Editing Pressure measurement (section)

==Static and dynamic pressure==
[[Static pressure]] is uniform in all directions, so pressure measurements are independent of direction in an immovable (static) fluid. Flow, however, applies additional pressure on surfaces perpendicular to the flow direction, while having little impact on surfaces parallel to the flow direction. This directional component of pressure in a moving (dynamic) fluid is called [[dynamic pressure]]. An instrument facing the flow direction measures the sum of the static and dynamic pressures; this measurement is called the [[Total pressure (fluids)|total pressure]] or [[stagnation pressure]]. Since dynamic pressure is referenced to static pressure, it is neither gauge nor absolute; it is a differential pressure.

While static gauge pressure is of primary importance to determining net loads on pipe walls, dynamic pressure is used to measure flow rates and airspeed. Dynamic pressure can be measured by taking the differential pressure between instruments parallel and perpendicular to the flow. [[Pitot tube|Pitot-static tube]]s, for example perform this measurement on airplanes to determine airspeed. The presence of the measuring instrument inevitably acts to divert flow and create turbulence, so its shape is critical to accuracy and the calibration curves are often non-linear.

'''Example:''' A water tank has a pressure of 10 atm. The atmospheric pressure is 1 atm. What is the gauge pressure?

P_g = P_a - P_v<br>
= 10 atm - 1 atm<br>
= 9 atm

Therefore, the gauge pressure is 9&nbsp;atm.