This ChairPro Safety Alarm System comes with a firm foundation with a built in sensor. The Skil-Care Leveling Pad Alarm System combines an alarm with a firm foundation for all cushions. Designed to fit wheelchairs 16" -18" wide, the Skil-Care Leveling Pad eliminates sling-seat hammocking by making wheelchair sling seats flat and level with the chair frame to provide a firm foundation for the addition of a Skil-Care cushion (sold separately). The solid closed cell foam is covered with a incontinent proof vinyl for an added level of antibacterial protection.