Absolute and Gauge Pressure

Every point in a fluid has an absolute pressure: the full, real pressure felt there, counting every contribution. Near Earth's surface, that includes the weight of the atmosphere itself, about 101325 Pa, pressing down on everything. Most pressure gauges, though, are built to ignore that background atmospheric push. A gauge reads only the difference between the pressure inside whatever it's measuring and the atmospheric pressure outside pushing back on the gauge. That difference is called gauge pressure, and it's related to absolute pressure by one simple equation: absolute pressure = gauge pressure + atmospheric pressure.

This is why a tire gauge reads zero on a flat tire. A flat tire isn't a vacuum. Air still fills it at roughly atmospheric pressure, pushing out just as hard as the atmosphere pushes in. The gauge reports zero because there's no difference between inside and outside, not because there's no air at all. A true vacuum, with zero absolute pressure, would actually show a negative gauge reading, about -101325 Pa, since the gauge is comparing against full atmospheric pressure.

This matters directly for the pressure-with-depth idea that comes right before it. When you find the pressure at the bottom of a pool using P = atmospheric pressure + (density)(g)(depth), that result is absolute pressure, because it includes the atmosphere sitting on top of the water. The (density)(g)(depth) part alone is the gauge pressure, the extra squeeze from the water itself. Keeping these two labeled clearly becomes essential once buoyancy and flowing fluids start combining pressures from multiple sources.