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Pounds per square inch
 
 
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The pound per square inch or, more accurately, pound-force per square inch (symbol: psi or lbf/in² or lbf/in²) is a unit of pressure
Pressure

Pressure is the force per unit area applied to an object in a direction surface normal to the surface. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure....
 or of stress
Stress (physics)

In continuum mechanics, stress is a measure of the average amount of force exerted per unit area. It is a measure of the intensity of the total internal forces acting within a body across imaginary internal surfaces, as a reaction to external applied forces and body forces....
 based on avoirdupois
Avoirdupois

The avoirdupois system is a system of Units of measurement based on a pound of sixteen ounces. It is the everyday system of weight used in the United States....
 units. It is the pressure resulting from a force of one pound-force
Pound-force

The pound-force or simply pound is a Units of measurement of force....
 applied to an area of one square inch
Square inch

A square inch is a Units of measurement of area, equal to the area of a Square with sides of one inch. The following symbols are used to denote square inches:...
:

: pascal (Pa) is the SI
Si

Si, si, or SI may refer to :...
 unit of pressure.

r abbreviations are used that append a modifier to "psi".






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Psidial
The pound per square inch or, more accurately, pound-force per square inch (symbol: psi or lbf/in² or lbf/in²) is a unit of pressure
Pressure

Pressure is the force per unit area applied to an object in a direction surface normal to the surface. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure....
 or of stress
Stress (physics)

In continuum mechanics, stress is a measure of the average amount of force exerted per unit area. It is a measure of the intensity of the total internal forces acting within a body across imaginary internal surfaces, as a reaction to external applied forces and body forces....
 based on avoirdupois
Avoirdupois

The avoirdupois system is a system of Units of measurement based on a pound of sixteen ounces. It is the everyday system of weight used in the United States....
 units. It is the pressure resulting from a force of one pound-force
Pound-force

The pound-force or simply pound is a Units of measurement of force....
 applied to an area of one square inch
Square inch

A square inch is a Units of measurement of area, equal to the area of a Square with sides of one inch. The following symbols are used to denote square inches:...
:

: pascal (Pa) is the SI
Si

Si, si, or SI may refer to :...
 unit of pressure.

Relation to other measures

Other abbreviations are used that append a modifier to "psi". However, the US National Institute of Standards and Technology
National Institute of Standards and Technology

The National Institute of Standards and Technology , known between 1901 and 1988 as the National Bureau of Standards , is a measurement standards laboratory which is a non-regulatory agency of the United States Department of Commerce....
 recommends that, to avoid confusion, any modifiers be instead applied to the quantity being measured rather than the unit of measure For example, "Pg = 100 psi" rather than "P = 100 psig".
  • psia (pounds-force per square inch absolute) — gauge pressure plus local atmospheric pressure. Replace "x psia" with "Pa = x psi".
  • psid (psi difference) — difference between two pressures. Replace "x psid" with "?P = x psi".
  • psig (pounds-force per square inch gauge). Replace "x psig" with "Pg = x psi".
  • psivg (psi vented gauge) — difference between the measuring point and the local pressure. Replace "x psivg" with "Pvg = x psi".
  • psisg (psi sealed gauge) — difference between a chamber of air sealed at atmospheric pressure and the pressure at the measuring point. Replace "x psisg" with "Psg = x psi".


psig


Psig (pound-force per square inch gauge) is a unit of pressure relative to the surrounding atmosphere. By contrast, psia measures pressure relative to a vacuum (such as that in space).

At sea level, Earth
Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
's atmosphere
Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low....
 actually exerts a pressure of 14.7 psi (see below). Humans do not feel this pressure because internal pressure of liquid in their bodies matches the external pressure. If a pressure gauge is calibrated to read zero in space, then at sea level on Earth it would read 14.7 psi. Thus a reading of 30 psig on a tire gauge represents an absolute pressure of 44.7 psi.

Psi is often used incorrectly instead of psig.

ksi and kip

The ksi (kip
Kip (unit)

In the United States, a kip is a unit of force that equals 1,000 pound-force, used primarily by architects and engineers to measure engineering loads....
, "kilo-pound[-force] per square inch") is 1000 psi, combining the prefix kilo with the psi abbreviation. It is occasionally used in materials science
Materials science

Materials science or materials engineering is an interdisciplinary field involving the properties of matter and its applications to various areas of science and engineering....
 and mechanical engineering
Mechanical engineering

Mechanical Engineering is an engineering discipline that involves the application of physics#branches of physics for analysis, design, manufacturing, and maintenance of machine....
 to specify stress
Stress (physics)

In continuum mechanics, stress is a measure of the average amount of force exerted per unit area. It is a measure of the intensity of the total internal forces acting within a body across imaginary internal surfaces, as a reaction to external applied forces and body forces....
 and Young's modulus
Young's modulus

In solid mechanics, Young's modulus is a measure of the stiffness of an isotropic elastic material. It is also known as the Young modulus, modulus of elasticity, elastic modulus or tensile modulus....
.

Magnitude

  • Atmospheric pressure at sea level (standard): Pa = 14.7 psi
  • Automobile tire overpressure (common): Pg = 32 psi
  • Bike tire overpressure (common): Pg = 65 psi
  • Air brake (rail)
    Air brake (rail)

    An air brake is a conveyance brake applied by means of Gas compressor. Modern trains rely upon a fail-safe air brake system that is based upon a design patented by George Westinghouse on March 5, 1872....
     or Air brake (road vehicle)
    Air brake (road vehicle)

    Air brakes are used in trucks, buses, Trailer , and semi-trailers. George Westinghouse first developed Air brake for use in railway service. A safer air brake was patented by him on March 5, 1872....
     reservoir overpressure (common): 90 psi = Pg = 120 psi
  • Road racing bicycle tire overpressure: Pg = 120 psi
  • Natural gas pipelines: 800 to 1000 psi
  • Full SCBA Self Contained Breathing Apparatus for toxic atmospheres: 2216 psi
  • Full scuba
    Scuba set

    A scuba set is an independent breathing set that provides a scuba diver with the breathing gas necessary to breathe underwater during scuba diving....
     tank overpressure (common): Pg = 3,000 psi
  • Commercial jet airliner hydraulic pressure: 3000 psi
  • Airbus A380
    Airbus A380

    The Airbus A380 is a Double-deck aircraft, wide-body, four-engine jet airliner manufactured by the European corporation Airbus, a subsidiary of EADS....
     hydraulic system: 5000 psi


Conversions


See also

  • Conversion of units
    Conversion of units

    Conversion of units refers to conversion factors between different units of measurement for the same quantity....
  • Pressure
    Pressure

    Pressure is the force per unit area applied to an object in a direction surface normal to the surface. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure....


External links