# Extra Column Dispersion - Dispersion in Tubular Conduits > Page 23

Now, the peak Variance in volume
units is (_{}) is given by

_{}

_{}

_{} (10)

Where (n) is the number of theoretical plates in the tube.

Now, = H. Thus, replacing () by (H), and substituting for (H) from equation (9),

_{}

_{ }

_{ } (11)

Bearing in mind that the flow rate

(12)

Equation (12)
clearly indicates the variables that control the dispersion that arises from tubular conduits. The flow rate, (Q),
is chosen to provide the maximum *column* efficiency and thus cannot be
used as a variable to control tube dispersion. Similarly, the diffusivity of
the solute, (D_{m}),is determined by the nature of the
sample and the mobile phase and is also not a variable available for dispersion
control. The major factor effecting tube dispersion is the tube radius. The
dispersion increases as the fourth power of the tube radius and thus, a
reduction in the tube radius by a factor of 2 will reduce the dispersion by a
factor of sixteen.