Kidde 21008873 Battery Operated Digital Display KN-Copp-B-LPM Carbon Monoxide Alarm Digital Display - Displays the level of carbon monoxide the unit is sensing. Shows CO level in PPM (parts per million). Updates digital reading every 15 seconds allowing you to see if levels change. Peak Level Memory displays the highest CO concentration measured since the last reset Battery Operated - Provides best protection during power outages. Battery Safeguard makes it difficult to close the cover without batteries in the unit. Low Battery Signal alerts user when the batteries need to be replaced. Slide-Out Battery Door Cover gives immediate access to batteries Test/Reset Button - Tests units electronic circuitry & horn and resets the unit during alarm. Tests CO alarm circuit operation and allows you to immediately silence the alarm LED's - Green LED for normal operation, Red LED for alarm Provides peace of mind against the silent killer and a digital display