What Does the Air Weigh?

[rsterne]

Ever wondered how much of the energy in a PCP goes into accelerating the air along with the bullet?.... Lloyd just added that calculation to his Internal Ballistics Spreadsheet, and it's quite an eye-opener.... I'll just give you a few of examples....

.25 cal with a 20" barrel, running regulated on 2000 psi, 25.4 gr. pellet at 900 fps, with an efficiency of 1.5 FPE/CI.... Weight of air in barrel = 9.15 gr. (36% of pellet weight)....

Same gun, but keep the valve open until the pellet reaches the muzzle.... Velocity 1079 fps, efficiency drops to 0.66 FPE/CI.... Weight of air in barrel = 29.9 gr. (more than the pellet)....

.308 cal with a 24" barrel, running on 3000 psi, 120 gr. bullet at 900 fps, with an efficiency of 1.0 FPE/CI.... Weight of air in barrel = 64.7 gr. (54%)....

If you wonder why Helium makes such a big difference in velocity.... take the last example, and replace the air with Helium (no other changes)....

.30 cal with a 24" barrel, 3000 psi of Helium.... That 120 gr. bullet will now hit 1033 fps, at an efficiency of 1.22 FPE/CI.... because the mass of He is only 9.62 gr. (8%)...

Bob

[rsterne]

Here is another visualization I just did.... It's only approximate, and only in 2 dimensions, but it will give you an idea what happens when you compress air....



The black dots represent an air molecule (oxygen or nitrogen).... They average about 4 Angstroms across (0.0000004 mm, and yeah, they are not spheres).... In a liquid (lower left), they are pretty much touching, but vibrating and changing positions, not locked into a crystal structure like in a solid.... As a gas, at 1 bar pressure, they average about 40 Angstroms apart (upper part of the diagram).... As you increase the pressure, they get closer together.... At 125 bar (1800 psi, lower right), they are about 8 Angstroms apart, about 1/5th the spacing at 1 bar (roughly enough room for one more molecule between each pair).... Since we are working in 3 dimensions, the volume occupied by a given number of molecules is the cube of the spacing.... so we have (1/5 x 1/5 x 1/5) = 1/125th the volume.... because the pressure is 125 times as great....

You may have heard me state that at very high pressures the pressure goes up faster than Boyle's Law for ideal gasses predicts.... What is happening is that the space between the molecules gets so small that they start developing additional pressure from hitting or interfering with each other, and not just the walls of the container.... It is called the VanDerWaals effect, and this diagram shows you how that might be occurring.... Now imagine each and every one of those molecules travelling in random directions at an average speed of 1650 fps.... like a crowded pool table gone mad.... in 3 dimensions....

Bob

[Whitewolf]

Interesting analogy in itself. Even though frictional forces, temperature and elevation are left out of the equation. It does definitely get the point across upon the lighter mass of gases used..