I need a measuring gadget.

I have a difficult time determining the size (mm) of beads. I can tell the difference between 6mm and 8mm. The problem is with 2.4mm and 2.5mm or 3mm. I wish there were a bead sorting gadget similar to a coin separator. It would be nice to toss the beads in the top and the sorter would dispense them to the appropriate container.