# Plate Theory and Extensions - The Compressibility of the Mobile Phase > Page 24

Equation (23)
allows the ratio _{} to be calculated along the length
of the column for different inlet/outlet pressure ratios.
Using equation

(23), the
values of _{} at different points along the
column were

calculated and the results shown
as curves relating _{} to fraction of the column length
are shown in Figure 4.

**Figure 4.
Graph Showing Change of Pressure Along a GC Column for Different Inlet/Outlet
Pressure Ratios**

** **

At low
inlet/outlet pressure ratios (*i.e., **g* < 2), the pressure falls almost linearly
from the inlet to the outlet of the column. At high inlet/outlet pressure
ratios, (*i.e*., g > 2), the
relationship is no longer linear and the pressure falls very rapidly in the
latter half of the column. It is obvious that the average flow rate can not be
used for measuring retention volumes and a correction factor must be employed.