A manometer water column is the partially water-filled tube inside a manometer, a gauge used for indicating pressure. The water column is usually shaped like the letter "U" and therefore called a U-tube.
Manometers measure liquid or gas pressure by determining the difference between levels in the two arms of the U-tube. Pressure is exerted on one arm of the column, which causes the liquid or gas to move upward toward the other column. There are numerical pressure markings on each arm, and when a liquid is used, it is often colored to allow the observer to easily determine the reading. One arm of the tube is connected to a gas or liquid source with an air-tight seal, and the other arm is open to the atmosphere.
The mathematical calculation used to determine pressure with a manometer is that the pressure differential is equal to the height difference between the liquid or gas in the two arms multiplied by the acceleration due to gravity multiplied by the density of the fluid or gas. Since acceleration due to gravity and the density of the fluid or gas are constant, known values, the observer can determine pressure differential directly by calculating the height differential between the liquid or gas in the two arms.