gauge pressure
Summary
Gauge pressure is pressure measured relative to the surrounding atmospheric pressure: Pgauge = Pabs − Patm. A gauge open to the air reads zero. The most testable point is that gas laws need absolute pressure, so add Patm to a gauge value first.
Detail
Meaning: Gauge pressure is fluid pressure measured relative to the surrounding atmosphere. Absolute pressure is measured relative to a perfect vacuum. A gauge open to the air reads zero, and a negative gauge value means the pressure is below atmospheric, as in suction through a straw.
Equation: Pgauge = Pabs − Patm, all in pascals (Pa), with Patm about 1.01 × 10⁵ Pa. At depth h in an open liquid, Pgauge = ρgh, where ρ is density (kg/m³), g is 9.8 m/s², and h is depth (m).
On the MCAT: Passages use tires, blood pressure, manometers and submerged objects. Expect to convert a gauge reading to absolute pressure before using PV = nRT, or to compare pressures at two depths. When the atmosphere pushes on both sides of a wall or piston, Patm cancels, so gauge pressure gives the net force per area.
Common confusion: A gauge reading of zero is not zero pressure; it means about 1 atm absolute. Gas laws and pressure ratios need absolute pressure, so plugging a gauge value into PV = nRT gives a wrong answer.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- College Physics (Knight, Jones, Field)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.