Integrating equation  over the control volume containing node N, and using Gauss’ theorem i.e.,
Yields the following result,
Where, A is the cross-sectional area of the control volume.
The equation must also satisfy the continuity equation, i.e.,
Now let us define variables F and D to represent the convection mass flux and diffusion conductance at cell faces,
: and      ()
Hence, equations () and () transform into the following equations:
Where, the lower case letters denote the values at the faces and the upper case letters denote that at the nodes.
We also define a non-dimensional parameter Péclet number (Pe) as a measure of the relative strengths of convection and diffusion,
For a low Peclet number (|Pe|<2) the flow is characterized as dominated by diffusion. For large Peclet number the flow is dominated by convection.