# Extra Column Dispersion - Dispersion in Tubular Conduits > Page 25

As already
discussed, the diameter of the connecting tube, however, should not be made less
than 0.012 cm, (0.005 in. I. D.), not because of the pressure drop that will
occur across it, but because tubes of less diameter *will
easily become blocked. *Employing
equation (12) the volume variance and the volume standard deviation
contribution from connecting tubes of different lengths can be calculated and
the results are shown in figure 9A and
9B. The tube radius is assumed
to be 0.012 cm, the flow rate 1 ml per minute, and the diffusivity of
the solute in the mobile phase 2.5 x 10^{-5}
cm^{2}/sec.

**Figure 9
The Volume Variance and the Volume Standard Deviation Contribution from
Connecting Tubes of Different Lengths**