# Pascal (unit)

Last updated

Pascal A pressure gauge reading in psi (red scale) and kPa (black scale)
General information
Unit system SI unit
Unit of Pressure or stress
SymbolPa
Named after Blaise Pascal
Conversions
1 Pa in ...... is equal to ...
SI base units:    kg m −1 s −2
US customary units:   1.450 × 10−4 psi
atmosphere:   9.869 × 10−6 atm
bar:   10−5 bar

The pascal (symbol: Pa) is the SI derived unit of pressure used to quantify internal pressure, stress, Young's modulus and ultimate tensile strength. It is defined as one newton per square metre.  It is named after the French polymath Blaise Pascal.

SI derived units are units of measurement derived from the seven base units specified by the International System of Units (SI). They are either dimensionless or can be expressed as a product of one or more of the base units, possibly scaled by an appropriate power of exponentiation. Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure is the pressure relative to the ambient pressure. Internal pressure is a measure of how the internal energy of a system changes when it expands or contracts at constant temperature. It has the same dimensions as pressure, the SI unit of which is the pascal.

## Contents

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.

The unit of measurement called standard atmosphere (atm) is defined as 101325 Pa.  Meteorological reports in the United States typically state atmospheric pressure in millibars.   In Canada these reports are given in kilopascals. 

The standard atmosphere is a unit of pressure defined as 101325 Pa. It is sometimes used as a reference or standard pressure. It is approximately equal to the atmospheric pressure at sea level. Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw modest progress in the field after weather observation networks were formed across broad regions. Prior attempts at prediction of weather depended on historical data. It was not until after the elucidation of the laws of physics and more particularly, the development of the computer, allowing for the automated solution of a great many equations that model the weather, in the latter half of the 20th century that significant breakthroughs in weather forecasting were achieved. An important domain of weather forecasting is marine weather forecasting as it relates to maritime and coastal safety, in which weather effects also include atmospheric interactions with large bodies of water.

Atmospheric pressure, sometimes also called barometric pressure, is the pressure within the atmosphere of Earth. The standard atmosphere is a unit of pressure defined as 1013.25 mbar (101325 Pa), equivalent to 760 mm Hg (torr), 29.9212 inches Hg, or 14.696 psi. The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth, that is, the Earth's atmospheric pressure at sea level is approximately 1 atm.

## Etymology

The unit is named after Blaise Pascal, noted for his contributions to hydrodynamics and hydrostatics, and experiments with a barometer. The name pascal was adopted for the SI unit newton per square metre (N/m2) by the 14th General Conference on Weights and Measures in 1971. Blaise Pascal was a French mathematician, physicist, inventor, writer and Catholic theologian. He was a child prodigy who was educated by his father, a tax collector in Rouen. Pascal's earliest work was in the natural and applied sciences where he made important contributions to the study of fluids, and clarified the concepts of pressure and vacuum by generalising the work of Evangelista Torricelli. Pascal also wrote in defence of the scientific method.

A barometer is a scientific instrument used to measure air pressure. Pressure tendency can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis to help find surface troughs, pressure systems and frontal boundaries.

The General Conference on Weights and Measures is the supreme authority of the International Bureau of Weights and Measures, the inter-governmental organization established in 1875 under the terms of the Metre Convention through which Member States act together on matters related to measurement science and measurement standards. The CGPM is made up of delegates of the governments of the Member States and observers from the Associates of the CGPM. Under its authority, the International Committee for Weights and Measures executes an exclusive direction and supervision of the BIPM.

## Definition

The pascal can be expressed using SI derived units, or alternatively solely SI base units, as: The SI base units are seven units of measure defined by the International System of Units as a basic set from which all other SI units can be derived. The units and their physical quantities are the second for time, the metre for measurement of length, the kilogram for mass, the ampere for electric current, the kelvin for temperature, the mole for amount of substance, and the candela for luminous intensity.

${\rm {1~Pa=1~{\frac {N}{m^{2}}}=1~{\frac {kg}{m\cdot s^{2}}}=1~{\frac {J}{m^{3}}}}}$ where N is the newton, m is the metre, kg is the kilogram, s is the second, and J is the joule. The newton is the International System of Units (SI) derived unit of force. It is named after Isaac Newton in recognition of his work on classical mechanics, specifically Newton's second law of motion.

The metre or meter is the base unit of length in the International System of Units (SI). The SI unit symbol is m. The metre is defined as the length of the path travelled by light in a vacuum in 1/299 792 458 of a second. The kilogram or kilogramme is the base unit of mass in the International System of Units (SI). Since 20 May 2019, it has been defined in terms of fundamental physical constants. Prior to 20 May 2019, it was defined by a platinum alloy cylinder, the International Prototype Kilogram, manufactured in 1889, and carefully stored in Saint-Cloud, a suburb of Paris.

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 101325 Pa (101.325 kPa).  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's ISO 2787 (pneumatic tools and compressors), ISO 2533 (aerospace) and ISO 5024 (petroleum). In contrast, International Union of Pure and Applied Chemistry (IUPAC) recommends the use of 100 kPa as a standard pressure when reporting the properties of substances. 

Unicode has dedicated code-points U+33A9SQUARE PA and U+33AASQUARE KPA in the CJK Compatibility block, but these exist only for backward-compatibility with some older ideographic character-sets and are therefore deprecated.  

## 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 or the US customary system, including the United States.

Geophysicists use the gigapascal (GPa) in measuring or calculating tectonic stresses and pressures within the Earth.

Medical elastography measures tissue stiffness non-invasively with ultrasound or magnetic resonance imaging, and often displays the Young's modulus or shear modulus of tissue in kilopascals.

In materials science and engineering, the pascal measures the stiffness, tensile strength and compressive strength of materials. In engineering use, because the pascal represents a very small quantity, the megapascal (MPa) is the preferred unit for these uses.

Approximate Young's modulus for common substances 
MaterialYoung's modulus
nylon 6 2–4 GPa
hemp fibre35 GPa
aluminium 69 GPa
tooth enamel 83 GPa
copper 117 GPa
structural steel 200 GPa
diamond 1220 GPa

The pascal is also equivalent to the SI unit of energy density, J/m3. This applies not only to the thermodynamics of pressurised gases, but also to the energy density of electric, magnetic, and gravitational fields.

In measurements of sound pressure or loudness of sound, one pascal is equal to 94 decibels SPL. The quietest sound a human can hear, known as the threshold of hearing, is 0 dB  SPL, or 20 µPa.

The airtightness of buildings is measured at 50 Pa. 

### Hectopascal and millibar units

The units of atmospheric pressure commonly used in meteorology 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 use kilopascals (kPa). In many other fields of science, the SI is preferred, which means Pa with a prefix (in multiples of 1000) is preferred.  

Many countries also use the millibars. In practically all other fields, the kilopascal (1000 pascals) is used instead.[ citation needed ]

## Related Research Articles The Metre Convention, also known as the Treaty of the Metre, is an international treaty that was signed in Paris on 20 May 1875 by representatives of 17 nations. The treaty created the International Bureau of Weights and Measures (BIPM), an intergovernmental organization under the authority of the General Conference on Weights and Measures (CGPM) and the supervision of the International Committee for Weights and Measures (CIPM), that coordinates international metrology and the development of the metric system. The International System of Units is the modern form of the metric system, and is the most widely used system of measurement. It comprises a coherent system of units of measurement built on seven base units, which are the second, metre, kilogram, ampere, kelvin, mole, candela, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units. The system also specifies names for 22 derived units, such as lumen and watt, for other common physical quantities.

The torr is a unit of pressure based on an absolute scale, now defined as exactly 1/760 of a standard atmosphere. Thus one torr is exactly 101325/760 pascals (≈ 133.32 Pa). The square metre or square meter is the SI derived unit of area with symbol m2.

The Dobson unit (DU) is a unit of measurement of the amount of a trace gas in a vertical column through the Earth's atmosphere. It originated, and continues to be primarily used in respect to, atmospheric ozone, whose total column amount, usually termed "total ozone", and sometimes "column abundance", is dominated by the high concentrations of ozone in the stratospheric ozone layer.

Hecto- is a decimal unit prefix in the metric system denoting a factor of one hundred. It was adopted as a multiplier in 1795, and comes from the Greek ἑκατόν hekaton, meaning "hundred". In 19th century English it was sometimes spelled hecato, in line with a puristic opinion by Thomas Young. Its unit symbol as an SI prefix in the International System of Units (SI) is the lower case letter h. The bar is a metric unit of pressure, but is not approved as part of the International System of Units (SI). It is defined as exactly equal to 100,000 Pa, which is slightly less than the current average atmospheric pressure on Earth at sea level.

The kilogram-force, or kilopond, is a gravitational metric unit of force. It is equal to the magnitude of the force exerted on one kilogram of mass in a 9.80665 m/s2 gravitational field. Therefore, one kilogram-force is by definition equal to 9.80665 N. Similarly, a gram-force is 9.80665 mN, and a milligram-force is 9.80665 μN. One kilogram-force is approximately 2.204622 pound-force.

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 133.322387415 pascals. It is denoted by the symbol mmHg or mm Hg. A kilogram-force per centimetre square (kgf/cm2), often just kilogram per square centimetre (kg/cm2), or kilopond per centimetre square is a deprecated unit of pressure using metric units. It is not a part of the International System of Units (SI), the modern metric system. 1 kgf/cm2 equals 98.0665 kPa (kilopascals).

The standard acceleration due to gravity, sometimes abbreviated as standard gravity, usually denoted by ɡ0 or ɡn, is the nominal gravitational acceleration of an object in a vacuum near the surface of the Earth. It is defined by standard as 9.80665 m/s2. This value was established by the 3rd CGPM and used to define the standard weight of an object as the product of its mass and this nominal acceleration. The acceleration of a body near the surface of the Earth is due to the combined effects of gravity and centrifugal acceleration from the rotation of the Earth ; the total is about 0.5% greater at the poles than at the Equator.

Airwatt or air watt is a measurement unit of the effectiveness of vacuum cleaners which refers to airflow and the amount of power (watts) a vacuum cleaner produces and uses. It can also be referred to as a measurement of the energy per unit time of the air flowing through an opening, which is related to the energy that electricity carries through the power cable (wattage).

The ambient pressure on an object is the pressure of the surrounding medium, such as a gas or liquid, in contact with the object. The Celsius scale, also known as the centigrade scale, is a temperature scale used by the International System of Units (SI). As an SI derived unit, it is used by all countries except the United States, the Bahamas, Belize, the Cayman Islands and Liberia. It is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale. The degree Celsius can refer to a specific temperature on the Celsius scale or a unit to indicate a difference between two temperatures or an uncertainty. Before being renamed to honor Anders Celsius in 1948, the unit was called centigrade, from the Latin centum, which means 100, and gradus, which means steps. The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units. It is the pressure resulting from a force of one pound-force applied to an area of one square inch. In SI units, 1 psi is approximately equal to 6895 N/m2. The metric system was developed during the French Revolution to replace the various measures previously used in France. The metre is the unit of length in the metric system and was originally based on the dimensions of the earth, as far as it could be measured at the time. The litre, is the unit of volume and was defined as one thousandth of a cubic metre. The metric unit of mass is the kilogram and it was defined as the mass of one litre of water. The metric system was, in the words of French philosopher Marquis de Condorcet, "for all people for all time".

1. International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), p. 118, ISBN   92-822-2213-6, archived (PDF) from the original on 14 August 2017
2. "Definition of the standard atmosphere". BIPM . Retrieved 16 February 2015.
3. bipm.fr Archived 30 June 2007 at the Wayback Machine
4. Table 3 (Section 2.2.2) Archived 18 June 2007 at the Wayback Machine , SI Brochure, International Bureau of Weights and Measures
5. "Resolution 4 of the 10th meeting of the CGPM". Conférence Générale des Poids et Mesures (CGPM). 1954. Retrieved 5 April 2010.
6. IUPAC.org, Gold Book, Standard Pressure
7. "CJK Compatibility" (PDF). 2015. Retrieved 21 February 2016.
8. "The Unicode Standard, Version 8.0.0". Mountain View, CA: The Unicode Consortium. 2015. ISBN   978-1-936213-10-8 . Retrieved 21 February 2016.
9. "Tensile Modulus - Modulus of Elasticity or Young's Modulus - for some common Materials" . Retrieved 16 February 2015.
10. "Chapter 7 ResNet Standards: ResNet National Standard for Home Energy Audits" (PDF). ResNet. 2010. Archived from the original (PDF) on 26 July 2011. Retrieved 3 March 2011.
11. "KNMI - Weer - Waarnemingen" . Retrieved 4 December 2016.
12. "Comment convertir la pression? - IRM" . Retrieved 4 December 2016.
13. "Japan Meteorological Agency - Weather Maps" . Retrieved 4 December 2016.
14. MDD Archived 6 May 2006 at the Wayback Machine
15. United Kingdom, Met Office. "Key to symbols and terms" . Retrieved 4 December 2016.
16. CTV News, weather; current conditions in Montreal Archived 4 June 2011 at the Wayback Machine
17. Canada, Environment. "Montréal, QC - 7 Day Forecast - Environment Canada" . Retrieved 4 December 2016.