Sedimentation equilibrium refers to one type of experiment performed on the ultracentrifuge. In this experiment the a relatively low centrifugal field is set up, and the solution is allowed to come to equilibrium. At equilibrium, the flow of solute due to sedimentation is equal to the flow of solute due to diffusion. This sets up a stable exponential concentration gradient. This gradient may be analyzed to give the reduced apparent molecular weights, σ's, of the solutes.
The Sednterp program helps to interpret σ's found by other programs (such as NONLIN). The primary interpretation of σ is to compute the molecular weight of the solute particle. The molecular weight, in turn, is necessary for further interpretations.