Subsection 3.2.3 Gage Pressure
Since the absolute atmospheric pressure is always present, and acts on everything equally, we normally don’t even notice it. Pressures are usually measured up or down from atmospheric pressure, and an ordinary pressure gage reads zero when it is subjected to atmospheric pressure only. Pressures measured in this way is called the gage pressure. It is important to distinguish between gage pressure and the absolute pressure described in the next section.
