Pressure

Introduction to pressure - relationship psi and Pa- 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 expressed as:

p = F / A         (1)

where

p = pressure (lb/in2 (psi) or lb/ft2 (psf), N/m2 or kg/ms2 (Pa))

F = force (1), N)

A = area (in2 or ft2, m2)

1) In the English Engineering System special care must be taken for the force unit. The basic unit for mass is the pound mass (lbm) and the unit for the force is the pound (lb) or pound force (lbf).

relationships between absolute pressure and gauge pressure

Absolute Pressure

The absolute pressure - pabs - 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

pg = ps - patm         (2)

where

pg = gauge pressure

ps = system pressure

patm = 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 273oK (0oC) and is 1.01325 bar or 101325 Pa (absolute). The temperature of 293oK (20oC) is also used.

In imperial units the Standard Atmospheric Pressure is 14.696 psi.

  • 1 atm = 1.01325 bar = 101.3 kPa = 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 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 - 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 / 760th 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 Engineering Toolbox Apps

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Online Pressure Units Calculator

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

Value (use period as decimal point)

Pa (N/m2) bar atmosphere mm Hg mm H2O m H2O kg/cm2 pound square feet pound square inches (psi) inches Hg inches H2O

Related Topics

  • Basics - Basic Information as SI-system, Unit converters, Physical constants
  • Fluid Mechanics - The study of fluids - liquids and gases. Involves various properties of the fluid, such as velocity, pressure, density and temperature, as functions of space and time.
  • Process Control - Instrumentation and process control systems - engineering and documentation
  • Gas and Compressed Air - Gas properties, capacities of pipelines, sizing of relief valves - air, LNG, LPG and more

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