An equation for the amount of gas that flows through a tube in free molecule flow, q√(2π)Δpd³/ 6l √ρ, where q is the volume of gas measured at unit pressure that flows through the tube per second, Δp is the difference between the pressures at the ends of the tube, d is the inside diameter of the tube, l is the length of the tube, and ρ is the density of the gas at unit pressure.