Bel-Art Triple Holder Clamp for PiRack Polypropylene Pipettor Holder System Hold a single pipette with one clamp or add a triple clamp or extenders to increase the number of holders at a work location. utility tray mounts between two clamps; ideal for storing pipetting supplies or other small items perfectly sized for no wire half racks that hold tubes, vials, mini tubes and cuvettes series 18748 attach to any shelf or benchtop edge up to 2.