Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

# 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/in2(psi), lb/ft2(psf), N/m2, kg/ms2(Pa))

F = force (N) 1)

A = area (in2, ft2, m2)

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 ( lbf ).

### 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 oC) and is 1.01325 bar or 101325 Pa (absolute) . The temperature of 293 o K (20 oC) is sometimes used.

In imperial units the Standard Atmospheric Pressure is 14.696 psi.

• 1 atm = 1.01325 bar = 101.3 kPa = 1.013 105 Pa = 14.696 psi (lbf /in2)= 760 mmHg =10.33 mH2O = 760 torr = 29.92 inHg = 1013 mbar = 1.0332 kgf /cm2= 33.90 ftH2O

### 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 cm2 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

### Related Mobile Apps from The Engineering ToolBox

- free apps for offline use on mobile devices.

## Related Topics

• ### Basics

Basic engineering data. SI-system, unit converters, physical constants, drawing scales and more.
• ### Fluid Mechanics

The study of fluids - liquids and gases. Involving velocity, pressure, density and temperature as functions of space and time.
• ### Gases and Compressed Air

Properties of air, LNG, LPG and other common gases. Pipeline capacities and sizing of relief valves.
• ### Process Control Systems

Instrumentation, design and documentation of process control systems.

## Related Documents

• ### ABS Pipes - Pressure Ratings

Pressure ratings of ABS 1208, ABS 1210, ABS 1316 and ABS 2112.
• ### Acetone - Thermophysical Properties

Chemical, physical and thermal properties of acetone, also called 2-propanone, dimethyl ketone and pyroacetic acid. Phase diagram included.
• ### Air - Specific Heat vs. Pressure at Constant Temperature

Figures and tables with isobaric (Cp) and isochoric (Cv) specific heat of air at constant temperature and pressure ranging 0.01 to 10000 bara.
• ### Atmospheric Pressure vs. Elevation above Sea Level

Elevation above sea level - in feet and meter - with barometric and atmospheric pressure - inches mercury, psia, kg/cm2 and kPa.
• ### Barometer - Altitude Compensation

Elevation compensating manometer.
• ### Benzene - Thermophysical properties

Chemical, physical and thermal properties of benzene, also called benzol. Phase diagram included.
• ### Compression and Expansion of Gases

Isothermal and isentropic gas compression and expansion processes.
• ### Duct Systems - Pressure Classifications

Duct systems are commonly divided into three pressure classifications.
• ### Hydraulic Force vs. Pascal's Law

Pascal's law and the hydraulic force acting in fluids.
• ### Hydrostatic Pressure vs. Depth

Depth and hydrostatic pressure.
• ### International Standard Atmosphere

International standard atmosphere in elevation -2000 to 30000 metre - pressure, temperature, density, viscosity, thermal conductivity and velocity of sound.
• ### Pressure Energy

Calculate the potential of pressure energy in a incompressible fluid.
• ### Pressure to Head Unit Converter

Pressure vs. head units - like lb/in2, atm, inches mercury, bars, Pa and more.
• ### Pressure Units - Online Converter

Convert between pressure units like Pa,  bar, atmosphere, pound square feet, psi and more.
• ### Pressure Units Converter

Convert from kPa to psi and inches of Water and Mercury.
• ### SI System

An introduction to the SI metric system.
• ### Steam Flash Generation (bar)

The amount of flash steam generated depends on steam pressure and pressure in the condensate lines.
• ### Steam Pipes - Online Pressure drop Calculator

Calculate pressure drops in steam distribution pipe lines.
• ### U.S. Standard Atmosphere vs. Altitude

Properties of the US standard atmosphere ranging -5000 to 250000 ft altitude.
• ### Unit Converter with commonly used Units

Common converting units for Acceleration, Area, Density, Energy, Energy per unit mass, Force, Heat flow rate, Heat flux, Heat generation per unit volume and many more.

## Search

Search is the most efficient way to navigate the Engineering ToolBox.

## Engineering ToolBox - SketchUp Extension - Online 3D modeling!

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with older versions of the amazing SketchUp Make and the newer "up to date" SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp Make/Pro from the Extension Warehouse !

We don't collect information from our users. More about

We use a third-party to provide monetization technologies for our site. You can review their privacy and cookie policy here.

You can change your privacy settings by clicking the following button: .

## Citation

• The Engineering ToolBox (2004). Pressure. [online] Available at: https://www.engineeringtoolbox.com/pressure-d_587.html [Accessed Day Month Year].

Modify the access date according your visit.

7.25.9

.