# Pressure

##
Introduction to pressure - relationship *psi* and *Pa*- online pressure units converter

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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 expressed as:

p = F / A(1)

where

p= pressure (lb/in^{2}(psi) or lb/ft^{2}(psf), N/m^{2}or kg/ms^{2}(Pa))

F= force (^{1)}, N)

A= area (in^{2}or 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 slugs 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 calculation involving the gas laws 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

p_{s}= system pressure

p_{atm}=atmospheric pressure

### Atmospheric Pressure

Atmospheric pressure is pressure in the surrounding air at - or "close" to - the surface of the earth. The atmospheric pressure vary with temperature and altitude above sea level.

#### Standard Atmospheric Pressure

**Standard Atmospheric Pressure** (*atm*) is used as a reference for 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 also 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 = 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 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 - 101,325 Pa**1 mm Hg - 133 Pa**1 inch Hg - 3,386 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*

**Pounds per square inch** (psi) was common in U.K. but has now been replaced in almost every country except in the U.S. by the 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 common 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

There are 1,000 **millibar** (mbar) in one bar, a unit common in meteorology and weather applications.

1 millibar = 0.001 bar = 0.750 torr = 100 Pa

### Related Mobile Apps from The Engineering ToolBox

- free apps for offline use on mobile devices.

### Online Pressure Units Calculator

The calculator below can used to convert between some common pressure units:

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