3. Working
It consist of a short converging conical tube which
has a total included angle of 21 ± 1° leading to a
cylindrical portion of short length, known as throat.
It s followed by diverging section known as diffuser
having total included angel 5°to 7°.
The pressure difference measure between the entry
section 1 and the throat section 2, usually by means
of a U-tube manometer.
The axis of venturimeter may be horizontal or
inclined or vertical.
Area of flow in divergent section is increased
gradually to avoid separation of flow and reduce
friction losses.
4. The value of coefficient of discharge varies from 0.95
to 0.99.
Disadvantages of Venturi meter:
Highly expensive
Occupies considerable space
Cannot be altered for measuring pressure
beyond a maximum velocity.
5. Orifice meter
• It may be installed in pipeline with a
minimum of trouble and expense
6. Working
Orifice meter is device used to determine the rate of
flow through pipe.
It consist of flat circular plate which has a sharp
edged circular hole called orifice.
It is fixed concentric to pipe.
The orifice diameter is generally kept half of the
diameter of the pipe.
It is based on the same principle as explained in
venturimeter.
The value of Cd varies between 0.60 to 0.65.
It is a economical and less space is requred for
fitting.
7. Difference between an Orifice
meter and Venturimeter
• For venturi tube, there is no contraction-
– Area of the jet is always equal to the area of the throat
and is fixed
While for the orifice the area of the jet is a variable
(less than Ao, the area of the orifice)
• For venturitube
Cd = Cv (Cc=1.0)
While for the orifice, it is much more affected by
variations in Cc than it is by variations in Cv
8. Notches
A notches is defined as an opening in the side of a
vessel or tank in such a manner that the liquid
surface in the tank.
A notch is generally made of metallic plate.
It is used for measuring the discharge rate of a liquid
from a small channel or tank.
9. Weir
A weir is basically an obstruction in the flow path in an open
channel.
The weir will cause an increase in the water depth as the
water flows over the weir.
In general, the greater the flow rate, the greater will be the
increase in depth of flow, The height of water above the top of
the weir is the measurement usually used to correlate with
flow rate.
10. Pitot tube
A pitot tube is a simple device used for meausuring
the velocity of a flow at the required point in a pipe or
steam.
It is also called as impact tube or stagnation tube.
11. Operation
• The basic pitot tube consists of a tube pointing
directly into the fluid flow. As this tube contains
fluid, a pressure can be measured; the moving
fluid is brought to rest (stagnates) as there is no
outlet to allow flow to continue.
• This pressure is the stagnation pressure of the
fluid, also known as the total pressure or
(particularly in aviation) the pitot pressure.
• The measured stagnation pressure cannot itself be
used to determine the fluid flow velocity (airspeed
in aviation). However, Beronulli’s equation states:
• Stagnation pressure =
static pressure + dynamic pressure
