SI derived unit of pressure
The
pascal
(symbol:
Pa
) is the unit of
pressure
in the
International System of Units (SI)
. It is also used to quantify
internal pressure
,
stress
,
Young's modulus
, and
ultimate tensile strength
. The unit, named after
Blaise Pascal
, is an
SI coherent derived unit
defined as one
newton
per
square metre
(N/m
2
).
[1]
It is also equivalent to 10
barye
(10 Ba) in the
CGS
system. 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 101,325 Pa.
[2]
Meteorological
observations typically report
atmospheric pressure
in hectopascals per the recommendation of the
World Meteorological Organization
, 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
[3]
or millibars (hectopascals).
[4]
[5]
In Canada these reports are given in kilopascals.
[6]
Etymology
[
edit
]
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/m
2
) by the 14th
General Conference on Weights and Measures
in 1971.
[7]
[8]
Definition
[
edit
]
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
.
[9]
One pascal is the pressure exerted by a force of magnitude one newton perpendicularly upon an area of one square metre.
Standard units
[
edit
]
The unit of measurement called an
atmosphere or a standard atmosphere
(atm) is
101
325
Pa (101.325 kPa).
[10]
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.
[11]
Unicode
has dedicated code-points
U+33A9
㎩
SQUARE PA
and
U+33AA
㎪
SQUARE KPA
in the
CJK Compatibility
block, but these exist only for backward-compatibility with some older ideographic character-sets and are therefore
deprecated
.
[12]
[13]
Uses
[
edit
]
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 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
,
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.
[15]
In medicine, blood pressure is measured in
millimeters of mercury
(mmHg, very close to one
Torr
). The normal adult blood pressure is less than 120 mmHg systolic BP (SBP) and less than 80 mmHg diastolic BP (DBP).
[16]
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
[
edit
]
The units of atmospheric pressure commonly used in
meteorology
were formerly the
bar
(100,000 Pa), which is 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.
[17]
[18]
[19]
[20]
[21]
[22]
[23]
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.
[24]
[25]
Many countries also use millibars. In practically all other fields, the kilopascal is used instead.
[26]
Multiples and submultiples
[
edit
]
Decimal multiples and submultiples are formed using standard
SI units
.
Multiples
|
Submultiples
|
Value
|
Name
|
Symbol
|
Value
|
Name
|
Symbol
|
10
1
Pa
|
decapascal
|
daPa
|
10
?1
Pa
|
decipascal
|
dPa
|
10
2
Pa
|
hectopascal
|
hPa
|
10
?2
Pa
|
centipascal
|
cPa
|
10
3
Pa
|
kilopascal
|
kPa
|
10
?3
Pa
|
millipascal
|
mPa
|
10
5
Pa
|
bar (non-SI unit)
|
bar
|
|
|
|
10
6
Pa
|
megapascal
|
MPa
|
10
?6
Pa
|
micropascal
|
μPa
|
10
9
Pa
|
gigapascal
|
GPa
|
10
?9
Pa
|
nanopascal
|
nPa
|
10
12
Pa
|
terapascal
|
TPa
|
10
?12
Pa
|
picopascal
|
pPa
|
10
15
Pa
|
petapascal
|
PPa
|
10
?15
Pa
|
femtopascal
|
fPa
|
10
18
Pa
|
exapascal
|
EPa
|
10
?18
Pa
|
attopascal
|
aPa
|
10
21
Pa
|
zettapascal
|
ZPa
|
10
?21
Pa
|
zeptopascal
|
zPa
|
10
24
Pa
|
yottapascal
|
YPa
|
10
?24
Pa
|
yoctopascal
|
yPa
|
10
27
Pa
|
ronnapascal
|
RPa
|
10
?27
Pa
|
rontopascal
|
rPa
|
10
30
Pa
|
quettapascal
|
QPa
|
10
?30
Pa
|
quectopascal
|
qPa
|
See also
[
edit
]
References
[
edit
]
- ^
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 4 June 2021
, retrieved
16 December
2021
- ^
"Definition of the standard atmosphere"
.
BIPM
. Retrieved
16 February
2015
.
- ^
"National Weather Service glossary page on inches of mercury"
.
- ^
"US government atmospheric pressure map"
.
- ^
"The Weather Channel"
.
- ^
Canada, Environment (16 April 2013).
"Canadian Weather - Environment Canada"
.
weather.gc.ca
.
- ^
bipm.fr
.
Archived
30 June 2007 at the
Wayback Machine
.
- ^
Minutes of the 14. General Conference on Weights and Measures
, 1971, p. 78.
- ^
Table 3 (Section 2.2.2)
.
Archived
18 June 2007 at the
Wayback Machine
.
SI Brochure
.
International Bureau of Weights and Measures
.
- ^
"Resolution 4 of the 10th meeting of the CGPM"
.
Conference Generale des Poids et Mesures
. 1954. Archived from
the original
on 30 March 2021
. Retrieved
5 April
2010
.
- ^
IUPAC.org, Gold Book,
Standard Pressure
- ^
"CJK Compatibility"
(PDF)
. 2015
. Retrieved
21 February
2016
.
- ^
The Unicode Standard, Version 8.0.0
. Mountain View, CA: The Unicode Consortium. 2015.
ISBN
978-1-936213-10-8
. Retrieved
21 February
2016
.
- ^
"Tensile Modulus ? Modulus of Elasticity or Young's Modulus ? for some common Materials"
. Retrieved
16 February
2015
.
- ^
"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
.
- ^
"BP Guideline | Target:BP"
.
American Heart Association
. Retrieved
18 May
2020
.
- ^
"KNMI ? Weer ? Waarnemingen"
. Retrieved
4 December
2016
.
- ^
"Comment convertir la pression? ? IRM"
. Retrieved
4 December
2016
.
- ^
"DWD"
. Archived from
the original
on 10 February 2008
. Retrieved
20 December
2006
.
- ^
"Japan Meteorological Agency ? Weather Maps"
. Retrieved
4 December
2016
.
- ^
"MDD"
. Archived from
the original
on 6 May 2006.
- ^
NOAA
- ^
United Kingdom, Met Office.
"Key to symbols and terms"
. Retrieved
4 December
2016
.
- ^
"CTV News, weather; current conditions in Montreal"
. Archived from
the original
on 4 June 2011.
- ^
Canada, Environment.
"Montreal, QC ? 7 Day Forecast ? Environment Canada"
. Archived from
the original
on 30 November 2017
. Retrieved
4 December
2016
.
- ^
Ambler Thompson (Editor)
Guide for the Use of the International System of Units (SI) (rev. ): The ...
, p. 66, at
Google Books