A small amount of polymer-particles with diameters in the 100 um range can be used as an ensemble of "microdivers" to study, sedimentation velocities of polydisperse submicron paiticles in concentrated opaque suspensions. This tracer technique, based on Berg's (1941) diver method, is simple, measures velocities directly, and does not require underlying theory, beyond conservation of momentum. It confirms that the polymer tracers reside at the position in the suspension where the suspension density corresponds to their own density. Aqueous inorganic suspensions of 250 - 750 nm alpha -alumina particles were studied, which is electrostatically, stabilized with nitric acid in water, using - 100 mum polymethylmetacrylate and polystyrene particles as tracers. Experiments were compared with predictions by, state-of-the-art continuum models for multicomponent sedimentation to underpin the validity, of the tracer technique.