The pascal (symbol: Pa) is the unit of
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
in the
International System of Units (SI), and is also used to quantify
internal pressure,
stress
Stress may refer to:
Science and medicine
* Stress (biology), an organism's response to a stressor such as an environmental condition
* Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...
,
Young's modulus
Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied le ...
, and
ultimate tensile strength. The unit, named after
Blaise Pascal, is defined as one
newton per
square metre and is equivalent to 10
barye (Ba) in the
CGS system. The unit of measurement called
standard atmosphere (atm) is defined as 101,325 Pa.
Common multiple units of the pascal are the hectopascal (1 hPa = 100 Pa), which is equal to one
millibar, and the kilopascal (1 kPa = 1000 Pa), which is equal to one centibar.
Meteorological observations typically report
atmospheric pressure
Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...
in hectopascals per the recommendation of the
World Meteorological Organization
The World Meteorological Organization (WMO) is a specialized agency of the United Nations responsible for promoting international cooperation on atmospheric science, climatology, hydrology and geophysics.
The WMO originated from the Intern ...
, thus a standard atmosphere (atm) or typical sea-level air pressure is about 1013 hPa. Reports in the United States typically use
inches of mercury
Inch of mercury (inHg and ″Hg) is a non- SI unit of measurement for pressure. It is used for barometric pressure in weather reports, refrigeration and aviation in the United States.
It is the pressure exerted by a column of mercury in heigh ...
or
millibars
The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is defined as exactly equal to 100,000 Pa (100 kPa), or slightly less than the current average atmospheric pressure on Earth at sea lev ...
(hectopascals). In Canada these reports are given in kilopascals.
Etymology
The unit is named after
Blaise Pascal, noted for his contributions to hydrodynamics and hydrostatics, and experiments with a
barometer
A barometer is a scientific instrument that is used to measure air pressure in a certain environment. Pressure tendency can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis ...
. The name pascal was adopted for the SI unit newton per square metre (N/m
2) by the 14th
General Conference on Weights and Measures in 1971.
[Minutes of the 14. General Conference on Weights and Measures](_blank)
1971, page 78
Definition
The pascal can be expressed using
SI derived units, or alternatively solely
SI base units, as:
:
where N is the
newton, m is the
metre
The metre (British spelling) or meter (American spelling; see spelling differences) (from the French unit , from the Greek noun , "measure"), symbol m, is the primary unit of length in the International System of Units (SI), though its prefi ...
, kg is the
kilogram, s is the
second, and J is the
joule
The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applie ...
.
One pascal is the pressure exerted by a force of magnitude one newton perpendicularly upon an area of one square metre.
Standard units
The unit of measurement called an
atmosphere or a standard atmosphere (atm) is . This value is often used as a reference pressure and specified as such in some national and international standards, such as the
International Organization for Standardization
The International Organization for Standardization (ISO ) is an international standard development organization composed of representatives from the national standards organizations of member countries. Membership requirements are given in Art ...
's
ISO 2787 (pneumatic tools and compressors),
ISO 2533 (aerospace) and
ISO 5024
ISO is the most common abbreviation for the International Organization for Standardization.
ISO or Iso may also refer to: Business and finance
* Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007
* Is ...
(petroleum). In contrast,
International Union of Pure and Applied Chemistry
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
(IUPAC) recommends the use of 100 kPa as a standard pressure when reporting the properties of substances.
Unicode
Unicode, formally The Unicode Standard,The formal version reference is is an information technology standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems. The standard, wh ...
has dedicated code-points and in the
CJK Compatibility
CJK Compatibility is a Unicode block containing square symbols (both CJK and Latin alphanumeric) encoded for compatibility with East Asian character sets. In Unicode 1.0, it was divided into two blocks, named CJK Squared Words (U+3300–U+337F) ...
block, but these exist only for backward-compatibility with some older ideographic character-sets and are therefore
deprecate
In several fields, especially computing, deprecation is the discouragement of use of some terminology, feature, design, or practice, typically because it has been superseded or is no longer considered efficient or safe, without completely removing ...
d.
Uses
The pascal (Pa) or kilopascal (kPa) as a unit of pressure measurement is widely used throughout the world and has largely replaced the
pounds per square inch (psi) unit, except in some countries that still use the
imperial measurement system
The imperial system of units, imperial system or imperial units (also known as British Imperial or Exchequer Standards of 1826) is the system of units first defined in the British Weights and Measures Act 1824 and continued to be developed thro ...
or the
US customary system
United States customary units form a system of measurement units commonly used in the United States and U.S. territories since being standardized and adopted in 1832. The United States customary system (USCS or USC) developed from English unit ...
, including the United States.
Geophysicists use the gigapascal (GPa) in measuring or calculating tectonic stresses and pressures within the
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
.
Medical
elastography measures tissue stiffness non-invasively with
ultrasound
Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies ...
or
magnetic resonance imaging, and often displays the
Young's modulus
Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied le ...
or
shear modulus
In materials science, shear modulus or modulus of rigidity, denoted by ''G'', or sometimes ''S'' or ''μ'', is a measure of the elastic shear stiffness of a material and is defined as the ratio of shear stress to the shear strain:
:G \ \stackre ...
of tissue in kilopascals.
In
materials science and
engineering
Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad range of more speciali ...
, the pascal measures the
stiffness,
tensile strength
Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials t ...
and
compressive strength of materials. In engineering the megapascal (MPa) is the preferred unit for these uses, because the pascal represents a very small quantity.
The pascal is also equivalent to the SI unit of
energy density, the joule per cubic metre. This applies not only to the thermodynamics of pressurised gases, but also to the energy density of
electric
Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by ...
,
magnetic, and
gravitational fields.
The pascal is used to measure
sound pressure.
Loudness is the subjective experience of sound pressure and is measured as a
sound pressure level (SPL) on a logarithmic scale of the sound pressure relative to some reference pressure. For sound in air, a pressure of 20 μPa is considered to be at the
threshold of hearing for humans and is a common reference pressure, so that its SPL is zero.
The airtightness of buildings is measured at 50 Pa.
In medicine, blood pressure is measured in
millimeters of mercury
A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high, and currently defined as exactly pascals. It is denoted mmHg or mm Hg.
Although not an S ...
(mmHg, very close to one
Torr
The torr (symbol: Torr) is a unit of pressure based on an absolute scale, defined as exactly of a standard atmosphere (). Thus one torr is exactly (≈ ).
Historically, one torr was intended to be the same as one " millimeter of merc ...
). The normal adult blood pressure is less than 120 mmHg systolic BP (SBP) and less than 80 mmHg diastolic BP (DBP). Convert mmHg to SI units as follows: 1 mmHg = 0.13332 kPa. Hence normal blood pressure in SI units is less than 16.0 kPa SBP and less than 10.7 kPa DBP. These values are similar to the pressure of water column of average human height; so pressure has to be measured on arm roughly at the level of the heart.
Hectopascal and millibar units
The units of atmospheric pressure commonly used in
meteorology
Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did no ...
were formerly the
bar, which was close to the average air pressure on Earth, and the millibar. Since the introduction of
SI units, meteorologists generally measure pressures in hectopascals (hPa) unit, equal to 100 pascals or 1 millibar. Exceptions include Canada, which uses kilopascals (kPa). In many other fields of science, prefixes that are a power of 1000 are preferred, which excludes the hectopascal from use.
Many countries also use millibars. In practically all other fields, the kilopascal (1000 pascals) is used instead.
[Ambler Thompson (Editor) ]
Multiples and submultiples
Decimal multiples and sub-multiples are formed using standard
SI units.
See also
*
Atmospheric pressure
Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...
which gives the usage of the hbar and the mbar
*
Centimetre of water
A centimetre or millimetre of water (US spelling ''centimeter'' or ''millimeter of water'') are less commonly used measures of pressure derived from pressure head.
Centimetre of water
A ''centimetre of water'' (US spelling ''centimeter of water ...
*
Meteorology
Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did no ...
*
Metric prefix
A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decadic. Each prefix has a unique symbol that is prepended to any unit symbol. The pr ...
*
Orders of magnitude (pressure)
*
Pascal's law
Pascal's law (also Pascal's principle or the principle of transmission of fluid-pressure) is a principle in fluid mechanics given by Blaise Pascal that states that a pressure change at any point in a confined incompressible fluid is transmitted ...
*
Pressure measurement
Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressu ...
References
External links
{{SI units
SI derived units
Units of pressure
Blaise Pascal