Perigo Under Drain Test Screens are an essential tool that every field technician and owner or operator of sand media filters should have. The screens are Stainless Steel and made to fit a 4" groove connection. They are used to locate underdrain failures in sand media filter tanks. A screen is placed at the discharge location at each tank in the system and will catch sand that is being passed. These screens are designed for troubleshooting only and should not be used for prolonged periods of time.