The manometer is a tool of measure of the pressure of fluids. Practically, most of these types measure a relative pressure that is the difference between the atmospheric pressure in the point of measure and the pressure of the environment in which the measure is desired. There are different types of manometer for different usages, these are: U manometers, diaphragm manometers and Bourdon manometers.  

U MANOMETERS                                                                                        
They are constituted by a pipe (usually transparent) bent to U and filled with a liquid of known density. An extremity of the pipe is open to the atmosphere, while the other extremity is in direct connection with the environment of measure. The liquid contained in the pipe will move up in one of the two branches of the U in such a value that the difference of weight between the two columns of liquid balance exactly the existent pressure (or depression) in the measured environment. For their simplicity these types of manometers are not subject to damages, however their resolution is not very elevated due to the phenomenon of the meniscus (a sort of superficial basin of a liquid which is in any container).

DIAPHRAGM MANOMETERS                                                                              
They are also called membrane manometers becouse the distorting element is an undulate membrane. The membrane separates the environment of measure from the outside, and it will inflate if the pressure that must be measured is greater than the atmospheric pressure, and vice versa.

BOURDON MANOMETERS                                                                      
Usually, they are constituted by a pipe of elliptic section and whose axis is collocated along a circumference, but it can also assume the form of a spiral. A pipe of this form tends to increase his ray of bending increasing the pressure inside the tube. The measurement of the ray gives the measure of the pressure.