Pressure
Introduction to pressure  online pressure units converter.
The pressure in a fluid is defined as
"the normal force per unit area exerted on a imaginary or real plane surface in a fluid or a gas"
The equation for pressure can be expressed as:
p = F / A (1)
where
p = pressure (lb/in^{2} (psi), lb/ft^{2} (psf), N/m^{2}, kg/ms^{2} (Pa))
F = force (N) ^{1)}
A = area (in^{2}, ft^{2}, m^{2})
^{1)} In the Imperial  English Engineering System special care must be taken for the force unit. The basic unit for mass is slug and the unit for force is pound (lb) or pound force (lb_{f}).
Absolute Pressure
The absolute pressure  p_{abs}  is measured relative to the absolute zero pressure  the pressure that would occur at absolute vacuum. All calculations involving the gas law requires pressure (and temperature) to be in absolute units.
Gauge Pressure
A gauge is often used to measure the pressure difference between a system and the surrounding atmosphere. This pressure is often called the gauge pressure and can be expressed as
p_{g} = p_{s}  p_{atm} (2)
where
p_{g} = gauge pressure (Pa, psi)
p_{s} = system pressure (Pa, psi)
p_{atm} = atmospheric pressure (Pa, psi)
Atmospheric Pressure
Atmospheric pressure is the pressure in the surrounding air at  or "close" to  the surface of the earth. The atmospheric pressure varies with temperature and altitude above sea level.
Standard Atmospheric Pressure
The Standard Atmospheric Pressure (atm) is normally used as the reference when listing gas densities and volumes. The Standard Atmospheric Pressure is defined at sealevel at 273^{o}K (0^{o}C) and is 1.01325 bar or 101325 Pa (absolute). The temperature of 293^{o}K (20^{o}C) is sometimes used.
In imperial units the Standard Atmospheric Pressure is 14.696 psi.
 1 atm = 1.01325 bar = 101.3 kPa = 1.013 10^{5} Pa = 14.696 psi (lb_{f}/in^{2})= 760 mmHg =10.33 mH_{2}O = 760 torr = 29.92 inHg = 1013 mbar = 1.0332 kg_{f}/cm^{2} = 33.90 ftH_{2}O
Pressure Units
Since 1 Pa is a small pressure unit the unit hectoPascal (hPa) is widely used, especially in meteorology. The unit kiloPascal (kPa) is commonly used in the design of technical applications  like HVAC systems, piping systems and similar.
 1 hectoPascal = 100 Pascal = 1 millibar
 1 kiloPascal = 1000 Pascal
Some Pressure Levels
 10 Pa  the pressure below 1 mm of water
 1 kPa  approximately the pressure exerted by a 10 g of mass on a 1 cm^{2} area
 10 kPa  the pressure below 1 m of water, or the drop in air pressure when moving from sea level to 1000 m elevation
 10 MPa  nozzle pressure in a "high pressure" washer
 10 GPa  pressure enough to form diamonds
Some Alternative Units of Pressure
 1 bar  100,000 Pa
 1 millibar  100 Pa
 1 atmosphere  101325 Pa
 1 mm Hg  133 Pa
 1 inch Hg  3386 Pa
A torr (often used in vacuum applications) is named after Torricelli and is the pressure produced by a column of mercury 1 mm high  equals to 1 / 760^{th}^{ }of an atmosphere.
 1 atm = 760 torr = 14.696 psi = 1.013 bar
Pounds per square inch (psi) was commonly used in the U.K. but is now replaced in almost every country except in the US by SI units. Since atmospheric pressure is 14.696 psi  a column of air on a area of one square inch area from the Earth's surface to the space  weights 14.696 pounds.
The bar (bar) is commonly used in the industry. One bar is 100,000 Pa, and for most practical purposes can be approximated to one atmosphere even if
1 bar = 0.9869 atm = 14.5 psi
There are 1000 millibar (mbar) in one bar, a unit common in meteorology and weather applications.
1 millibar = 0.001 bar = 0.750 torr = 100 Pa
Download kPa to bar, psi, mmH2O and inH2O chart
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