Intermixing of two or more gases in available space is called diffusion. Diffusion of gas from high pressure to low pressure through an orifice is called effusion. The rate of effusion is : $\mathrm{r}=\frac{\mathrm{PA}}{\sqrt{2\mathrm{\pi RTm}}}$
A = Area of cross-section of orifice;  P = Pressure of gas filled in the cylinder 
T = Absolute temperature;   
R = Universal gas constant 
m = Molar mass of the gas
Effusion time of a gas is directly proportional to the square root of molar mass,
$\frac{{\mathrm{t}}_{\mathrm{effuse}}\mathrm{A}}{{\mathrm{t}}_{\mathrm{effus}}\mathrm{B}}=\sqrt{\frac{{\mathrm{m}}_{\mathrm{A}}}{{\mathrm{m}}_B}}$.
Rate of diffusion/effusion is experimentally measured by number of moles of gas or volume of gas diffused per unit time or distance travelled by the gas molecules per unit time.