# Extra Column Dispersion - Low Dispersion Connecting Tubes > Page 28

Equation (14) is the same as the function derived from the Golay equation for a simple straight tube when operated at low mobile phase velocities. However, at higher velocities, the momentum of the fluid as it changes direction while flowing round a curved tube, introduces the radial flow that breaks up the parabolic velocity profile. As a consequence, the dispersion is reduced. At high linear velocities, Tijssen deduced that,

_{} (15)

where (b) is a
constant for a given mobile phase and (f)
is the *coil* *aspect ratio*
(the ratio of the tube radius to the coil radius),

* i.e*., _{}

Equation (14)
shows that, at high velocities, (H) is now determined by (D_{m}) to the
power of 0.14 and is inversely proportional to the coil aspect ratio and linear
velocity. It follows, that, at low velocities, the band dispersion *increases*
with (u), whereas at high velocities, the band dispersion *decreases* with
(u). It follows that a curve relating (H) to (u) will show a maximum at a
certain value of (u). The equation giving the value of (u) at which (H) is a
maximum was deduced by Tijssen to be,