Designed to monitor the condition of your air filters and advise you when they need changing. There is a programmable change light indicator that will light when the programmed level is reached. The light can be programmed to come on from 0 - 40"H2O Normal is a setting of 16" or 18". After changing the filters, the gauge automatically resets its self. Requires a 1/4" air line. Mounts in a standard 2 1/16" hole.