, 1/3 reserve), suppose I go out with 3000psi (on a hp) I turn around at 2000 and get out at 1000.

so if an lp can only be filled to 2600, I have to turn around at roughly 1700-1800, get out of the water at 900ish.

So won't my dive time be shortened? or does the pressure guage just go down slower with an lp?

 

[CD_in_Chitown]

The volume is larger usually

My 3500 psi HP100s hold 100 cu ft at the full 3500

LP95s hold 95 cu ft at 2640 (or thereabouts), while a PST LP95 will hold like 104 cu ft at 3450psi, this is known as a + hydro, such a hydro certifies the tank to working pressure +10%.

As an aside the tank neck is different depending on the working pressure 200 or 300 bar may ring a bell? LP tanks use the 3/4" threads in the neck (which is the same as your AL80 btw) and HP tanks use a 5/8" threads on the neck. These numbers may be off, I usually have seen them designated in NPT instead of inches but not sure what NPT means. On a DIN valve the LP connection point (200 bar) is not as deep as an HP DIN (300 bar).

 

[FIXXERVI6]

high pressure, low pressure who cares, I'll take either

run the high pressure tanks at 3600 /3 1200 psi before turn

run the low pressure tanks at 3600 /3 1200 psi before turn

E8/130 vs LP 104, close enough, I can get the same volume of gas out of both.

I wish everything was liquid internal volume

 

[CD_in_Chitown]

run the high pressure tanks at 3600 /3 1200 psi before turn

run the low pressure tanks at 3600 /3 1200 psi before turn

Do you want to check that math? Isn't the rule of thirds more like

3600/3 = 1200, meaning 1200 out, 1200 back, and 1200 in reserve?

 

[FIXXERVI6]

an example

LP 108 has 4.1 cubic feet of gas per 100 psi of pressure, (roughly)
E8 130 has 3.8 cubic feet of gas per 100 psi of pressure, (roughly)
HP 100 has 2.9 cubic feet of gas per 100 psi of pressure, (roughly)

so while the total volume of tank at operation pressure on a 108 vs 100 is about the same, the internal volume is quite a bit different, this is where over pressuring LP tanks shows its wonders.

HP 100 pumped to 3600 has roughly 105 cubic feet of gas
LP 108 pumped to 3600 has roughly 147 cubic feet of gas

so while both tanks are rated for about the 100 cubes range, the 108 holds a lot more gas at the same pressure as the 100, so if you pump a HP 100 to operating pressure and a LP 108 to operating pressure, the needle moves slower on the LP 108.

 

[FIXXERVI6]

Do you want to check that math? Isn't the rule of thirds more like

3600/3 = 1200, meaning 1200 out, 1200 back, and 1200 in reserve?

yea thats what I said, 1200 psi before turn you use 1200 psi, then turn, if I was not clear, sorry, 3600 /3 indicates rules of 3rds to me and = 1200 is how much I can use I just didn't write it all out for everyone.

 

[CD_in_Chitown]

yea thats what I said, 1200 psi before turn you use 1200 psi, then turn, if I was not clear, sorry, 3600 /3 indicates rules of 3rds to me and = 1200 is how much I can use I just didn't write it all out for everyone.

Alright, I see your shorthand now. I gotta read slower.

 

[akscubainst]

So to answer the question:

An HP 80 (3500) is about 44 psi per Cu Ft
An LP 85 (2640) is about 31.

If you're just dividing by 3 the 80/3 rounds to 27*44= 1188psi 85/3=28*31=868psi.

So your 3rds are different numbers but the actual Cubic feet used/reserve is very close to the same.

Dave

 

[limeyx]

yea thats what I said, 1200 psi before turn you use 1200 psi, then turn, if I was not clear, sorry, 3600 /3 indicates rules of 3rds to me and = 1200 is how much I can use I just didn't write it all out for everyone.

of course, the real rule of 1/3's takes into account

1) amount of gas to get you to the surface once you get back to the entry point
2) is dependent on the heaviest breather breathing from the smallest tank for the return portion of the dive.

Right?

 

[scubamanny]

In Europe LP steel tanks are routinely filled to 4000psi(the metric equivalent). Aside from DOT regulations, is there any reason why this should not be the case in the USA? Any Comments?