Pressure (symbol: p or P) is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. The standard SI unit for pressure measurement is the Pascal (Pa). Different types of pressure measurement exist. These include gauge pressure, vacuum, absolute, barometric, and differential pressure.
Even the highest quality instruments are subject to drift over time, resulting in inaccurate measurements and substandard performance. It is very important that all instruments are calibrated by trained, competent and approved personnel.