Maytronics Dolphin Spring Cartridge Filter Panel [Qty 4] (9991433 ASSY) lighting Flow meters are useful because Flow meters are useful because they make it easier to confirm that circulation or feed conditions are where they should be during normal operation or service checks A check valve is useful because it helps keep water moving in the intended direction while reducing the chance of reverse flow through the line these filter panels help support dependable debris capture and better overall robotic cleaning performance and this Maytronics Dolphin version is supplied in a quantity of four using the 9991433 ASSY reference