Differential Pressure measurement is an important of any instrumentation system. Differential Pressure is the difference in pressure between two points. If the pressure in one point is 10 kg/cm2 and the pressure at another point is 15 kg/cm2. The differential pressure will be 5 kg/cm2 .
Differential Pressure is measured across equipment which have both an inlet and an outlet. Filters, coolers, heaters are equipment across which the differential pressure is measured. 
A simple method of measuring the differential pressure is by using a U-tube manometer which consists of a U tube . One end of the U tube is connected to the inlet of the equipment while the other end is connected to the outlet. The differential pressure is measured by measuring the difference in levels between the two limbs of the U-tube.
Special sensors which measure the differential pressure are also available.
The differential pressure should be as low as possible. If the differential pressure across any equipment increases, it may indicate choking of the inlet.
The differential pressure can also be used to measure the flow through a pipe.