Pressure is the force that a fluid exerts on a solid surface divided by the area of the surface.
Fluids exert forces on solid surfaces because their molecules are constantly colliding with the solid surface.
One interesting aspect of pressure is that it always acts in a direction perpendicular or normal to the solid surface.
If the surface is curved, then pressure acts in a direction perpendicular to a plane that is tangent to the curved surface.
The units for pressure are the units of force per length squared.
In the SI system, the standard unit is a N/m2 which is known as a Pascal.
A Pascal is a rather small amount of pressure, so kPa and MPa turn out to be more common units in practice.
Two unit conversions that you might like to memorize are given here.
1 atm is 101.325 kPA and 1 bar is 100 KPa. These will come in handy more than once or twice in this course.
The standard unit for pressure in the AE System is the lbf/ft2.
The most important conversion to remember is that 1 atm = 14.696 lbf/ft2,.
In general, lbf/ft2, is abbreviated as a “psi”.
But, the symbol “psi” is not adequate.
You must use “psia” when referring to an absolute pressure and “psig” when referring to a gage pressure.
In fact, you must distinguish between absolute and gage pressure with ANY system of units !
So, what is the difference between absolute and gage pressure anyway ?
Flip the page and let’s see.