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.
The following are true concerning manometers:
If pressure is applied at one end of a manometer, the liquid is forced toward the other end.
The difference in the heights of the two columns is a measure of the pressure applied.
The greater the pressure, the larger the difference in liquid level in the "legs" of the manometer.
Mercury is 13.6 times heavier than water. Therefore, the height of a mercury column is much less than the height of a water column that measures the same pressure.
The U-tube manometer is a very common measurement device and as long as the tube is not blocked, it is extremely accurate. In fact, U-tube manometers are often used to calibrate electronic pressure transmitters used on plant equipment.