The Morrison Fig. 548 pressure/vacuum vent with gauge hatch is designed for "normal" venting of Aboveground Storage Tanks. It allows tanks to "breathe" during filling and withdrawing operations. Pressure/ Vacuum poppets seal vapors in the tank when pressure is equalized. This vent must be used in conjunction with an emergency vent and it is recommended that the opening pressure setting is set below that of the emergency vent so the normal vent relieves first.