Principle

• Orifice meter is a thin plate containing a narrow and sharp aperture

• When a fluid stream is allowed to pass through a narrow constriction the velocity of the fluid increase compared to     up stream

• This results in decrease in pressure drop and the difference in the pressure may be read from a manometer

• The velocity of the fluid at thin constriction may be written as

U0 =C 0 √ 2g ∆H

∆H = can be measured by manometer

C0 = constant

U0 = velocity of fluid at the point of orifice meter

Construction

• It is consider to be a thin plate containing a sharp aperture through which fluid flows

• Normally it is placed between long straight pipes

• For present discussion plate is introduced into pipe and manometer is connected at points A and B

Working

• Orifice meter is referred as the variable head meter, i.e it measure the variation in the pressure across a fixed construction placed in the path of flow

• When fluid is allowed to pass through the orifice the velocity of the fluid at point B increase, as a result at point A pressure will be increased.

• Difference in the pressure is measured by manometer

• Bernoulli’s equation is applied to point A and point B for experimental conditions

√U₀2 – U_A2 =C₀√2g. ∆H

U0 = velocity of fluid at orifice

UA = velocity of fluid at point A

C0 = constant

• If the diameter of the orifice is 1/5 or less of the pipe diameter then UA is neglected

Applications

• Velocity at either of the point A and B can be measured

• Volume of liquid flowing per hour can be determined by knowing the area of the cross section