chapter08 01 analog flow sensor

8/12/2019 Chapter08 01 Analog Flow Sensor

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FAB10402 SENSOR TECHNOLOGY.

Chapter_06-Cylinder sensor

Handout 1

INDUSTRIAL AUTOMATION AND NETWORKING SECTION

Flow sensors

This chapter will explain the different types of flow sensors and the theory of operation of

each type of sensor. The flow sensor can be classified into 2 types. They are:

Intrusive flow sensor The sensor disturbs the flow of fluid that it is measuring.

Non-intrusive flow sensor The sensor can measure the flow of the fluid without

disturbing it.

Flow technology

Fluid is the term that describes any substance that flows. Liquids such as water or hydraulic

fluid and gasses such as oxygen or nitrogen are all considered fluids since they flow.

The flow rate of a fluid as it flows through a pipe can be calculated by the formula Q = V x A

where

Q = liquid flow through a pipe

V = average velocity of the flow

A = cross-sectional area of the pipe

Example : Determine the flow of hydraulic fluid through a 2 in. diameter pipe that has an

average velocity of 60 in. per second.

Solution:

( )

.14.3

.114.32

2

insqA

inA

rA

=

=

=

sec/.4.118

.14.360

incuQ

inpsiQ

VQ

=

=

=


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From the diagram above we can see that the fluids is swirling as it flows. The swirling actions

tend to create opposition to the flow.

Calculating Flow from Pressure-Drop measurement

One of the easiest ways to determine the amount of flow is to place an obstruction in the flow

such as an orifice plate to create a pressure drop. These types of sensors are categorized as

obstructive flow sensors.

The figure above shows a diagram of the orifice plate that is mounted in a pipe. When fluid

begins to flow in the pipe, a pressure is created on the side identified as P1. When the fluid

flow through the orifice, a lower pressure is created on the opposite side of the orifice plate

identified as P2. The simplified formula using the pressure drop is:

21 PPkQ =

Where Q = flow in gallons/minute (gpm)

k = is the constant that is determined by the orifice plate.

P1 = is higher pressure in front of the orifice

P2 = is lower pressure behind the orifice

Front viewof orifice plate

Side view

Orifice plate

Flange plate

Fluid Flow

Test port P1 Test port P2


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Examples of Flow Meters that utilize a pressure drop

1. Venturi Flow meter

A venture is a point in a pipe that has been narrowed so the flow is restricted slightly. In

figure below notice that a high pressure port is provided in front of the point where the

venture is narrowed down and a low pressure port is provided directly after the point where

the pipe is narrowed down.

The high pressure port is provided to sample the increased fluid pressure where it will

increase slightly because of the restriction caused by the venture, and the low pressure port

is provided to sample the pressure as it drop after it flows past the restriction caused by the

venture. Additional pressure ports are provided on both sides of the restriction in larger

venturies to allow an average pressure to be measured.

The venture is widely used because it has no moving parts and the small amount of

restriction it causes to create the pressure drop does not disturb the fluid flow too much.

This means that the venture can handle larger volumes of flow than most other types of flow

meters.

2. Flow Tubes

The flow tube is similar to the venture in that it places a restriction in the flow that creates a

pressure drop. Figure below shows an example of a flow tube that is located in a pipe.

Test port P1

Test port P2

Fluid Flow


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From this diagram we can see that the flow tubes looks like a short piece of pipe with a thick

wall. The front face of the pipe is rounded so that fluid is disturbed as little as possible when it

enters the tube. The diameter of the flow tube is approximately half the diameter of the

original pipe so that a pressure drop is created as the flow is diverted through the flow tube.

A pressure tap is provided prior to the flow tube to measure the higher pressure and directly

after the opening of the flow tube to measure the lower pressure. The difference in the

pressure must be measured by a pressure differential sensor

3. Pitot Tube

The pitot tube is a device that has two tubes that are placed in a fluid flow to sense the

impact pressure and the static pressure used to determine the amount of pressure drop.

Test port P1Test port P2

Fluid Flow FlowNozzle


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In the first example two tubes are connected side by side. One of the tubes has a hole in it

that faces the fluid flow (impact tube), while the other tube has a hole in it that faces away

from the fluid flow (static tube). The impact tube measures the higher pressure (impact

pressure), and the static tube measures the lower pressure (static pressure) as fluid flows

past the pitot tube.

Test port P1

Test port P2

Fluid Flow

Test port P1

Static tube

Test port P1

Test port P2

Fluid Flow

Test port P1

Static tube


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The second example shows the tubes that are mounted one inside the other. The inside tube

measures the impact pressure, and the outside tube measures the static pressure.

In both examples the ends of the pitot tube that are outside the pipe are connected with

plastic tubing to a sensor or instrument that can measure a very small pressure differential.

The difference between the impact pressure and the static pressure is very small so a

nanometer or ultralow differential pressure sensor must be used to measure the pressure

differential. The manometer can be used if only a visual indication is needed, and a

differential pressure sensor or transducer is required if the pressure drop is converted to an

electrical signal.

4. Elbow Meters

Another to create a pressure drop in a fluid system is to use an existing elbow in the piping

system. It has been determined that the pressure of fluid will show a slight pressure

differential as it passes through an elbow. The fluid that flows near the inside radius of the

elbow will have a slightly lower pressure than the fluid that flows on the outer radius of the

elbow. Figure below shows one of the examples of this type of meter.


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Two low pressure ports are provided on the inner side of the elbow to provide the lower

pressure reading, and two ports are provided on the outer side of the elbow to provided

higher pressure reading so that a better average of each pressure is determined. Theamount of pressure difference from this type of sensor is very small and a manometer or

ultralow differential pressure sensor must be used to sense it. The pressure difference is

used in a calculation to determine fluid flow. The amount of pressure difference will increase

when the fluid flow increases, and the pressure difference will decrease when the fluid flow

decreases.

5. Weirs

Another way to create a pressure difference in a flow of fluid is to place a weir directly in the

flow. A weir is a narrowing of an open channel to create an obstruction that is placed in the

flow to cause a pressure drop as fluid flow through it.

Test port P1

Test port P2

Fluid Flow


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When the fluid flow increases, the pressure drop through the weir will increase. One example

of this type of application would be a weir placed in a canal used to move cooling water to a

pond. Many industries use ponds for cooling water or to store waste water while it is being

treated. It is important to get an estimate of the amount of water that is flowing into the pond

if water-treatment chemicals are added, so a weir is used to measure flow. The pressure of

the water when it is obstructed by the weir will cause the velocity of flow to change slightly,

which will cause a slight pressure increase. When the water exits the weir, its pressure will

decrease slightly. This small amount of pressure difference can be used to calculate flow.

Weirs are also used to extensively in irrigation ditches to measure the amount of water flow.

In these applications the amount of flow is totalized so the water can be sold.

Velocity Flow Meters

Velocity flow meters use the change in velocity that occurs when fluid flow changes to

measure the amount of flow. Examples of the velocity flow meter are the paddlewheel flow

meter, the turbine flow meter, the vortex flow meter, the electromagnetic flow meter, and the

ultrasonic (Doppler) flow meter.

1. Paddlewheel Flow Meters

A paddlewheel flow meter uses a completely different way to determine the amount of fluid

flow than the pressure differential flow meter. This type of sensor has a paddlewheel that is

Fluid Flow

P1

Weir

Weir

P2

P1 P2


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placed directly in the fluid flow so that it can rotate freely as fluid passes it. The fastest the

fluid flows past it, the faster the wheel rotate.

Each paddle or web of paddlewheel has a magnet mounted in it so that a sinusoidal

waveform is produced when it passes the detector mounted in the head of the sensor. A

frequency-to-voltage converter is used to convert the sinusoidal signal to a variable-voltage

signal.

Figure above shows the paddlewheel flow meter mounted in a pipe. It is important to

understand that the paddlewheel flow meter is mounted in a straight run of piping where the

flow will be laminar. The paddlewheel is a low-cost sensor used in applications that do not

require a high degree of accuracy.

2. Turbine Flow Meters

Turbine flow meters are very similar to paddlewheel flow meter except the turbine flow meteris much more accurate. From the diagram below we can see that the turbine wheel is

mounted so that it is directly in the flow. Each of the vanes of the turbine wheel has a magnet

mounted in it so that when the vane spins under the magnetic pickup, an electric pulse is

generated. When fluid starts to flow, the wheel will begin to rotate. When the flow increases,

the wheel will rotate more quickly and the number of electric pulses will increase. The electric


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pulses can be averaged over time to provide a flow rate, or they can be totalized to

determine the total flow.

3. Vortex Flow Meter

The vortex flow meter is also a velocity-type flow meter that places a flow element in the flow

stream that is not streamline. When the flow stream strikes the flow element, a series of

vortices is produced (shedded). For this reason, this type of flow meter is sometimes called a

vortex shedding flow meter. When a vortex is produced, it causes the fluid to create a

swirling motion as it moves. A very sensitive electronic detector can detect the presence of

the vortices. The number of vortices that are produced is directly proportional to the flow rate.

This type of sensor is very accurate and it can also be used when the fluids has suspended

solids (slurries) moving in the flow. The number of vortices that are detected can be

averaged to produce a flow rate, or they can be added to produce a total flow. A frequency-

to-voltage circuit and amplifier must be used to increase the small-pulse signal from the

electronic detector. It is also important to understand that the vortex type flow meter work

best in very turbulence flow. Figure below shows the location of the flow element in a pipe.


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4. Electromagnetic Flow Meter

Another type of flow meter uses the operating principle of creating an electrical field in the

fluid and then measures the strength of the field. This type of flow meter takes advantage of

the principle of electromagnetic induction. As you know, a voltage can be induced in a

conductor when it passed through a magnetic field. In this case the fluid must electrically

conductive so that it will accept the magnetic field and act as conductor.

Flow

Flowelement

vortex


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Figure above shows an electromagnetic flow meter. The meter has two major parts. The first

part is a set of coils that creates the magnetic field. When the fluid flows past these coils, it

will act like an electrical conductor and will hold and electrical charge. The second part of the

flow meter is a set of electrodes that is used as the detector. The detector acts like a

voltmeter and measures the intensity of the electrical charge. The stronger the fluid flow, the

stronger the electrical charge.

This type of flow meter is extremely useful in that it does not need to place any disturbance in

the fluid flow, and it is also useful to measure the flow of very corrosive fluids. Early versions

of this type of meter consumed large amounts of electrical power, but newer versions use

pulse technology to limit the current supplied to the magnetizing coils so that the power

consumption is reduced.

5. Ultrasonic Flow Meters

The ultrasonic flow meter uses Doppler meters to measure the shift of a frequency signal that

is sent into the liquid flowing through a pipe. From the diagram in Figure above we can see

that the transmitting element injects a signal with a given frequency into the fluid flow.

Bubbles in the fluid or any suspended solids in the fluid reflect the signal back to a receiver.

The receiving element called Doppler meter is placed a short distance downstream and it

detects the frequency as the fluid flows past it. A special circuit called a time-of-travel meter,

measure the time delay or shift in the frequency that is caused by the fluid flow. The faster

the fluid flows, the more the frequency is shifted. This type of flow meter is useful because it

can measure the flow without creating any obstruction. It is important to understand that

some amount of suspended solids or bubbles must be present in the flow to get the best

reflection of the signal.

This type of meter is also used extensively as a portable flow meter. The portable flow meter

is used in troubleshooting to detect flow in complex hydraulic systems. When a hydraulic

systems start to fail, it generally leaks fluid past an inoperable valve back to the reservoir.

Since the hydraulic piping is made of metal or hoses, it is difficult to tell how much fluid flow is

being bypassed, so an ultrasonic flow meter can be temporarily placed around each segment


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of the hydraulic piping and the amount of flow in the main lines and the return lines to the

reservoir can be determined.

Positive Displacement Flow Meters

Another category of flow meters is called the positive displacement flow meter. This type of

flow meter is the most accurate in that the flow is broken into segments and each segment is

measured as the flow is moved. One example of a positive displacement flow meter uses a

piston pump. The volume of the piston is known, and all of the fluid flows through the piston

pump. This allows the total volume of fluid to be measured by counting the stroke of the

piston.

In example, if a piston has a volume of 0.1gal, and it makes 200 strokes per minute, the flow

would be 20gpm. Another type of positive displacement flow meter uses a set of oval gears

(lobes) that pump a specific volume of fluid each time the gears mesh. The faster the flow

moves, the faster the gears rotate. The numbers of rotations are counted and the flow rate

can be calculated because the volume of fluid that is pumped during each revolution of the

lobes is known. Fig. 10-67 shows a positive displacement flow meter.

The nutating disk is another type of positive displacement flow meter. The nutating disk is a

movable disk that is offset so that it makes a concentric circular motion each time it rotates.

The housing for the disk is perfectly round, so that each time the disk rotates, its oval path

will trap a specific amount of fluid. Since the volume of trapped fluid is known, the number of

revolutions the disks makes can be counted and the flow rate can be calculated.


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Mass Flow Meter

A mass flow meter is the most accurate type of flow meter designed to measure the flow of

gases as well as other fluids. Two types of mass flow meters are commonly used. The

Coriolis mass flow meter is so named because it uses the Coriolis phenomenon to measure

mass flow instead of volumetric flow, and the other is called a thermal mass flow meter.

The coriolis mass flow meter uses a U-shaped tube that is designed to vibrate up and down

at its natural frequency while all of the fluid flows through it. A strong magnet is used to make

the U-tube vibrate. Fig. 10-68 shows an example of the Coriolis mass flow meter in three

distinct stages. When the U-tube vibrates it will naturally move up and down. When fluid is

flowing through the tube, it will oppose the up and down movement, which will cause the U-

tube to twist. The amount of twist will be directly proportional to the amount of flow. Sensors

are located near the tube to detect the amount of twist and convert it to a usable variable-

voltage signal. This type of mass flow meter is useful to measure fluids whose viscosity

continually changes because they do not need to have pressure or temperature

compensation.

The thermal mass flow meter is usable for measuring the mass flow of gases. This type of

mass flow meter uses a thermal element whose temperature changes as fluids flows past it.

The amount of heat loss is directly proportional to the fluid flow. The thermal element is

mounted close to the fluid flow but it does not come directly into contact with the fluid. This

allows this type of mass flow meter to be used in virtually all types of applications where the

density, pressure, and viscosity may change. The flow meter uses an electronic package that

contains a flow analyzer, temperature compensator, and a signal conditioner to provide a

linear output signal.

Some Consideration of selecting Flowmeter :

1. Is the fluid a gas or liquid ?

2. Is the fluid a gas or Corrosive ?

3. Is the fluid electrically conductive or not ?

4. Does the fluid contain slurry or large solids ?

5. What is the fluid viscosity ?

6. What is the need for accuracy and repeatability ?

7. What is the cost ?


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Chapter 06 Cylinder sensor

Handout 1


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chapter08 01 analog flow sensor

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