•Reduction of cross-section of the flowing stream in passing through orifice increases the velocity head at the expense of pressure head

•Reduction of pressure between taps is measured using manometer

•Formation of Vena-contracta- Fluid stream separates from the downstream side of the orifice plate and forms a free-flowing jet in the downstream side.

•Orifice coefficients are more empirical than those for the Venturi meter.

•Orifice coefficient, generally, is 0.61 in case of flange taps and vena-contracta taps for NRe< 30,000.

•In the process of calculating fluid velocity with a orifice meter, the velocity of approach is not included.
 

Velocity through an orifice meter:
 
 

                       where,   Co - Orifice coefficient
      - Ratio of CS areas of upstream to that of down stream
                                Pa-Pb  - Pressure gradient across the orifice meter
    - Density of fluid