At point 1 and 2 in the gas column

and

(1)

Where  and  represent draft at point 1 and 2 in the gas column.  and .  are the atmospheric pressures and  and  absolute pressures at points 1 and 2 respectively.
Variations of pressure with height in a gas column are important. In a static column of fluid at constant temperature the pressure decreases with height due to gravity:

(2)

Neglecting the effect of density with Z, it follows for static systems

(3)

 is pressure at height  and  is pressure at arbitrary datum plane of zero height.  is the gas density.