Extra Column Dispersion - Low Dispersion Connecting Tubes > Page 31

Figure 11. Graph of Total Variance of Tube 1 m Long against Number of Coils


Scott and Simpson took a 1 m length of tubing, 0.010 in I.D., and measured the dispersion initially in the form of a straight tube and then after it had been made into coils of different diameter. This resulted in a set of tubes (all of the same linear length) but bent into coils of different radius. The variance (in ml2) is shown plotted against the number of turns in the coil in figure 11. This simple experiment demonstrates the profound effect of coiling the tube on the consequent dispersion, It is clear that the introduction of radial flow by the change in direction of the linear flow as it passes round the coils greatly increases the effective diffusivity and reduces the dispersion. After about 50 turns have been introduced into the coil, however, the radial flow effect appears to tend towards a maximum and further coiling has only a small effect on dispersion.