Rotor radius ($r$)

The rotor radius $r$ is the perpendicular distance from the rotation axis to a specified sample position, not the tube's length. A filled tube spans a range $r_{\min }\le r\le r_{\max }$. Consequently, one rotational speed produces different local centrifugal accelerations along that range. A reported relative centrifugal force needs a stated reference position, such as the outermost sample radius.