Flow Measurement Techno.
Fluid flowing in pipe lines creates different types of velocity profiles due to properties of fluid & friction with pipe internal surface; Depending on the Reynolds no. following are the velocity profile any fluid.
One of the the British mechanical engineer Osborne Reynolds in 1883 proposed a dimensionless unit called Reynolds no. it is nothing but the ration inertial forces to viscous forces.
Re = DVr/m
Where,
D is the pipe diameter,
V is the fluid velocity,
r is the fluid density and
m is the fluid viscosity.
Depending of Re no. it is observed,
For fluids of which Re <2,000 the flow is Laminar (Velocity profile is parabolic)

For fluids of which Re > 2,000 the flow is Turbulent (Velocity profile is Uniform)
Turbulent Flow : The pressure drop through a restriction is proportional to the square of the flow rate. So flow can be measured by taking the square root of a differential pressure cell output.
Laminar Flow: The relationship between flow and pressure drop is linear.
Generally the flow meters used form laminar flow measurements are
1. Capillary -type flow meters (For viscous fluids)
2. Diaphragm seal capillary type flow meter (For corrosive fluids)
Following points/properties of fluid are to be considered for selection of flow sensor,
1. Allowable pressure drop
2. Pressure @ max, min & normal flow
3. Temperature Normal & Max. operating
4. Viscosity
5. Conductivity
6. Density / (Specific gravity)
7. Clean/dirty fluid
8. Toxicity
Following are different types of flow sensor;
1. Orifice plate
2. Venturi tubes & nozzles
3. Pitot tubes
4. Elbow tapes
5. Target meters
6. Electromagnetic flow meters
7. Vortex meters
8. Variable Area meters
9. Positive Displacement Meters
10. Ultrasonic Meters.
11. Mass Flow Meters

Easy way for the selection of flow meters is using orientation table.