The 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/in2 (psi) or lb/ft2 (psf), N/m2 or kg/ms2 (Pa)]
F = force [1), N]
A = area [in2 or ft2, m2]
1) In the English Engineering System special care must be taken for the force unit. The basic unit for mass is the pound mass (lbm) and the unit for the force is the pound (lb) or pound force (lbf).
Absolute PressureThe absolute pressure - pa - is measured relative to the absolute zero pressure - the pressure that would occur at absolute vacuum. All calculation involving the gas laws requires pressure (and temperature) to be in absolute units. Gauge PressureA 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 aspg = ps - pa (2)
where
pg = gauge pressure
ps = system pressure
pa = atmospheric pressure
Atmospheric PressureAtmospheric pressure is pressure in the surrounding air at - or "close" to - the surface of the earth. The atmospheric pressure vary with temperature and altitude above sea level.Standard Atmospheric Pressure (ATM) is used as a reference for gas densities and volumes. The Standard Atmospheric Pressure is defined at sea-level at 273oK (0oC) and is 1.01325 bar or 101325 Pa (absolute). The temperature of 293oK (20oC) is also used.
In imperial units the Standard Atmospheric Pressure is 14.696 psi.
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.
Pounds per square inch (psi) was common in U.K. but has now been replaced in almost every country except in the U.S. by the SI units. Since atmospheric pressure is 14.696 psi - a column of air on a area of one square inch area from the Earth's surface to the space - weights 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 even if
1 Bar = 0.9869 ATM
There are 1,000 millibar (mbar) in one bar, a unit common in meteorology.
1 millibar = 0.001 bar = 0.750 torr = 100 Pa
A gauge pressure taken does not include atmospheric pressure, thus to find the absolute pressure you must add atmospheric pressure (14.7 PSI or 101325 Pa) to the gauge pressure.
Subtract atmospheric pressure from absolute pressure to get gauge pressure.
gauge
Gauge pressure is what you get when you take the reading from your tire pressure gauge. Absolute pressure is the pressure inside your tires plus the atmospheric pressure, which is roughly; 14.7 psi, 101.3 kPa (kilo-Pascals), or one atmosphere. Absolute pressure measures all of the pressure on your tires, inside and out, whereas gauge simply measures the pressure inside the tire.
i need to explain what an absolute pressure gauge measures
Absolute Pressure
absolute pressure; gauge pressure; atmospheric pressure...
if the gauge pressure is 206 kPa, absolute pressure is 307 kPa
If a gas has a gage pressure of 156 kPa its absolute pressure is approximately?
44.7 psi
The gauge pressure would be 448.955kPa.
Absolute
Bourdon's tube pressure gauge cannot be used to measure negative pressure. This is because absolute pressure must be measured and the Bourdon gauge only indicates the gauge pressure.
-14.7psig is the gauge pressure of an absolute vacuum.