Average Velocity of Sublimation
From a sublimation rate, one can calculate the average velocity of water vapor exiting a vial. This is not the same as molecular velocity, but is analogous to "wind speed". It is easy to obtain a sublimation rate in gm / (m2 x s). That value can be directly measured by opening the lyophilizer before sublimation is complete and measuring the weight remaining in a vial or collection of vials if an average is desired. Because sublimation is a zero order reaction, a one point (in time) measurement is sufficient to obtain the sublimation rate.
The expected value will be on the order of 0.02 to 0.08 gm/(cm2 x hr). Alternatively, the average sublimation rate can be measured from a graph of product ice temperature versus time. This assumes that the thermocouple temperature is coming from the bottom of the vial. The amount sublimed in grams is approximately the total water in the vial.
Assuming an ideal gas approximation for the amount of water vapor in any element of volume above the lyophile (cake), one can know how much water vapour can be in that volume element.