Calibration The Filter Minder ® air filter service indicator measures vacuum in the air induction system in 'inches of water vacuum'. The Filter Minder ® is calibrated per engine or vehicle manufacturer's recommendations. Most diesel engines operate at a maximum 20 to 25 inches water vacuum under full load. Gasoline engine calibrations can vary depending on the engine size and air cleaner design. Small gas engines (2-5 HP) should usually limit restriction to 3 inches maximum, 6-25 HP should limit filter restriction to 8" maximum, and most gasoline engine powered automobiles 20 inches maximum. Some vehicles with the Filter Minder ® as standard equipment may use a higher calibration than the above guidelines. If in doubt, contact Engineered Products Company for recommendations. Diesel suction fuel filters are usually monitored in inches of mercury. The maximum restriction is typically 10-12 inches of mercury. Engineered Products Co. 2940 Airport Blvd. Waterloo, IA 50703 Ph: (319) 234-0231 Fax: (319) 234-8922 info@filterminder.com