high pressure vs. low pressure

[dumpsterDiver]

In my experience many people will over fill their LP (2400 psi, 2640 psi with +) tanks as a standard practice. I, for what ever reason, feel comfortable to have the LP Faber tanks filled to 3100 or 3200, which is what I have my aluminum tanks filled to also.

In "the real world" LP tanks are commonly over-filled, hp tanks are commonly underfilled due to the compressor limitations and hot fills. A 3000 psi fill on a LP tank is a decent fill, 3000 in an HP (after it cools) is not.

 

[limeyx]

All good points made here by everyone! And you Fixxer....I want to party with you...3900 on LP...You Go Boy! :bang:

The original question was turn around point as it relates to pressure. The turn around point will change as the use of Hp or LP changes as well as the 1/3 rule. However, regardless of pressure, it still goes back to the volume of the tank and not the pressure per say. What we are trying to achieve is 1/3 of our gas based on the size of our tank as it relates to the tanks pressure. Am I makin any sense at all here? If not you all jump in and go with it.

An 80 cu/ft tank at 3000 will have a 1/3 at 1000 psi and a volume of about 26 cu/ft.
A 100 cu/ft tank at 3440 will have a 1/3 at 1146 psi and a volume of about 33 cu/ft.

If we took that same 100 cu/ft tank and could get 100 cu/ft at 3000 psi, our 1/3 would still be 1000 psi but we would now have about 33 cu/ft of gas instead of our 26 with our 80.

But you have to remember what the extra 1/3 is used for.

Imagine the "worst case"
you swim out to your maximum psi (1/3) and your buddy goes OOA. You now need to swim back to where you came from.

So now you have you and your buddy breathing off of your tank.

If your tank is smaller than theirs, or they breathe more gas per minute than you do, then you are in trouble.

SO you take the smallest tank, and take 1/3 of that as the maximum cubic feet anyone can use. This can then be converted to psi in a given persons tank *based on how many cft/min they breathe at depth*

all the initial examples on this thread showed just dividing the psi by 3 which doesn work.

You also need to consider getting back to the surface too once you get back (as you;d need to share air on the way up). For a shallow cave that might be negligible. For a deep wreck, it could be a lot of gas

 

[gracefulc]

OK, I appreciate everyone's input on this and trying to address some other questions regarding using HP vs LP tanks but my question is really not anything as complex as you all are going into...

Here is my question, perhaps phrased better this way: HP Steel 80 vs LP Steel 80; am I going to have any difference in my dive time?

Potential scenario: I'm diving an LP 80, pressure of 2600(we'll say that's full). My buddy is diving an HP 80, filled to 3400 (we'll say that is also full)
Since everyone always asks how much pressure you have (not how much cubic feet you have) when diving, at 1/3 of the way thru our tanks, I'm only going to have 1734 psi left while my buddy has 2267 psi. Doesn't this mean less bottom time? Or with the lower pressure is the SPG going to drop slower than the SPG of someone on an HP?

 

[FIXXERVI6]

OK, I appreciate everyone's input on this and trying to address some other questions regarding using HP vs LP tanks but my question is really not anything as complex as you all are going into...

Here is my question, perhaps phrased better this way: HP Steel 80 vs LP Steel 80; am I going to have any difference in my dive time?

Potential scenario: I'm diving an LP 80, pressure of 2600(we'll say that's full). My buddy is diving an HP 80, filled to 3400 (we'll say that is also full)
Since everyone always asks how much pressure you have (not how much cubic feet you have) when diving, at 1/3 of the way thru our tanks, I'm only going to have 1734 psi left while my buddy has 2267 psi. Doesn't this mean less bottom time? Or with the lower pressure is the SPG going to drop slower than the SPG of someone on an HP?

your making it too complicated, 80 cubic feet is 80 cubic feet, if a tank is rated for 80 cubic feet at 10,000 psi and its filled to capacity it contains the exact same volume of gas as an 80 cubic foot tank at 200 psi thats rated for 80 @ 200, 80 cubes is 80 cubes.

a HP 80 is 80 at 3442 (pst) 1/3 is about 1147 so your turn is about 2295 psi, your PSI is not your volume, the buddy on the LP tank's pressure guage will drop slow, the LP buddy will turn at 1760, if you have 2295 psi left in a HP 80 and your buddy has 1760 left in a LP 80 you are both at your turn based on 3rds. You seam to be hung up on the PSI forget about it, look at the volume.

Just go the easy route, you and your buddy buy LP 80's and pump them to 3600 and you'll be set.

 

[meekal]

I'm diving an LP 80, pressure of 2600(we'll say that's full).

that's actual 10% over fillled so you 'acutally' have 88 PSI..... minor detail but......


in summary
the LP is going to drop slower that the HP - so - assuming the same consumption rate, you will reach your respective 1/3's at the same 'time'

 

[FIXXERVI6]

that's actual 10% over fillled so you 'acutally' have 88 PSI..... minor detail but......


in summary
the LP is going to drop slower that the HP - so - assuming the same consumption rate, you will reach your respective 1/3's at the same 'time'

a LP 80 is 80 @ 2640, LP tanks are rated with cubic feet at 10% overfill

 

[DEEPLOU]

your making it too complicated, 80 cubic feet is 80 cubic feet, if a tank is rated for 80 cubic feet at 10,000 psi and its filled to capacity it contains the exact same volume of gas as an 80 cubic foot tank at 200 psi thats rated for 80 @ 200, 80 cubes is 80 cubes.

a HP 80 is 80 at 3442 (pst) 1/3 is about 1147 so your turn is about 2295 psi, your PSI is not your volume, the buddy on the LP tank's pressure guage will drop slow, the LP buddy will turn at 1760, if you have 2295 psi left in a HP 80 and your buddy has 1760 left in a LP 80 you are both at your turn based on 3rds. You seam to be hung up on the PSI forget about it, look at the volume.

Just go the easy route, you and your buddy buy LP 80's and pump them to 3600 and you'll be set.

I was about to say that (80 cuft is 80 cuft) when I came to your post.
Well said.

As to the labels LP vs HP. We have tanks rated at 2015, 2400, 2640 (2400 w/ +), 3000, 3200, 3300, 3442, 3500, 4350.

So who decides where the label LP or HP goes?

 

[meekal]

a LP 80 is 80 @ 2640, LP tanks are rated with cubic feet at 10% overfill

dang... you DO learn something new everyDay.... my bad.... thanks !

 

[meekal]

So who decides where the label LP or HP goes?

http://en.wikipedia.org/wiki/Atmospheric_pressure

Air masses are affected by the general atmospheric pressure within the mass, creating areas of high pressure (anti-cyclones) and low pressure (depressions).

oops... never mind... wrong forum.... hehehehehe

 

[meekal]

http://www.diveriteexpress.com/library/tanks.shtml#pressure - pretty good summary overall.... might help... can't hurt