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.

Table 1.1-1. Pressure units

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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