A manometer comprises a manometer body with an indicating scale with graduated markings of pressure units and a safety vent, a plunger with a annular groove and a tinted O-ring a cap mount, a primary box spring and an auxiliary box spring. By means of the tinted O-ring and the indicating scale with graduated markings of pressure units, the manometer innovatively functions as indication of air pressure measured. By means of the primary box spring and auxiliary box spring, the manometer precisely measures overall air pressure in combination of coarse air pressure variation responded by the primary box spring and fine air pressure variation responded by the auxiliary box spring. By means of the safety vent disposed on the peripheral of manometer body and the tinted O-ring on the plunger, the manometer provides a safety proactive means for input compressed air.