Gas Pressure: Pressure is defined as a force per unit area, that is a force divided by the area over which the force is distributed. It is difficult to measure force exerted by gas molecules.
Liquid Pressure: The pressure of liquid depends only on the height of the liquid column and the density of the liquid. Consider a liquid with density d, contained in a cylinder with cross-sectional area A, filled to a height h;
Barometric Pressure: The device used to measure the pressure exerted by the atmosphere is called a barometer.The height of mercury in a barometer, called the barometric pressure, varies with atmospheric conditions and with altitude.
The Standard Atmospheric Pressure (atm) is defined as the pressure exerted by a mercury column of exactly 760 mm in height, when density of Hg = 13.5951 g/cm3 at 0⁰C and the g = 9.80665 m/s2. Then, 1.00 atm = 760 mm Hg = 760 torr = 101.325 kPa = 1.01325 bar = 1013.25 mbar
Example: What is the height of a column of water that exerts the same pressure as a column of mercury 76.0 cm (760 mm) high ?