A rotameter is a flow measurement device consisting of a conical glass tube and a float, which indicates the flow rate based on its position. It requires no external power, is easy to manufacture, and operates linearly with flow rate changes. Other flow measurement devices discussed include venturimeters, electromagnetic flowmeters, and positive displacement flowmeters, each with unique working principles and applications.

1. Rotameter

2. Rotameter:
A rotameter is a device that
measures the flow rate of liquid or gas
in a closed tube. A rotameter
consists of a Conical Tube, typically
made of glass with a 'float', actually a
shaped weight, inside that is pushed
up by the Drag Force of the flow and
pulled down by Gravity. The position
of the float indicates the flow rate on a
marked scale.

3. Working of Rotameter:
A rotameter is basically constructed of a vertically
oriented glass tapered tube and a metering float which is free to
move within the tube. When there is no fluid flow, the float
rests freely at the bottom of the tube. When liquid enters the
bottom of the tube, the metering float begins to rise. The
position of the float changes directly with the flow rate. The
accurate position of the float is at the point where the
differential pressure between the upper and lower surfaces
balances the weight of the float. The greater the flow, the
higher the float gets lifted.

4. Advantages:
•A rotameter requires no external power or fuel, it uses only the
inherent properties of the fluid, along with gravity, to measure
flow rate.
•A rotameter is also a relatively simple device that can be mass
manufactured out of cheap materials, allowing for its
wide spread use.
•Since the area of the flow passage increases as the float moves
up the tube, the scale is approximately linear.

5. Railway rotameter
The New South Wales Government Railways constructed in 1903 a
device for measuring the length of its lines of railway. That
authority named the machine a Rotameter. It consisted of a four-wheel
trolley with an additional large fifth wheel which traveled
along the running surface of the rail. Its last recorded use was in the
1920s.

6. Venturimeter
It is a device, which is used for measuring the
rate of flow of fluid through a pipe.
It consists of an
• Inlet section followed by
• Convergent section
• A cylindrical throat and
• A gradually divergent cone.

7. WORKING.
AS THE CROSS SECTION OF THE THROAT
IS SMALLER THAN THE CROSS
SECTIONAL AREA OF THE INLET SECTION,
THE VELOCITY OF FLOW AT THE THROAT
WILL BECOME GREATER THAN THAT AT
THE INLET SECTION, ACCORDING TO
CONTINUITY EQUATION.

8. THE INCREASE IN THE VELOCITY OF FLOW AT THE
THROAT RESULT IN DECREASE IN PRESSURE AT THIS
SECTION. AS SUCH A PRESSURE IS DEVELOPED
BETWEEN THE INLET SECTION AND THE THROAT OF
VENTURIMETER.
THIS PRESSURE DIFFERENCE CAN BE DETERMINED
EITHER BY CONNECTING DIFFERENTIAL
MANOMETER BETWEEN THE PRESSURE TAPS
PROVIDED AT THESE SECTIONS OR BY CONNECTING
THE SEPARATE PRESSURE GAUGE AT EACH OF THE
PRESSURE TAPS. THE MEASURE OF THE PRESSURE
DIFFERENCE BETWEEN THESE SECTIONS ENABLES
THE RATE OF FLOW OF FLUID TO BE CALCULATED.

9. THE CROSS SECTIONAL AREA OF THE THROAT
OF VENTURIMETER SHOULD NOT BE REDUCED
UNTO A CERTAIN LIMIT, OTHERWISE THE
PRESSURE AT THIS SECTION DROPS BELOW
THE VAPOR PRESSURE OF THE FLOWING
FLUID THAN THE FLOWING FLUID MAY
VAPORIZE AND THE VAPOR POCKETS MAY BE
FORMED IN THE LIQUID AT THIS SECTION.

11. Electromagnetic Flowmeters
• Magnetic flowmeters have been widely used in industry
for many years.
• They are easy to install and use to the extent that
existing pipes in a process can be turned into meters
simply by adding external electrodes and suitable
magnets.
• They can measure reverse flows and are insensitive to
viscosity, density, and flow disturbances.
• Electromagnetic flowmeters can rapidly respond to
flow changes and they are linear devices for a wide
range of measurements.
• The induced voltages in an electromagnetic flowmeter
are linearly proportional to the mean velocity of liquids
or to the volumetric flow rates.

12. • The accuracy of these meters can be as low as 0.25% and, in most
applications, an accuracy of 1% is used.
• At worst, 5% accuracy is obtained in some difficult applications
where impurities of liquids and the contact resistances of the
electrodes are inferior as in the case of low-purity sodium liquid
solutions.
• Faraday’s Law of Induction
• This law states that if a conductor of length l (m) is moving with a
velocity v (m/s–1), perpendicular to a magnetic field of flux density
B (Tesla), then the induced voltage e across the ends of conductor
can be expressed by:
e = Blv

13. The velocity of the conductor is
proportional to the mean flow velocity
of the liquid.
Hence, the induced voltage becomes:
e = BDv

14. e = BDv
Q = Av = p
D2v
4
e BQ
D
= 4
p

15. Positive Displacement Flowmeters
• A positive displacement flowmeter, commonly called a PD
meter, measures the volume flow rate of a continuous flow
stream by momentarily entrapping a segment of the fluid into
a chamber of known volume and releasing that fluid back into
the flow stream on the discharge side of the meter.
• By monitoring the number of entrapments for a known period
of time or number of entrapments per unit time, the total
volume of flow or the flow rate of the stream can be
ascertained.
• The total volume and the flow rate can then be displayed
locally or transmitted to a remote monitoring station.

16. Sliding-vane type PD meter.
Tri-Rotor Type PD Meter
Birotor PD Meter
Piston Type PD Meter
Oval Gear PD Meter

17. Advantages PD Meters
• Advantages PD Meters
• High-quality, high accuracy, a wide range, and are
very reliable, insensitive to inlet flow profile
distortions, low pressure drop across the meter.
• Until the introduction of electronic correctors and
flow controls on other types of meters, PD meters
were most widely used in batch loading and
dispensing applications. All mechanical units can be
installed in remote locations.

18. Disadvantages PD Meters
• bulky, especially in the larger sizes.
• the fluid must be clean for measurement accuracy and
longevity of themeter.
• More accurate PD meters are quite expensive.
• Have high inertia of the moving parts; a sudden change in the
flow rate can damage the meter.
• Only for limited ranges of pressure and temperature
• Most PD meters require a good maintenance schedule and are
high repair and maintenance meters.
• Recurring costs in maintaining a positive displacement
flowmeter can be a significant factor in overall flowmeter cost.