Pressure

An introduction to pressure - the definition and presentation of common units as psi and Pa and the relationship between them - 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 (lb_m) and the unit for the force is the pound (lb) or pound force (lb_f).

Absolute Pressure

The absolute pressure - p_a - is measured relative to the absolute zero pressure - the pressure that would occur at absolute vacuum. In 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_a - p_o (2)

where

p_g = gauge pressure

p_o = atmospheric pressure

Atmospheric Pressure

The 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.

• Altitude and Air Density

Standard Atmospheric Pressure

The 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^oK (0^oC) and is 1.01325 bar or 101325 Pa (absolute). The temperature of 293^oK (20^oC) 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 (lb_f/in²)= 760 mmHg =10.33 mH₂O = 760 torr = 29.92 inHg = 1013 mbar = 1.0332 kg_f/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 designing 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 at a depth of 1 mm of water
• 1 kPa - Approximately the pressure exerted by a 10 g mass on a 1 cm² area
• 10 kPa - The pressure at a depth of 1 m of water, or
  the drop in air pressure when going from sea level to 1000 m elevation
• 10 MPa - A "high pressure" washer forces the water out of the nozzles at this pressure
• 10 GPa - This pressure forms 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 (torr) 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. The Normal atmospheric pressure is 14.696 psi, meaning that a column of air on one square inch in area rising from the Earth's atmosphere to 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.

1 millibar = 0.001 bar = 0.750 torr = 100 Pa

Pressure Units Online 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

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