Once every 8 seconds indicates that the sensing unit is dirty and needs cleaned. From the manual:
To clean the alarm, remove it from the mounting base. You can clean the interior using compressed air or a vacuum cleaner. Blow or vacuum through the openings around the perimeter of the alarm. The outside of the alarm can be wiped with a damp cloth. After cleaning, test the alarm by pushing the Test button. If cleaning does not restore the alarm to normal operation you need to replace the alarm.