Module 7: Energy conservation
  Lecture 22: Flow measuring devices
 
 
(b) Orifice meter: The instrument has an orifice–plate (circular plate with a small hole)  inserted across the section of the pipe or tube:
(Fig. 22c)

In such a device, the flow from the upstream–section accelerates as the flow-area decreases from the section 1 to 2 at the orifice. The flow continues to accelerate, or the main flow area continues to decrease till the section 2 and further downstream of the orifice at C, before it starts increasing downstream. The section at C contains the minimum flow area, known as vena–contracta.
Similar to the analysis carried out for venturimeter, one can calculate the ideal or  theoretical flow rate:

To account for viscous-losses, a parameter  is defined, so that
    and    
C is known as the discharge coefficient and   is the velocity-of-approach factor.
A single factor, flow coefficient   is used to express

K is reported in a graph supplied by the manufactures.