Consider a point (x) along the column,
The HETP equation, as derived by Van Deemter, is seen to apply only to a particular point distance (x) from the inlet of the column where the pressure is constant.
Now, applying the gas laws
Pxux
= Pouo
or 
where (Px) is the pressure at point (x) along the column,
(ux) is the linear velocity of the mobile phase at point (x),
