Measurements

Pressure

Pressure is a measurement of the ratio between the force applied to the surface area on which that force is applied. The SI pressure units are Pascals which are equivalent to  . Among many other units of pressure, some of the most common used within engineering include bar, atm, torr, psi, and mmHg. There are many different types of pressure as essentially anything can exert a force on a surface although within the context of chemical engineering fluid pressure is the most common which can be the pressure of either a liquid or gas.

Fluid pressure within a container can be defined by the equation  where P_atm is the atmospheric pressure exerted at the top of the container,  is the density of the fluid, g is the force of gravity, and h is the height of the fluid within the container. Atmospheric pressure is evidently the pressure within the atmosphere of the earth and has been measured to be 101325 Pa / 760 mmHg / 1.01325 bar / 14.696 psi at sea level. As shown in figure 1 atmospheric pressure is applied at the top of the container onto the fluid. The density of the fluid is another large factor as the higher the density of the fluid the higher the pressure will be. For example water is more dense than gasoline and will exert a lot more pressure. For a chemical engineer, gravity will be constant at 9.8 m/s² although perhaps in the future if there is a demand for water tanks on mars or the moon, this value will change. Height is another large factor in determining pressure and can often change. In figure 1, the height is measured from the top of the water down which means the pressure will be the greatest at level C and smallest at level A.