Fluid Properties: Force and Pressure

 

Force

     A force is a push or a pull, or more generally anything that can change an object’s speed or direction of motion. The International System of Units (SI) unit used to measure force is the Newton (symbol N).

F = ma

      where F stands for force in Newton, m stands for mass in Kg and a represents acceleration expressed as meters divided by seconds squared m/s².

Pressure

      Pressure is the ratio of force to the area over which the force acts.

      Mathematically, it can be expressed as:

     p =F/A

      where p is pressure, F is force, and A represents area. Pressure is usually expressed in Newton per square meter, given the name Pascal, and traditionally, it was expressed in pounds force per square inch (PSI).

Atmospheric Pressure

     Atmospheric pressure is defined as the pressure due to the weight of the atmosphere (air and water vapor) on the earth’s surface. Atmospheric pressure is determined by a mercury column barometer, that is why it is sometimes called as barometric pressure. The average atmospheric pressure at sea level has been defined as 1.01325 bars, or 14.696 pounds per square inch absolute (PSIA).

Absolute Pressure

      Absolute pressure can be given as gauge pressure plus barometric or atmospheric pressure. Absolute pressure is referenced against absolute zero pressure, or a complete vacuum. The units of absolute pressure are followed by suffix “a,” such as psia. If we hold an absolute pressure instrument in the open air, the reading should be well above zero, in the range of 14.7 to 12 psia.

Gauge and Vacuum Pressure

      Gauge pressure is referenced against the atmospheric pressure at the measurement point. The units of gauge pressure are followed by a “g,” such as psig. A gauge pressure instrument should always read zero when exposed to atmospheric pressure. Similarly, when the pressure falls below atmospheric, it is called vacuum pressure, sometimes it is also called negative gauge pressure.

      Based upon the above discussions, the following equations can be derived:

                                    P_abs = P_atm + P_guage

                                    P_abs  = P_atm -P_vacuum 

Where                          P_abs  = Absolute pressure

                                    P_atm  = Atmospheric pressure

                                    P_guage = Gauge pressure

                                    P_vacuum = Vacuum pressure (−ve gauge pressure).

Conversion of various units of pressure in Pascal