The document discusses various fluid flow measurement devices used throughout history and today. Early civilizations used simple obstructions in rivers to measure flow rates. Modern devices discussed include Venturi tubes, rotameters, pitot tubes, vortex flowmeters, calorimetric flowmeters, weirs, and orifice plates. Each functions using a different principle such as differential pressure, buoyancy, heat transfer, or vortex shedding to obtain a flow rate measurement. The document provides brief descriptions of the operating principles and applications of each type of flow meter.

1. FLOW MEASURING DEVICES

2. Fluid flow measurement can encompass a wide variety of fluids and applications. To meet this wide variety of
applications the instrumentation industry has, over many years, developed a wide variety of instruments. The
earliest known uses for flow come as early as the first recorded history. The ancient Sumerian cities of UR and
Kish, near the Tigris and Euphrates rivers (around 5000 B.C.) used water flow measurement to manage the flow
of water through the aqueducts feeding their cities. In this age the a simple obstruction was placed in the water
flow, and by measuring the height of the water flowing over the top of the obstruction, these early engineers could
determine how much water was flowing. In 1450 the Italian art architect Battista Alberti invented the first
mechanical anemometer. It consisted of a disk placed perpendicular to the wind, and the force of the wind caused
it to rotate. The angle of inclination of the disk would then indicate the wind velocity. This was the first recorded
instrument to measure wind speed. An English inventor, Robert Hooke reinvented this device in 1709, along with
the Mayan Indians around that same period of time. Today we would look down our noses at these crude methods
of flow measurement, but as you will see, these crude methods are still in use today.
Fluid Flow Measurement

3. VenturiTube
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Due to simplicity and dependability, the Venturi tube flowmeter is often used in applications
where it's necessary with higher Turn down rates, or lower pressure drops, than the orifice plate
can provide.
In the Venturi Tube the fluid flowrate is measured by reducing the cross sectional flow area in the
flow path, generating a pressure difference. After the constricted area, the fluid is passes through
a pressure recovery exit section, where up to 80% of the differential pressure generated at the
constricted area, is recovered.
With proper instrumentation and flow calibrating, the Venturi Tube flowrate can be reduced to
about 10% of its full scale range with proper accuracy. This provides a turndown rate 10:1.
Orifice, nozzle and Ventury meter.

4. .
VenturiTube
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8. Rotameter
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The rotameter consists of a vertically oriented glass (or plastic) tube with a larger end at the top, and a
metering float which is free to move within the tube. Fluid flow causes the float to rise in the tube as
the upward pressure differential and buoyancy of the fluid overcome the effect of gravity.
The float rises until the annular area between the float and tube increases sufficiently to allow a
state of dynamic equilibrium between the upward differential pressure and buoyancy factors, and
downward gravity factors.
The height of the float is an indication of the flow rate. The tube can be calibrated and graduated in
appropriate flow units.
The Rota meter meter typically have a Turn Down Ratio up to 12:1. The accuracy may be as good as
1% of full scale rating.
Magnetic floats can be used for alarm and signal transmission functions.

9. .
Rotameter
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10. PitotTube
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The pitot tube are one the most used (and cheapest) ways to measure fluid flow,
especially in air applications like ventilation and HVAC systems, even used in
airplanes for speed measurent.
The pitot tube measures the fluid flow velocity by converting the kinetic energy
of the flow into
potential energy.
The use of the pitot tube is restricted to point measuring. With the "annubar", or
multi-orifice pitot probe, the dynamic pressure can be measured across the
velocity profile, and the annubar obtains an averaging effect.

11. .
PitotTube
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12. Vortexflowmeter
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Principle of Vortex flowmeter - An introduction to the vortex
flowmeter principle.
• An obstruction in a fluid flow creates vortices in a downstream flow.
Every obstruction has a critical fluid flow speed at which vortex
shedding occurs. Vortex shedding is the instance where alternating low
pressure zones are generated in the downstream.

13. Vortex flowmeter
These alternating low pressure zones cause the obstruction to move towards the low
pressure zone.
With sensors gauging the vortices the strength of the flow can be measured.
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14. CalorimetricFlowmeter
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The calorimetric principle for fluid flow measurement is based on two temperature sensors in
close contact with the fluid but thermal insulated from each other.
One of the two sensors is constantly heated and the cooling effect of the flowing fluid is used to
monitor the flowrate. In a stationary (no flow) fluid condition there is a constant temperature
difference between the two temperature sensors. When the fluid flow increases, heat energy is
drawn from the heated sensor and the temperature difference between the sensors are reduced.
The reduction is proportional to the flow rate of the fluid.
Response times will vary due the thermal conductivity of the fluid. In general lower thermal
conductivity require higher velocity for proper measurement.
The calorimetric flow meter can achieve relatively high accuracy at low flow rates.

15. CalorimetricFlowmeter
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16. Weirs
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Weirs are structures consisting of an obstruction such as a dam or bulkhead placed
across the open channel with a specially shaped opening or notch. The flow rate over a
weir is a function of the head on the weir.
Common weir constructions are the rectangular weir, the triangular or v-notch weir,
and the broad-crested weir. Weirs are called sharp-crested if their crests are
constructed of thin metal plates, and broad-crested if they are made of wide timber or
concrete.

17. Weirs
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18. OrificePlate
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With an orifice plate, the fluid flow is measured through the difference in pressure from the
upstream side to the downstream side of a partially obstructed pipe. The plate obstructing the
flow offers a precisely measured obstruction that narrows the pipe and forces the flowing fluid
to constrict.
The orifice plates are simple, cheap and can be delivered for almost any application in any
material.
The Turndown rates for orifice plates are less than 5:1. Their accuracy are poor at low flow rates.
A high accuracy depend on an orifice plate in good shape, with a sharp edge to the upstream
side.
Wear reduces the accuracy.
Orifice, Nozzle and Ventury meter.

19. OrificePlate
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