A U tube manometer is used to measure the differential pressure or the pressure difference between two points. The U tube manometer is a simple and inexpensive instrument. It can be easily made in the field by using a tube and wooden board.
The U tube manometer consists of a tube bent in a U shape. The tube is made of transparent plastic. Glass tubes bent in a U shape can also be used. 
The U tube is fixed against a wooded board. A scale in centimeter is placed between the two limbs of the U tube to measure the levels. The vessel is then filled with a liquid usually water. Mercury is also used in certain situations.
One end of the U tube is connected to the inlet of an equipment while the other end is placed on the outlet.
If the pressure at both ends is equal, the water level in the two limbs will be equal. If the pressure at one end is more than the other, one limb will show a lower level to the other limb.
The difference between the levels (in cms) is noted.
The difference in pressure can be calculated from the following formula
Differential Pressure = Difference in levels x Density of the Liquid x Acceleration due to gravity
Inclined U tube Manometer
The Inclined U tube manometer is used to measure very low values of differential pressure. Inclining the U tube by a certain angle increases the accuracy. The difference in height increases due to the inclination and enables more accurate reading.
The formula for Differential Pressure then changes to
Differential Pressure = Difference in levels x Density of the Liquid x Acceleration due to gravity x Sin ?
where ? is the angle of inclination