Pascal (unit)


The pascal is the SI derived unit of pressure used to quantify internal pressure, stress, Young's modulus and ultimate tensile strength. The unit, named after Blaise Pascal, is defined as one newton per square metre. The unit of measurement called standard atmosphere is defined as.
Common multiple units of the pascal are the hectopascal, which is equal to one millibar, and the kilopascal, which is equal to one centibar. Meteorological forecasts typically report atmospheric pressure in hectopascals per the recommendation of the World Meteorological Organization. Forecasts in the United States typically use millibars, 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. The name pascal was adopted for the SI unit newton per square metre by the 14th General Conference on Weights and Measures in 1971.

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, kg is the kilogram, s is the second, and J is the joule.
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 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's ISO 2787, ISO 2533 and ISO 5024. In contrast, International Union of Pure and Applied Chemistry recommends the use of 100 kPa as a standard pressure when reporting the properties of substances.
Unicode has dedicated code-points and 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 or kilopascal as a unit of pressure measurement is widely used throughout the world and has largely replaced the pounds per square inch 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 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 is the preferred unit for these uses.
MaterialYoung's modulus
nylon 62–4 GPa
hemp fibre35 GPa
aluminium69 GPa
tooth enamel83 GPa
copper117 GPa
structural steel200 GPa
diamond1220 GPa

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, magnetic, and gravitational fields.
In measurements of sound pressure or loudness of sound, one pascal is equal to 94 decibels sound pressure level. 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.
In medicine, blood pressure is measured in millimeters of mercury. The normal adult blood pressure is < 120 mm Hg Systolic BP and < 80 mm Hg Diastolic BP. Convert mm Hg to SI units as follows: 1 mm Hg = 0.13332 kPa. Hence normal blood pressure in SI units is < 16.0 kPa SBP and < 10.7 kPa.

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 unit, equal to 100 pascals or 1 millibar. Exceptions include Canada, which uses kilopascals. 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 is used instead.