Variable head meters use different principles and designs to measure fluid flow velocity or discharge rate. Pitot tubes use stagnation pressure to measure flow velocity. They consist of a bent glass tube placed in flow, where the height of liquid rise indicates stagnation pressure head. Orifice meters measure flow rate using a differential manometer and the pressure drop across an orifice plate. Venturi meters also use differential pressure but have a converging-diverging nozzle shape to reduce head losses. Weirs and notches are open channel flow measurement devices where flow rate correlates to upstream water depth. Flumes are specially designed open channels also used for flow measurement.

1. VARIABLE HEAD METERS
 Guided By: Presented By : Shivang Bansal
Mr. Rahul Bhargava (0818CM141011)

2. Pitot tube :-

3. Working principle and construction :
The velocity distribution across a section can be obtained with help of a pitot
tube, which is one of the most accurate device for velocity measurement.
It consist of a glass tube in form of 90˚ bend of short length open at both of its
ends. It is placed in flow with its bent leg directed upstream so that a stagnation
point is created immediately in front of opening . The kinetic energy at this point
gets converted into potential energy causing the liquid to rise in vertical limb, to
a height equal to the stagnation pressure.
Applying Bernoulli's equation between stagnation point (s) & point(p) in
undistributed flow at the same horizontal plane, we get:
Po /W + v2 /2g = Ps /w
OR ho + v2/2g = hs
Po & Ps = pressure at point ‘P’ & point ‘S’
V = free flow velocity
ΔH = dynamic pressure
The height of liquid rise indicates stagnant pressure head . it can also be
measured with the help of piezometer.

4. Advantage:-
1. Negligible loss of heat when pitot tube is inserted in the
pipe.
2. Uses costly as compared to venturimeter or orificemeter.
3. Easy to install & remove from the pipeline.
Disadvantages:-
1. Poor accuracy.
2. Unsuitable for dirty or sticky liquids.
3. Requires high flow velocity of about 15m/s to produce
measureable heats.
4. Sensitive to upstream disturbance to reynolds number
changes and to dimension errors.
5. Cannot be used for industrial application which require
an instant readout.

5. Orificemeter:-
Is an opening usually round located in the side wall of tank or reservoir
for measuring flow of liquid. The main feature is that most of the
potential energy of liquid is converted into kinetic energy of free jet
issuing through the orifice.
Also used for flow measurement in a pipe.

6. It consist of a flat circular plate having a circular sharp edged hole
concentric with the pipe. The diameter of orifice may vary from 0.4-0.8
times the diameter of pipe but its value is generally chosen is 0.5.
A differential manometer is connected at section 1 which is at a distance to
1.5-2 times the pipe diameter upstream from the orifice plate , and at section
2 which is at a distance of about half the diameter of orifice from orifice
plate from downstream side.
Jet coming out of orifice plate gradually expands from vena contracta to fill
the pipe.
Figure shows the location of vena contracta point. A part of kinetic energy
of the jet is converted into point using eddy currents causing dissipation of
energy and loss of head.

7. Disadvantages:-
1. Low coefficient of discharge.
2. Poor pressure recovery.
3. Susceptible to inaccuracies resulting from erosion, corrosion and
sealing.
4. Lower physical strength orifices are likely to be damaged by pressure
transients.
1. Low initial cost.
2. Easy to install.
3. Simple and less expensive maintenance as compared with
venturimeter.
4. Requires less spaces as compared to a venturimeter.
5. Can be used in wide range of pipe sizes(1.25-1.50cm)
Advantages:-

8. Venturimeter:-

9. 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 is followed by diverging section known as diffuser having a total
included angle 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.
 The value of coefficient of discharge varies from 0.95 to 0.99.

10. Disadvantages of Venturimeter :
 Highly expensive.
 Occupies considerable space.
 Cannot be altered for measuring pressure beyond a
maximum velocity.

11. Difference between orifice meter and
venturi meter :
 The orifice plate can easily be changed to accomodate widely different
flow rates, whereas the throat diameter of a venturi is fixed, so that its
range of flow rates is circumscribed by the practical limits of Dp.
 The orifice meter has a large permanent loss of pressure because of the
presence of eddies on the downstream side of the orifice-plate; the
shape of the venturi meter prevents the formation os these eddies and
greatly reduces the permanent loss.
 The orifice is cheap and easy to install. The venturi meter is expensive,
as it must be carefully proportioned and fabricated. A home made
orifice is often entirely satisfactory, whereas a venturi meter is
practically always purchased from an instrument dealer.

12.  On the other hand, the head lost in the orifice for the same conditions
as in the venturi is many times greater. The power lost is proportionally
greater, and, when an orifice is inserted in a line carrying fluid
continuously over long periods of time, the cost of the power may be
out of all proportion to the saving in first cost. Orifices are therefore
best used for testing purposes or other cases where the power lost is not
a factor, as in steam lines.
 However, in spite of considerations of power loss, orifices are widely
used, partly because of their greater flexibility, because installing a new
orifice plate with a different opening is a simpler matter. The venturi
meter can not be so altered. Venturi meters are used only for
permanent installations.
 It should be noted that for a given pipe diameter and a given diameter
of orifice opening or venturi throat, the reading of the venturi meter for
a given velocity is to the reading of the orifice as (0.61/0.98)2, or
1:2.58.(i.e. orifice meter will show higher manometer reading for a given
velocity than venturi meter).

13. Weir :
 A weir 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.

14. Drawbacks of Weirs :
 A weir will artificially reduce the upstream water velocity,
which can lead to an increase in siltation.
 Weirs can also have an effect on local fauna.
 Even though the water around weirs can often appear
relatively calm, they can be extremely dangerous places to
boat, swim or wade, as the circulation patterns on the
downstream side – typically called a HYDRAULIC JUMP –
can submerge a person indefinitely.
 This phenomenon is so well known to persons who spend
time on rivers that they even have a rueful name for weirs :
“ Drowning Machines”.
 The weir can become a point where garbage and other
debits accumulate.

15. 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.

16. Flumes :
 A flume is a man made channel for water, in the form of an open
declined gravity channel whose walls are raised above the surrounding
terrain, in contrast to a trench or ditch.
 Some varieties of flumes are used in measuring water flow of a larger
channel.

17. THANK YOU