The kinetic limited mass flux due to evaporation at the liquid–vapor interface surrounded by an air– vapor mixture is given as: mass flux = 2σ/(2-σ) *(M/2piRT)^0.5 * (Pv,eq -Pv) where Pv,eq is the saturated vapor pressure of liquid and Pv is the partial pressure of vapor in the gas phase very close to the liquid vapor interface. The question is, in real evaporation process, there is a mass diffusion boundary layer very near the interface. In other words, how do we know Pv? Pv could be very small far away from the interface, and becomes bigger and bigger as it approaches the interface. If this Pv cannot be determined accurately, how is this equation useful?