Fresh air system: how to detect indoor pressure on site

At present, most of the fresh air systems we use follow the design idea of large air volume and micro positive pressure, but many people are also confused. How to detect the indoor pressure on site? If the fresh air system compares the air volume to the swing force of a weapon, the wind pressure is the sharpness of the weapon. After the fresh air system is turned on, is the indoor positive pressure or negative pressure? How big is the pressure difference? How to measure? Here we introduce a simple and easy measurement method for your reference only.