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Concept of Pressure
Pressure is one of the most important concepts in the study of hydrostatic. To understand what is pressure, take the example of a box of a weight of 10N on a table area of 2 m². This box, as we know, applies a certain force on the table, and sometimes we need to know the strength that each part of that area is supporting. The ratio of the force that the block applies on the table and the area of the table that is supporting it is
10 N / 2 m² = 5 N/m²
This means that each m² of the table is being compressed with a force of 5N. This force is shared equally by the surface, hence each 1 m² of the surface withstand a force of 5N (10N / 2). So, we define pressure as 'the amount of force that is applied over a given area divided by the size of this area' and can be stated as follows:
P = F/A, where: F = the applied force and A = the area that the force is beeng applied.
Pressure is a scalar quantity and its unit in the International System of Units (SI) is the Pa, named after Blaise Pascal. The unit of force is Newton (N) and the unit area is m², both in SI. So, the ratio between force and area results in N/m².
1 N/m² = 1 Pascal = 1 Pa
Other usual units are millibars, psi (pound per square inch), torr (mmHg), atm (atmospheric pressure) and bar.
Atmospheric pressure: the weight of the Earth's atmosphere on the surface. It always decreases with altitude.
Hydrostatic pressure: the stress, exerted equally in all directions at points within a confined fluid. ‘Hydrostatic pressure’ refers to the pressure that a liquid exerts when it is at rest. The pressure at a given depth in a static liquid is a result the weight of the liquid acting on a unit area at that depth plus any pressure acting on the surface of the liquid.
The pressure exerted by a confined gas results from the average effect of the forces produced on the walls of the container by the continual, numerous collisions by gas molecules.
Pressure Conversion Table
Bellow is a handy conversion table. These units are mostly used in industrial environment.
|From / To||Bar||Mbar||Pa||KPa||MPa (torr)||Kgf/cm2||Mm hg||mm wc||PSI||1" wc|
|1 mm Hg||0.00133||1.333||133.63||0.13332||0.1333x10-3||0.001359||1||13.5951||0.01934||0.53524|
|1 mm wc||0.000098||0.098||9.807||0.009807||9.807106||0.001||0.07356||1||0.0012||39.3701x103|
Pressure Unit Conversion Factors
The following list shows each pressure unit and its equivalent value converted into Pascals (Pa) (the SI unit for measuring pressure).
|Unit of Pressure||Value in Pascals (SI unit for pressure)|
|psipfsilb/in², orlbf/in² (Pounds per Sq. Inch or Pounds Force per Sq. Inch)||6894.75729316836...|
|mb or mbr (Millibar)||100|
|N/m² (Newton per Square Metre)||1|
|Pa (Pascal)||1 (SI unit for pressure)|
|kg/cm² or kgf/cm² (Kilogram per Square Centimeter)||98,066.5|
|atm (Standard Atmosphere)||101,325|
|at (Technical Atmosphere)||98,066.5|
|dyn/cm², dyne/cm² or dynes/cm² (Dyne per Square Centimeter)||0.1|
|oz/in² or ozf/in²||430.9223308230...|
|tsi or tfsi (Tons per Sq.e Inch or Tons Force per Sq.e Inch (uk, long))||15,444,256.33669...|
|tsi or tfsi ( Tons per Sq. Inch or Tons Force per Sq. Inch (usa, short))||13,789,514.58633...|
|tsf or tfsf (Tons per Sq. Foot or Tons Force per Sq. Foot (usa, short))||95,760.51796067...|
|psf or pfsf (Pounds per Sq. Foot or Pounds Force per Square Foot)||47.88025898033...|
|g/cm² or gf/cm² (Gram per Sq. Centimetre or Gram Force per Sq. cm)||98.0665|
[...] means a rounded value.