  The structure and dynamics of steady state sedimentation of semi-concentrated
($\phi=0.10$) monodisperse spheres are studied in liquid fluidized beds. Laser
turbidity and particle imaging methods are used to measure the particle
velocity fluctuations and the steady state concentration profiles. Using a wide
range of particle and system sizes, we find that the measured gradients $\nabla
\phi$, the fluctuation magnitudes $\sigma_v$, and their spatial correlation
lengths $\xi$, are not uniform in the columns - they all show strongly
$z-$dependent profiles. These profiles also display a scaling in which results
from different particle sizes collapse together when plotted in the forms
$-a\nabla \phi(z)$, $\xi(z)/a$, and $\sigma_v(z)/v_p$, demonstrating the
universality of the particle dynamics and structure in steady state
sedimentation. Our results are also used to test a recently proposed model for
the correlation lengths $\xi(z)$ in terms of the concentration stratification
$\nabla \phi(z)$ [P.J. Mucha and M.P. Brenner, Phys. Fluids {\bf 15}, 1305
(2003)], $\xi(z)=c_0 a[\phi S(\phi)]^{1/5}[-a\nabla\phi(z)]^{-2/5}$. We find
that the correlation lengths predicted by this model are in very good agreement
with our measured values, showing that the origin of the fluctuation length
$\xi$ lies with the concentration stratification $\nabla \phi$.
