Advanced Intelligence Detects Both Fast Flaming and Slow Smoldering Fires. AI-Artificial Intelligence Techniques Used to Classify Smoke and Carbon Monoxide Profiles. Microprocessor Virtually Eliminates Nuisance Alarms Caused by Everyday Cooking Smoke, Steam, etc. Detection of Polyurethane Foam in both Slow Smoldering and Fast Flaming Fires. Immunity to High-Voltage Current Surges. No Batteries to change. Lithium-Free 10-year Permanent Power. Meets New UL 217 8th Edition Requirements.Multi-criteria detectionSmoke and carbon monoxide sensors conform to new UL 217 8th Edition standardsHardwired with lithium-free 10-year battery backupTamper-proof design prevents battery removalNo batteries to change-everLithium-free 10-year battery backupAlkaline batteries are safer and more eco-friendlyLED status indicatorInter-connectable up to 24 USI unitsBase diameter: 5-5/8 in.