Orificemeter
Construction: An
orificemeter provides a simpler and cheaper arrangement for
the measurement of fow through a pipe. An orificemeter is
essentially a thin circular plate with a sharp edged concentric
circular hole in it.
Working:
 The orifice plate, being fixed at a section
of the pipe, (Fig. 15.3) creates an obstruction to the flow
by providing an opening in the form of an orifice to the
flow passage.
Fig 15.3 Flow
through an Orificemeter
 The area A_{0} of the orifice
is much smaller than the crosssectional area of the pipe.
The flow from an upstream section, where it is uniform,
adjusts itself in such a way that it contracts until a section
downstream the orifice plate is reached, where the vena
contracta is formed, and then expands to fill the passage
of the pipe.
 One of the pressure tapings is usually
provided at a distance of one diameter upstream the orifice
plate where the flow is almost uniform (Sec. 11) and the
other at a distance of half a diameter downstream the orifice
plate.
 Considering the fluid to be ideal and
the downstream pressure taping to be at the vena contracta
(Sec. cc), we can write, by applying Bernoulli’s
theorem between Sec. 11 and Sec. cc,

(15.10) 
where
and
are the piezometric pressures at Sec.11 and cc respectively.
 From the equation of continuity,

(15.11) 
where A_{c} is the area of the vena contracta.
 With the help of Eq. (15.11), Eq. (15.10)
can be written as,

(15.12) 
