HP vs. LP

[trymixdiver]

i had a set of double HP 100s.

I didnt think about it long enough and i realized i got the wrong tanks. When i did a few boat dives i was getting crappy fills and may as well have done single tank dives. Needless to say i sold them to a friend who wanted them and i got my 112's. The OMS tanks have done well, im glad i have them despite the paint issue. Mine are 8 yrs old and have never failed a vis or hydro.

Andy

get the LP

 

[omar]

The Ideal Gas Law assumes several factors about the molecules of gas. The volume of the molecules is considered to be zero, and the molecules collide in completely elastic collisions. Attractive and repulsive forces between the molecules are therefore considered negligible.

Gases, however, are never perfectly in the ideal state. All atoms of every gas have mass and volume. When pressure is low and temperature is low, gases behave similarly to gases in the ideal state. When pressure and temperature increase, gases deviate farther from the ideal state.

Pressure is a convenient but indirect way to measure the quantity of oxygen , air or helium added to a scuba tank. Typically the pressures are calculated using or mixing tables or the ideal gas laws. The calculations are not complex or difficult, the accuracy of the mix depends, to a very large extent, on the skill and care of the operator to make the necessary adjustments for temperature increase, and for the different compressibility of the gases.

An examination of compressibility factors of the typical scuba gases, oxygen, helium and air for varying pressures and temperatures will indicate that the ideal gas equation is a very simplistic approach for estimating final gas contents in a mix.

For a visual look at the compressibility of gases go to:
http://www.atomox.com/gas_graph.html

If you want to see how to deal with the compressibility of the gases go to:
http://www.atomox.com/mixing.html

Or if you are lazy and want it done for you, a spreadsheet that will show the compressibility factors and the difference between real and ideal gas calculations can be downloaded here:
http://www.atomox.com/mix_calc.html

omar

 

[roakey]

Originally posted by jetblast00
LP104's hold 104 cu ft. of gas at 2400psi
with a 10% overfill rating, you get to fill them to 2640psi
2 * 104 == 208 + 10% = 228.8 cu ft of gas (yes, ideal gas laws...)

Never underestimate the marketing department's abaility to take advantage of numbers. 104s hold 104 WITH the 10% overfill (2640 PSI).

If you lose your + rating, you now have 95s.

This was the old steel 72 problem, once you lost the 10% rating, you had 65s.

Roak

 

[jetblast00]

Originally posted by roakey

Never underestimate the marketing department's abaility to take advantage of numbers. 104s hold 104 WITH the 10% overfill (2640 PSI).

If you lose your + rating, you now have 95s.

This was the old steel 72 problem, once you lost the 10% rating, you had 65s.

Roak

Really? I believe you, but if you get 104 cu ft. at the working pressure, why is the working pressure on every LP tank manufacturer's info (not just PST) that I have seen listed as 2400?

the 2640 wouldn't then be overfill, it would be the working pressure...

 

[Rick Murchison]

Originally posted by jetblast00



Really? I believe you, but if you get 104 cu ft. at the working pressure, why is the working pressure on every LP tank manufacturer's info (not just PST) that I have seen listed as 2400?

the 2640 wouldn't then be overfill, it would be the working pressure...

Because 104 is a bigger number than 95, and people will pay more for 104 than 95.
An AL 80 holds 80 at 3100, with a working pressure of 3000, by the way. Small compressor manufacturers claim 4.2 SCFM fill rate for their 3 SCFM compressors..
It goes on and on...
Rick

 

[padiscubapro]

Originally posted by jetblast00
Of course, DIR says LP 104's... you already know that.

LP104's hold 104 cu ft. of gas at 2400psi
with a 10% overfill rating, you get to fill them to 2640psi
2 * 104 == 208 + 10% = 228.8 cu ft of gas (yes, ideal gas laws...)

as oppsed to HP100's
2 * 100 = 200 cu ft. gas no overfill.

you probably know the above too...

I ussually fill my LP 104's to 3000psi, but I have seen people plug the burst disks and fill them to 3500psi too. (I start to get nervous around there though. I wouldn't do it to my own tanks, and really wouldn't recommend this...)
104 cu ft / 2400psi * 3000psi == 130 cu ft
130 * 2 == 260 cu ft of gass

104 cu ft / 2400psi * 3500psi == 151.7 cu ft
151.7 cu ft * 2 == 303.4 cu ft of gass

yea, the LP's are a bit bigger, and the HP's do weigh less, but the HP's are more negative in the water.

With Tech 1, it probably means helium soon (do they do that in Tech 1?) which means you have to start thinking about real gas laws anyway if you're going to be filling them yourself. (I've never blended helium, but I have read that it's molecular size means real gas laws come into play more than with O2 or N2...)

Sorry you have some mistakes..
the rated pressure is 2400, but the stated capacity is only true with a 10% overfill.. a 104 is only 104@2640 - neglecting real gas laws.. If you take that into account your actual volume is dependent on what percentage the oxygen, nitrogen and (if present helium is)

 

[jetblast00]

Originally posted by padiscubapro


Sorry you have some mistakes..
the rated pressure is 2400, but the stated capacity is only true with a 10% overfill.. a 104 is only 104@2640 - neglecting real gas laws.. If you take that into account your actual volume is dependent on what percentage the oxygen, nitrogen and (if present helium is)

Yes, I stated I was assuming ideal gas laws... But then roak pointed out the same issue of the 104 being at 2640. This I wasn't previously aware of...

So... let's try this again, for LP104 doubles:
104 cu ft / 2640psi * 2400psi == 94.5 cu ft
94.5 * 2 == 189 cu ft of gas

104 cu ft / 2640psi * 2640psi == 104 cu ft
104 * 2 == 208 cu ft of gas

104 cu ft / 2640psi * 3000psi == 118.2 cu ft
118.2 * 2 == 236.4 cu ft of gas

104 cu ft / 2640psi * 3500psi == 137.9 cu ft
137.9 cu ft * 2 == 275.8 cu ft of gas

Of course, the above is assuming ideal gas laws, which should be pretty close to the actuals for air or EANx, He in the mix will make the actual volumes a little less.
Is this correct now?

 

[padiscubapro]

Originally posted by jetblast00


Yes, I stated I was assuming ideal gas laws... But then roak pointed out the same issue of the 104 being at 2640. This I wasn't previously aware of...

So... let's try this again, for LP104 doubles:
104 cu ft / 2640psi * 2400psi == 94.5 cu ft
94.5 * 2 == 189 cu ft of gas

104 cu ft / 2640psi * 2640psi == 104 cu ft
104 * 2 == 208 cu ft of gas

104 cu ft / 2640psi * 3000psi == 118.2 cu ft
118.2 * 2 == 236.4 cu ft of gas

104 cu ft / 2640psi * 3500psi == 137.9 cu ft
137.9 cu ft * 2 == 275.8 cu ft of gas

Of course, the above is assuming ideal gas laws, which should be pretty close to the actuals for air or EANx, He in the mix will make the actual volumes a little less.
Is this correct now?

I wasn't trying to be anal, The 2400 thing is a common misconception many people don;t realize if they don't get a 10% "overfill" they aren't at the "rated" volume..

 

[devjr]

Jet, the relative volumes of specific gases will always be those specified for the mix. True, there is no corresponding volume and pressure for different compressed gases. Some pack more free gas volume, and some less, even when measuring identical tank pressures. The mixers adjust volume by calculating and adjusting the partial pressures using compressibility coefficients to determine the exact pressures needed to balance the respective volumes.

Otherwise, yes, a snootfull of He will have less volume than some other gases, and one way to match them would be to raise the He pressure. This can have feedback problems because the overall tank pressure is raised. Most mixers probably raise one and lower another depending on the gas.

We've gotten off on a tangent, but what the heck.

 

[neil]

Originally posted by VTWarrenG

HP tanks put more stress on regulators. As I understand, regs are made to work properly at WELL over 3000 psi.

Warren,
I don't think so.
The only closing force in your first stage is intermediate pressure. In a balanced diaphragm reg, the IP is the same no matter what the tank pressure is (of course, within the range of the reg's design). The seat is held closed by the spring behind it, not air pressure. I doubt very much (and have never noticed in regs I've rebuilt) that HP air rushing past the seat does any damage to it. So I wouldn't worry about using the HP tanks in terms of reg seat longevity.

Neil