An introduction to
pressure - the definition and presentation of common units as psi and Pa and
the relationship between them:
Pressure in a fluid is
defined as *"the normal force per unit area exerted on a imaginary or real
plane surface in a fluid or a gas"*.
The equation for
pressure can expressed as:
p = F/A (1)
where
p* *
= pressure [lb/in^{2} (psi)
or lb/ft^{2} (psf) or N/m^{2} or kg/m^{2}
(Pa)]
*F*
= force [lb^{(1)}, N]
*A*
= area [in^{2} or ft^{2}, m^{2}]
^{(1) }
In the
English Engineering System special care must be taken for the force
unit. The basic unit for mass is the pound mass (lb_{m}) and the
unit for the force is the pound (lb) or pound force (lb_{f}).
##### Absolute Pressure
The **absolute
pressure** - *p*_{a}_{
}- is measured relative to
the *absolute* *zero pressure *- the pressure that would occur at
absolute vacuum.
##### Gauge Pressure
A **gauge** is often used to measure the
pressure difference between a system and the surrounding atmosphere. This
pressure is often called the **gauge pressure** and can be expressed as:
*p*_{g} = p_{a}
- p_{o}_{ }
(2)
where
*p*_{g}*
= *
gauge pressure
*p*_{o}*
= *atmospheric pressure
###### Atmospheric Pressure
The atmospheric
pressure is the pressure in the surrounding air. It varies with temperature
and altitude above sea level.
##### Standard Atmospheric Pressure
The Standard Atmospheric Pressure (atm) is
used as reference for gas densities and volumes. The Standard Atmospheric
Pressure is defined at sea-level at 273^{o}K (0^{o}C) and is
1.01325 bar** **or** **101325 Pa (absolute). The temperature of 293^{o}K
(20^{o}C) is also used.
In imperial units the
Standard Atmospheric Pressure is 14.696 psi.
101.3 kPa = 1 atm = 14.696 psi = 760 mmHg = 760 torr
Pressure Units
Since 1 Pa is a small pressure unit, the unit hectopascal
(hPa) is more widely used, especially in meteorology. The unit kilopascal (kPa)
is commonly used in technical applications as HVAC systems, piping or
similar.
1 hectopascal = 100 pascal = 1 millibar
1 kilopascal = 1000 pascal
##### Various Pressure Values
10 Pa - The pressure at a depth of 1 mm of
water
1 kPa - Approximately the pressure exerted by a 10 g mass on a 1 cm^{2}
area
10 kPa - The pressure at a depth of 1 m of water, or
the drop in air pressure when going from sea level to 1000 m
elevation
10 MPa - A "high pressure" washer forces the
water out of the nozzles at this pressure
10 GPa - This pressure forms diamonds
**Alternative Units of Pressure**
1 bar - 100,000 Pa
1 millibar - 100 Pa
1 atmosphere - 101,325 Pa
1 mm Hg - 133 Pa
1 inch Hg - 3,386 Pa
A **torr **(torr) is**
**named after Torricelli and is the pressure produced by a column of
mercury 1 mm high equals to 1/760th of an atmosphere. 1 atm = 760 torr =
14.696 psi
**Pounds per square
inch** (psi) was common in U.K. but
has now been replaced in almost every country, except the U.S., by the SI
units. Normal atmospheric pressure is 14.696 psi, meaning that a column of
air on one square inch in area rising from the Earth's atmosphere to space
weighs 14.696 pounds.
The bar (bar) is common
in the industry. One bar is 100,000 Pa, and for most practical purposes can
be approximated to one atmosphere. 1 Bar = 0.9869 atm.
There are 1,000
millibar (mbar) in one bar. The unit is common in meteorology. One millibar
= 0.001 bar = 0.750 torr = 100 Pa. |