air reserve on lp96 vs n80

[FIXXERVI6]

OK, thinking about going to a LP 95 or LP 104 steel tank. My question is this. if on an aluminum tank, your reserve is suppose to be 500psi... well look at this below

My current tank:
N80
77.4 cube feet (an al80's internal space)
3300 PSI

so

3300/77.4 = 42.64 PSI to make 1 cubic feet of air
500 psi reserve = 11.73 cubic feet of air in reserve

so on a LP95
2400/95 = 25.26 PSI to make 1 cubic feet of air
296 psi reserve = 11.73 cubic feet of air in reserve

The usuable air in the 80 after reserve is about 65 cubic feet, the useable air in the 95 with a 500psi reserve is about 75 cubice feet, only 10 cube gain, however, if you keep the same cubic feet in reserve, and not psi, the lp95 will have 83 cubic feet of usible air.

These numbers are rounded some so they are not 100% dead on.

Should a person run a lp 95 down to 300psi for reserve, or stick with the 500psi and lose that 8 cube of air. ?

or are my calculations wrong?

 

[docmartin]

good point. rationally your thinking is right on. but you are up against the immutable law of round numbers. the 500psi reserve is somewhat arbitrary anyway but it is nice and round. after all in europe they will tell you the reserve is 50 bar which equals 725 psi. 300psi just does not look as pretty. the other factor to consider may be the accuracy of the gauge. if you breathe the tank down to 300psi you better have an accurate gauge. i think it is not that uncommon that they are a couple of hundred psi off in the lower pressure range.

 

[MikeFerrara]

Why 500 psi?

You need enough to get you and a buddy to the surface by whatever path makes the most sense in the environment. It might be 500 psi (in some tank) or it might be 1500 psi.

Wouldn't you think they'd teach this to folks in class?

 

[MechDiver]

FIXXERVI6 once bubbled...

so on a LP95
2400/95 = 25.26 PSI to make 1 cubic feet of air
296 psi reserve = 11.73 cubic feet of air in reserve

LP tanks are at capacity at rated pressure - 2640psi - not the 2400 you sited above. I consider 3000psi a normal fill in my LP95s.

And, as others have already said, the 500psi is not "unusable" air.

MD

 

[FIXXERVI6]

Thats why I am asking, for OW that that is 60' and shallower 500psi is plenty, granted, I'm not talking about taking this thing into a cave and sucking it down to 300psi or taking it into a wreck or something, I'm trying to compare a 95 to an 80, I'm just using this example as an example. the amount of breathable gas you have is not psi, its cubic feet, thats why I did the conversion, 300 psi in a lp95 is the same amount of air as in a standard 80 at 500 psi.

 

[5615mike]

MikeFerrara once bubbled...
Why 500 psi?

You need enough to get you and a buddy to the surface by whatever path makes the most sense in the environment. It might be 500 psi (in some tank) or it might be 1500 psi.

Wouldn't you think they'd teach this to folks in class?

Up through my AOW specifics such as this were never discussed. So, have others had such specifics discussed up through including AOW? Size of tanks, PSI vs. cft have never been brought up. Shed some light on this Mike. Are you saying you teach this info at this level? Is it a requirement or are you.....if you are.......doing it b/c you believe it is valuable info for students? I'm interested to know if I was short changed on my education to that point.

 

[FIXXERVI6]

2640 with the + rating, so the say 95 cubes at 2640 with the +

sneaky bastards

 

[FIXXERVI6]

so 325 psi in a LP95 gives the same cubic feet of air as in an 80 at 500 psi.

So if I'm not diving in an overhead and stuff is not an issue, 325 psi in reserve on a lp 95 should be fine.

 

[FIXXERVI6]

in a LP104 300psi would give the same volume of gas, so for safetys' sake we say 350 psi reserve in a lp95 and 325 in a 104, does that sound right?

I stated above in my calculations that the reserve is not usable gas.

 

[MechDiver]

FIXXERVI6 once bubbled...
in a LP104 300psi would give the same volume of gas, so for safetys' sake we say 350 psi reserve in a lp95 and 325 in a 104, does that sound right?

I stated above in my calculations that the reserve is not usable gas.

My point was that any gas in the tank is usable. The only unusable gas is whatever your first stage can't suck out of the tank.

And "psi" does equal usable gas. If you don't have any, you don't breathe. Also, IMO, if you're that worried about 50 or 100 psi difference in your tank at the end of the dive, you need to re-evaluate your diving style. Thats what gas management is about.

MD