Filling an AL 80 from a larger tank???

[knotical]

By Jove Charlie99, you’ve got it!

I first had to Google a bit on “Van der Waals” and “air”, but now I’m on board.

Pretty dramatic difference.

Thank you!

 

[scubafool]

OK, now I am not so much confused as I am ignorant. Ideal gas curves wern't covered in my OW course. :

 

[Charlie99]

OK, now I am not so much confused as I am ignorant. Ideal gas curves wern't covered in my OW course. :

the Ideal Gas Law, aka the General Gas Law is a combination of what is taught in most scuba courses as Boyle's Law and Charles's Law, with a touch of Dalton's for good luck.

PV=nRT or PV=NkT are the typical forms.

P is absolute pressure.
V is volume of the container.
n and N are number of moles or molecules of the gas
R and k are constants (k is the common Boltzmann's constant, R= k * Avogadro's number).
T is absolute temp.

http://www.babilim.co.uk/pages/gas_laws.html is a good discussion, so I won't bother repeating any of it.

The helium analyzer designer/manufacturer, atomox, has some good info also, including a table that shows how various gases deviate from the straightline pressure vs free volume plot. On the Atomox "Z-tables", an ideal gas would have 1.000 for every entry.

 

[lamont]

this is what i get using the beattie-bridgeman equations of state. i'm treating the O2 and N2 in the air as non-interacting, I don't have exact coefficients for air:

444 cu ft tank @ 4500 psi = 523.025133672509 moles, 43.5848163031276 L
after al80 fill = 431.849130602394 moles, 3615.80613773164 psi
next fill = 2835.36483059597

What i did was:

- find the number of moles in 444 cu ft of air at 1 atm
- find the water volume of the tank by packing that many moles to 4500 psi
- find the number of moles in 77.4 cu ft of air at 1am
- find the water volume of an AL80 by packing that many moles to 3000 psi
- take storage tank moles minus al80 moles to find moles left after one fill
- take that many moles and the combined water volumes to find pressure after next fill

roughly:

my $tankfullmoles = get_bb_air_moles(1.0, cuft_to_L(444), 293, 1e-9);
my $tankwatervol = get_bb_air_volume($tankfullmoles, psi_to_atm(4500), 293, 1e-9);
my $al80moles = get_bb_air_moles(1.0, cuft_to_L(77.4), 293, 1e-9);
my $al80watervol = get_bb_air_volume($al80moles, psi_to_atm(3000), 293, 1e-9);
my $afterfillmoles = $tankfullmoles - $al80moles;
my $nextfillpress = get_bb_air_pressure($afterfillmoles, $al80watervol + $tankwatervol, 293);

EDIT: and i'm ignoring gauge vs. absolute pressures...

EDIT2: with the ideal gas law and the same numbers and procedure i get 2945 psi

EDIT3: i notice now that my compressibility factors for He, N2 and O2 are spot-on in agreement with the atomox site, but I disagree quite a lot on air, so I'm probably doing something wrong...

 

[fmw625]

WHAT! They didn't teach you this in OW? LOL

 

[lamont]

I found b-b coefficients for air and plugged those in and I get these numbers now:

444 cu ft tank @ 4500 psi = 518.409855821682 moles, 44.2397499619546 L
after al80 fill = 427.216849388056 moles, 3557.51206113902 psi
next fill = 2768.8274366624

i still can't regenerate the numbers for air on that atomox chart though, which is very odd...

 

[scubafool]

I guess I'm just a :dunce:. lamont, your explanation made everything just as clear as, well, mud. :confused_

I think I had better go read charlie's links. See ya in an hour or two.

 

[Charlie99]

this is what i get using the beattie-bridgeman equations of state. i'm treating the O2 and N2 in the air as non-interacting, I don't have exact coefficients for air:EDIT3: i notice now that my compressibility factors for He, N2 and O2 are spot-on in agreement with the atomox site, but I disagree quite a lot on air, so I'm probably doing something wrong...

Did you forget that air has 0.94% argon? Not sure that it matters, but considering how nasty argon is from both a narcotic effect and its effect on decompression, its strange how much we ignore almost 1% of what we breathe.

Anyway, I think I'll just go back to the first line of my first post:

The math below ignores the errors caused by the pressure-volume curve of air not being a perfect straight line.

and leave the rest up to you.

 

[lamont]

Did you forget that air has 0.94% argon? Not sure that it matters, but considering how nasty argon is from both a narcotic effect and its effect on decompression, its strange how much we ignore almost 1% of what we breathe.

Anyway, I think I'll just go back to the first line of my first post: and leave the rest up to you.

For the first pass, I was ignoring Ar, but for the second pass I'm just reading off of experimental tables for "air", which I would hope includes Ar and all the rest...

 

[jbd]

Therefore something in your original post doesn’t seem to compute. Did you overfill the first AL80? Are the gauges accurate? Was the supply tank full to start? etc. etc.

Based on all the math posted, what I saw on the guages, possible effects of temperature changes, and the scale on the guage(0-8500) I think it all comes pretty close.

Just wish I had the spare cash for the other two or three 444's to complete the set up.

Huge thanks to Charlie99 and lamont for all the math!!!!