# 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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