foot of water column
The foot of water column is the pressure at the bottom of a one foot column of water, conventionally taken with water at 1000 kg/m³ and standard gravity, giving about 2989.07 pascals. It is a conventional unit rather than an exact one, because real water density changes with temperature and different reference temperatures are in use. It is the natural unit of pump head, where a pump that lifts water 100 ft is described as producing 100 ft of head regardless of the fluid's pressure in psi.
Watch out: Head in feet of water is only equal to pressure when the fluid is water. A centrifugal pump develops a head in feet that is nearly independent of fluid density, but the pressure it produces is proportional to density, so pumping a fluid of specific gravity 1.3 gives 30 percent more pressure for the same head. Also check the reference temperature: values quoted at 39.2 °F, 60 °F and 68 °F differ in the fourth digit.
| 1 ft H₂O | 2.9890669 kPa |