With the optional extension package for series connection, the two pressure tanks of the mixed-bed polisher can be operated in series. The sequence of the flow can be freely selected, the switchover is manual. In addition to the connecting piping in PP and two additional ball valves per pressure tank, the extension package includes an additional Thornton conductivity measuring cell between the pressure tanks, which is connected to the 2-channel conductivity measuring device of the Polisher MB.