Chapter 5 : Applications of Equations of Motion and Mechanical Energy
Lecture 15 :

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 A0 of the orifice is much smaller than the cross-sectional 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. 1-1) 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. c-c), we can write, by applying Bernoulli’s theorem between Sec. 1-1 and Sec. c-c,
 (15.10)

where and  are the piezometric pressures at Sec.1-1 and c-c respectively.

• From the equation of continuity,
 (15.11)

where Ac is the area of the vena contracta.

• With the help of Eq. (15.11), Eq. (15.10) can be written as,
 (15.12)