## Pressure

Pressure is the application of force to a surface, and the concentration of that force on a given area. A finger can be pressed against a wall without making any lasting impression; however, the same finger pushing a thumbtack can easily damage the wall, even though the applied force is the same, because the point concentrates that force on a smaller area. More formally, pressure (symbol: p or P) is the measure of the normal component of force that acts on a unit area.

 1 Pa Pascal (SI unit) = 1 Newton/m2 (= N/m2) 1 kPa Kilopascal (SI unit) = 1,000 N/m2 1 MPa Megapascal (SI unit) = 1,000,000 N/m2 100,000 Pa 100 kPa 1 bar bar (accepted by SI) = 0.1 MPa 750.06 mm Hg 14.5 psi 1.013 bar 760 mm Hg 1 atm physical atmosphere 1.033 kp/cm2 (= at) = 14.696 psi 1 ata atmospheres absolute 10.08 metres sea water 33.07 feet sea water (= fsw) 33.90 feet fresh water 1 mm Hg millimetres of mercury = 133.32 Pa 1 psi pounds per square inch = 0.069 bar 1 psig psi gauge pressure

In the literature different pressure units are mentioned even though there have been international agreements regarding standardized nomenclature for many years. Following this international standardization (SI) the units 'Pascal' [Pa], 'Kilopascal' [kPa] or 'Megapascal' [MPa] should be used (SI units), and the unit 'Bar' [bar] is accepted. Nevertheless in hyperbaric medicine you still will find old units (ata) or imperial units (psi, fsw). In many countries, mm Hg is still used for blood pressure and blood gases.

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