# Dispersion in Chromatography Columns - The Van Deemter Equation > Page 52

Consequently, g = 0.747

Giddings (11) also determined that, for a well packed column, (g) should be about 0.6 so again the dispersion from longitudinal diffusion as measured. confirms that the column was reasonably well packed.

For a packed
column, the particle size has a profound effect on the minimum value of the
HETP of a column and thus the maximum efficiency attainable. It would also
indicate that the highest column efficiency would be obtained from the smallest
particles. However, this assumes that an unlimited pressure is available and
the apparatus can tolerate such pressures. Within practical limits of pressure,
the smaller the particle diameter, the smaller will be the minimum HETP and
thus, the larger the number of plates per unit length obtainable from the
column. However, the total number of theoretical plates that can be obtained,
will depend on the *length of the column* which, in turn, must take into
account the *available inlet pressure.*

The optimum mobile phase velocity can be obtained by differentiating equation (33) with respect to (u) and equating to zero, thus,

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and
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