Manometers
are simple instruments used to measure pressure. They consist of U-shaped tubes filled with mercury or water and are open at both ends.
Shown here are two manometers. The one on the left contains water and the other contains mercury. The level in both legs is equal because the liquid seeks equilibrium in the tubes.
Now see what happens if one side of each manometer is connected to a 1 psi (0.07 bar) source of air pressure.
The mercury moved up its column exactly 2.04 inches (51.81 millimeters). However, the water was blown completely out of its manometer tube. The reason for this is that 1 psi is equal to 2.04" Hg (51.8 mm Hg) and 27.71" H2O (703.8 mm H2O). The manometers were not long enough to measure the pressure using water.