Adsorption-desorption conductance, transit

A useful concept is that of transit conductance, which is a measure of the time it takes to get a specific molecule from one place to another. In mass flow control, the adsorption-desorption mechanism may delay the effect of changes in the MFC as a function of position, particularly if the chamber is crowded. ... 

Conductance, transit (vacuum technology) The ability of a specific atom or molecule to pass from one end of a path to the other. For some species this may involve adsorption-desorption events (e.g. water vapor) while for other species (e.g. Argon) it only involves non-interactive collisions. 

Asymmetry of the response curve to the point of the exposition end reflects the different nature of the exposition and relaxation output signals. A transition from an exposition into relaxation phase corresponds to a return of gas-sensitive matter contact with the initial atmosphere. A variety of processes take place simultaneously in that phase. They may include oxidation of adsorbed molecules by the air oxygen, desorption of the previously adsorbed molecules, competitive adsorption of the ambient atmosphere components. These circumstances cause a complicated shape of the relaxation curve. In general, its course reflects the dynamics of the surface concentration of conductivity clusters. Almost all relaxation curves are characterized by presence of a maximum. It is often more prominent that the corresponding exposition maximum. The origin of this phenomenon is determined by higher conductivity of clusters formed by the oxidized molecules of compounds adsorbed during the exposition phase. 

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