The digital readout of the Kidde Plug-In Digital Carbon Monoxide Alarm with 9-Volt Backup continuously displays carbon monoxide levels from 30 to 999 parts per million (PPM) and will sound an 85dB alarm when dangerous levels are reached. The carbon monoxide level sensed is continuously displayed and updated every 15-seconds and the peak level memory shows the highest level of carbon monoxide present since the last time the unit was reset or unplugged. Protection continues during power outages thanks to a 9-Volt battery backup.