how much air in the tank?

[Bogie]

This thread gives me gas.

 

[Teamcasa]

The calculations are fun but since most divers rely solely on what their SPG or computer says, that's all it will be, just for fun information.

 

[bfisher]

Math is good and all, but I've got my own answer. When breathing from a regulator when it starts to feel like I'm sucking a milk shake though a straw I know it's getting low.

 

[Garrobo]

My formulae is a lot easier. With an AL80 I get 45+ minutes bottom time on a shallow dive and about 25+ bottom time minutes on a deep dive wreck dive. If I am deep I go up at 1000psi, shallow at least 500. Works every time.

 

[Gombessa]

My understanding is that you can ignore deviations from the ideal gas laws for nitrox, but when helium comes into the equation, the low compressibility requires gas blenders to take into account the actual behavior from the real gas laws.

For those who don't want to put up with the forum's cranky formatting of tab-separated tables, this chart from Vance Harlow's Oxygen Hacker's Companion may be easier to understand.

From this chart, you can see why EAN36 compresses better than does air - not all gasses compress less than the ideal gas laws describe, and not at all pressures.

 

[oldnotdead]

I'll admit it, I'm a mouth breather!

 

[Crush]

My understanding is that you can ignore deviations from the ideal gas laws for nitrox, but when helium comes into the equation, the low compressibility requires gas blenders to take into account the actual behavior from the real gas laws.

For those who don't want to put up with the forum's cranky formatting of tab-separated tables, this chart from Vance Harlow's Oxygen Hacker's Companion may be easier to understand.

From this chart, you can see why EAN36 compresses better than does air - not all gasses compress less than the ideal gas laws describe, and not at all pressures.

The differences between the compressibility of O2 and N2 can be seen in the (empirical) van der Waals gas coefficients. The "volume" of the gas particles indicates incompressibility. For N2, b[N2] = 0.0387 while for O2, b[O2] = 0.0319. N2 is less "compressible" at higher pressures (where the b term dominates). Also note that the attractiveness of these gases, given by a, is such that N2 is less attractive to N2 (a[N2] = 1.370) while O2 is more attractive to O2 (a[O2] = 1.382). For He: a[He] = 0.034 (attractiveness, dominates at low P) and b[He] = 0.024 (incompressibility, dominates at high P). He is more easily compressed at high P, but it is not as easily compressed (compared to less ideal gases) at low P.

 

[watboy]

You observed as in took measurements? Or ran calculations under the two models?

Consider yourself blessed.

1. The pressure gauge is right. The assumption that the pressure is directly proportional to the amount of gas is incorrect, but that error is, in fact, small.

2. At low pressures real gasses are more "compact" than ideal gasses. This means that, at pressures below say 3000 psi, there are more gas particles in your tank than is predicted by the ideal gas law.

3. At high pressures real gasses are less "compact" than ideal gasses. This means that, at pressures above say 3000 psi, there are fewer gas particles in your tank than is predicted by the ideal gas law.

A practical example: which contains more gas mix - one "80L tank" charged to 2000 psi or one "40L tank," charged to 4000 psi? Answer: one "80L tank" charged to 2000 psi since at pressures above 3000 psi the gas gets less compressible. It took fewer gas particles to raise the pressure from 2000 psi to 4000 psi than it took to raise it from 0 to 2000 psi. The pressure went up to 4000 psi, it is just that it took fewer gas particles to achieve that than you would have thought. Putting less gas in to raise the pressure from 2000 to 4000 psi means you get less gas out as you reduce the pressure (by breathing) from 4000 psi to 2000 psi.

 

[koozemani]

Huh?!? Is the right answer purple.

 

[LoonDiver]

The "Basic" Scubing Diving forum??? I already knew I don't know much...now I think I'm in big trouble...