Static Fluid Pressure

The pressure in a static fluid arises from the weight of the fluid and is given by the expression

The pressure from the weight of a column of liquid of area A and height h is

The most remarkable thing about this expression is what it does not include. The fluid pressure at a given depth does not depend upon the total mass or total volume of the liquid. The above pressure expression is easy to see for the straight, unobstructed column, but not obvious for the cases of different geometry which are shown.

Because of the ease of visualizing a column height of a known liquid, it has become common practice to state all kinds of pressures in column height units, like mmHg or cm H₂O, etc. Pressures are often measured by manometers in terms of a liquid column height.

Fluid Pressure Calculation

Discussion

Fluid column height in the relationship is often used for the measurement of pressure. After entering the relevant data, any one of the highlighted quantities below can be calculated by clicking on it. Pressure difference = density x g x height If the fluid density is ρ = gm/cm3 = kg/m3 and the column height is h = m = x 10^ m h = ft then the pressure difference is ΔP = kPa ΔP = lb/in2 ΔP = mmHg= inches Hg ΔP = atmos ΔP = inches water= cm water

Note that this static fluid pressure is dependent on density and depth only; it is independent of total mass, weight, volume, etc. of the fluid.