# Pressure

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 sea-level at * 273 ^{ o } K (0 ^{ o } C) * and is

*or*

**1.01325 bar***101325 Pa (absolute)*. The temperature of

*293*is sometimes used.

^{ o }K (20^{ o }C)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*area^{ 2 }*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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