19IN Lec 3.pptx

SUBJECT-INSTRUMENTATION & CONTROL
TRANSDUCERS FOR VARIOUS MEASUREMENTS
1

Text Books
2
• Measurements and Instrumentation Principles
By-Alan S Morris

1. Transducers for Pressure measurements
• Units for Pressure
• Pressure Measurement Scales
• Pressure transducers
2. Temperature Transducers
 Thermocouple
 Thermistor
 RTD
3. Transducers for Level measurements
• Level Measurements methods
4. Transducers for Flow measurements
• Flow characteristics
• Flow Measurements Methods/Transducers
OUTLINE
TRANSDUCERS FOR VARIOUS MEASUREMENTS

• The basic SI unit of pressure is the Pascal, which is N/m2.
• Another common unit used in vacuum is Torr (Torricelli).
• Torr (Torricelli): pressure exerted by a 1mm-Hg of mercury or
1/760 of an atmosphere.
• Atmosphere Pressure: force per unit area exerted against a
surface by the weight of air above that surface.
• 1 atm = 101.325 kPa = 760 Torr
• 1 atm= 14.696 psi
• 1 psi=6,894.76Pa
• Another commonly used unit is bar, which is 100K pascal.
• Bar : atmospheric pressure on earth at sea level.
1. TRANSDUCERS FOR PRESSURE MEASUREMENTS
UNITS FOR PRESSURE

• Pressure is an expression of the force and is usually stated in
terms of
force per unit area
P=F/A
• Pressure are exerted by gases, vapors and liquids.
• The instruments that were generally use, however, record as the
difference between the two pressure (the pressure exerted by a
fluid of interest and the ambient atmospheric pressure).

PRESSURE UNITS REALTIONSHIP
Unit Pascal
(Pa)
bar
atmosphere
(atm)
Torr
pound-force per
square inch (psi)
1 Pa 1 10−5 9.8692×10−6 7.5006×10−3 145.04×10−6
1 bar 100,000 1 0.98692 750.06 14.504
1 atm 101,325 1.01325 1 760 14.696
1 Torr 133.322 1.3332×10−3 1.3158×10−3 1 19.337×10−3
1 psi 6,894.76 68.948×10−3 68.046×10−3 51.715 1

Five basic scales are used to measure the pressure:
• Atmosphere pressure (low vacuums measurements)
• Gauge pressure
• Absolute pressure (high vacuums measurements)
• Differential pressure
• Vacuum pressure
PRESSURE MEASUREMENT SCALES

• The atmospheric pressure is the pressure
that an area experiences due to
the force exerted by the atmosphere. Thus, it
is the air pressure exerted upon the earth.
• Atmospheric pressure is approximately 14.7 psi
at sea level and decreases as height increases
(earth level).
• For measuring low vacuum below 1 atm,
atmospheric pressure is taken as “ZERO”, with
unit in-Hg (inch of Hg) to indicate the vacuum (1
atm = 29.92 in-Hg) without the negative sign.
• Hg stand for hydrargyrum (chemical name of
mercury).
• Compound vacuum pressure gauge can measure
pressures both above and below one
atmosphere.
ATMOSPHERIC PRESSURE SCALE
Compound vacuum
pressure gauge

• Gauge pressure scales use atmospheric
pressure as a reference point and extends in
the positive direction.
• If the sensing element is exposed to the
atmosphere, it registers zero pressure.
• Gauge pressure is zero referenced against
ambient air pressure, so it is equal to
absolute pressure minus atmospheric
pressure.
• Negative signs are usually omitted.
• Gauge pressure is measured in psig (pounds
per square inch, gage).
• Examples: Tire pressure, blood pressure
GAUGE PRESSURE SCALE

• For very accurate readings, especially at very low pressures, a gauge
that uses total vacuum as the zero point may be used, giving
pressure readings in an absolute scale.
• Any pressure measured above the absolute zero of pressure is
termed as an Absolute pressure.
• A Pressure of Absolute zero can exist only in complete vacuum. So, it
is equal to gauge pressure + atmospheric pressure.
• Absolute pressure is always used to measure high vacuums, where
Torr and bar are the most used units.
• Absolute pressures are always indicated by positive numbers.
• If the sensing element is exposed to the atmosphere, it will register
14.7 psia (pounds per square inch, absolute).
ABSOLUTE PRESSURE SCALE

• Differential pressure is used to express the difference between
two measured pressures.
• It is determined by subtracting the lower reading from the
higher reading.
• Differential pressures are commonly used in industrial process
systems.
DIFFERENTIAL PRESSURE SCALE

• Pressures below atmospheric pressure are called vacuum
pressures and are measured by vacuum gages that indicate
the difference between the atmospheric pressure and the
absolute pressure.
• The vacuum pressure scale ranges from atmospheric
pressure (as a reference point) to absolute zero pressure.
• A vacuum gage will read zero when measuring atmospheric
pressure and 29.92 in Hg when measuring a complete
vacuum.
VACCUUM PRESSURE SCALE

Pgage = Pabs − Patm
Gauge pressure
Pvac = Patm − Pabs
Vacuum pressure
Pabs = Patm + Pgage
Absolute pressure
Mathematical relationship between pressure scales

• The actual pressure at a given position is called the
absolute pressure, and it is measured relative to
absolute vacuum (i.e., absolute zero pressure).
• Most pressure-measuring devices, however, are
calibrated to read zero in the atmosphere, and so
they indicate the difference between the absolute
pressure and the local atmospheric pressure. This
difference is called the gauge pressure.
• Pressures below atmospheric pressure are called
vacuum pressures and are measured by vacuum
gauges that indicate the difference between the
atmospheric pressure and the absolute pressure.
Summary

 Upon applying pressure, the closed end of the Bourdon tube is displaced.
When a pressure difference exists b/w the inside and outside, tube tends to
straighten out and the end moves.
 Thus, the pressure is converted into a small displacement. The closed end of
the Bourdon tube is connected through mechanical linkage to a sector-pinion
gearing arrangement. The gearing arrangement amplifies the small
displacement and makes the pointer to rotate through a large angle. The dial
scale on the gauge body displays the pressure.
15
(1) BOURDON PRESSURE GAUGE

 A variable capacitance pressure transducer is as shown
in Figure in the next slide.
 The movable plate in the capacitor is the diaphragm.
When the pressure is applied on the diaphragm it
deflects and changes its position, due to which the
distance between the plates is changed.
 The change in capacitance between a metal diaphragm
and a fixed metal plate is measured and calibrated to
the change in pressure.
 These pressure transducers are generally very stable
and linear. They can withstand vibrations. But they are
sensitive to high temperatures. Their performance is
also affected by the dirt and dust as they change the
dielectric constant. 16
(2) CAPACITIVE PRESSURE TRANSDUCER

17
Fig. Variable capacitance pressure transducer

WORKING PRINCIPLE
 A capacitor consists of two conductors (plates) that are
electrically isolated from one another by a nonconductor
(dielectric).
 When the two conductors are at different potentials (voltages),
the system can store an electric charge. The storage
capability of a capacitor is measured in farads.The principle of
operation of capacitive transducers is based upon the
equation for capacitance of a parallel plate capacitor as
shown below
Where, A = Overlapping area of plates; m2,
d = Distance between two plates; m,
e = Permittivity (dielectric constant); F/m.

WORKING PRINCIPLE
 The capacitive transducers work on the principle of change in capacitance of
the capacitor. In most of the cases the above changes are caused by the
physical variables, such as, displacement, force or pressure. Variation in
capacitance is also there when the dielectric medium between the plates
change, as in the case of measurement of liquid or gas levels.
 Therefore, the capacitive transducers are commonly used for different
measurement, by employing the following effects as shown in Fig.
i. Change in capacitance due to change in overlapping area of plates.
ii. Change in capacitance due to change in distance between the two
plates.
iii. Change in capacitance due to change in dielectric between the two
plates
In capacitive transducers, pressure is utilized to vary any of the above-
mentioned factors which will cause change in capacitance and that is a
measureable by any suitable electric bridge circuit and is proportional to the
pressure.

WORKING PRINCIPLE
Fig. Variable capacitive transducer varies; (a) area of overlap,
(b) distance between plates, (c) amount of dielectric between plates

Applications
 If the pressure you want to measure or control is not affected by changes
in atmospheric pressure, as
 when measuring hydraulic pressure,
 pneumatic pressure,
 the level of liquid in an open tank,
then you need to use a sensor that measures gauge pressure.
 If you want to measure pressure that is affected by atmospheric pressure
— in closed systems or sealed containers,
 when assessing altitude in aeronautic applications,
 when you need to measure the atmospheric pressure, itself,
then you require an absolute pressure gauge.
Absolute pressure gauges are typically used in research and scientific
laboratories where fluctuating atmospheric pressure can become an issue
and in aeronautics where precise measurements are critical to determine
altitude.
The food packaging industry makes extensive use of absolute pressure
gauges to assess vacuum independently from atmospheric pressure. This is
a critical application as only very good vacuum packaging can ensure food
quality over a certain period of time.

• Temperature is a scalar quantity that determines the direction of
heat flow between two bodies.
• Temperature measurement is very important in all spheres of
life and especially in the process industries.
• Temperature can be measured by simple devices called
thermometers.
• Thermometers are of two types
1. Liquid-in-Glass thermometers
2. Rotary thermometers
2. TEMPERATURE TRANSDUCERS

• The liquid in glass thermometer, is the most commonly used device to
measure temperature and it is inexpensive to make and easy to use.
• The liquid in glass thermometer has a glass bulb attached to a sealed glass
tube (also called the stem or capillary tube). A very thin opening, called a bore,
exists from the bulb and extends to the centre of the tube.
• The bulb is typically filled with either mercury (special metal that is a liquid at
ordinary, everyday temperatures) or red-coloured alcohol and is free to expand
and rise up into the tube when the temperature increases, and to contract and
move down the tube when the temperature decreases.
• Working Principle
• The principle used to measure temperature is that of the apparent thermal
expansion of the liquid. Matter (Solids, liquids & gases) expands on
heating, this is known as thermal expansion. The rate of expansion in
gases is greater than liquids and the rate of expansion in liquids is greater
than solids.

Fig. liquid-in-glass thermometer

• Liquids used in glass thermometers

• Making a Celsius (centigrade) thermometer is easy, because it's
based on the temperatures of ice and boiling water. These are
called the two fixed points.
• We know ice has a temperature close to 0°C while water boils at
100°C. If we dip our thermometer in some ice, we can observe
where the mercury level comes to and mark the lowest point on
our scale, which will be roughly 0°C.
• Similarly, if we dip the thermometer in boiling water, we can
wait for the mercury to rise and then make a mark equivalent to
100°C. All we must do then is divide the scale between these
two fixed points

• Rotary Thermometers are based on bimetallic strip.
• A bimetallic strip is made up of metal A and B, having different thermal expansion co-
efficient. The two metals are bonded together.
• These thermometers use the following two principles:
1. All metals change in dimension, that is expand or contract when there is a change in
temperature.
2. The rate at which this expansion or contraction takes place depend on the temperature
co-efficient of expansion of the metal and this temperature coefficient of expansion is
different for different metals. Hence, the difference in thermal expansion rates is used
to produce deflections which is proportional to temperature changes.

• Its one end is fixed, and the other end is attached to a
pointer which moves on the scale.
• So, when temperature rises, the coil expands, and the
pointer moves rightward.
• When temperature falls the coil contracts, and the pointer
moves leftward. In this way we can measure temperature.

• Temperature sensors/Transducers which generate output signals
in one of two ways are:
1. Through a change in voltage (output)
• Thermocouple
2. Through a change in resistance
• RTD
• Thermistor

• Commonly found temperature sensing device in industrial
production.
• A thermocouple converts thermal energy into electrical energy.
• Thermocouples can measure temperature at a point in a range
of -250C to +3500C.
• Thermocouples are a very important class of device as they
provide the most common used method of measuring
temperatures in industry.
THERMOCOUPLE

• Thermocouple working principle is based on the Seebeck effect,
discovered by Thomas Seebeck in 1822.
• Electrons flow from one wire to other, due to different energy
potentials of alloys.
• As temperature changes, current flows.
• Voltage is measured between the two alloys (small voltage,
less than 10 mV)
• Seebeck effect: when any conductor is subjected to thermal
gradient, it generates a voltage.
OR
• A temperature gradient along a conductor creates an EMF.
• It was also noticed that different metal combinations have a
different voltage difference.
THERMOCOUPLE-SEEBECK EFFECT

• If two conductors of different materials are joined at one point,
an EMF is created between the open ends which is dependent
upon the temperature of the junction.
• As T1 increases, so does Voltage (V).
• EMF also depends on the temperature of the open ends T2.
• The junction is placed in the process, the other end is in iced
water at 0°C. This is called the reference junction.
• In a circuit composed of two different metals A and B, if joints
are at different temperatures T1 & T2, an electrical voltage E
proportional to T2-T1 is present.
THERMOCOUPLE-SEEBECK EFFECT

 Thermocouple is made by two different metal wires joined at one
end, this joint end is placed in a temperature zone where temperature
should be measured called “hot zone” and the other end of
thermocouple where two metal wire are open(not connected or
joined) is placed in a low room temperature called “cold zone or
reference temperature.”
 Now two ends of this metal pair are placed in two different
temperature zone. A net thermoelectric voltage is generated
according to the temperature difference between two ends. This
voltage is measured in the open pair placed in cold zone or reference
zone.
THERMOCOUPLE-Working Principle

• A set of metal couples are commonly used in industry
to maximize sensitivity and linearity in their range
following an international standard that defines also
their expected uncertainty.
• thermocouple materials are classified as
• BASE METAL : Types E, J, K, N, and T
• NOBLE METAL: Types R, S, and B
THERMOCOUPLE-Metals

THERMOCOUPLE- Material characteristics

Alloys used:
• Constantan: 55% Copper and 45% Nickel
• Cromel: 90% Ni + 10% Cr
• Alumel: 95% Ni + 2% Mn + 2% Al + 1% Silicon
• Nicrosil: 14.4%Cr +1.4 Silicon + 0.1% Mn + Ni
• Nisil: same as Nicrosil but different %

• Type K: ‘General Purpose' and low-cost thermocouple, very
popular
• Type J: Limited range (-40 to +750°C), less popular than type K.
• Type E: High output (68 mV/°C) well suited to low temperature
(cryogenic) use.
• Type N: High stability and resistance to high temperature
oxidation, designed as an 'improved' type K, it’s becoming more
popular.
• Type T: They are used for moist or sub-zero temperature
monitoring applications because of superior corrosion resistance

Figure 3: A graph of the temperature sensitivity of various
thermocouples. K: chromel/alumel, T: Copper/Constantan, J:
Iron/Constantan, E: Chromel/Constantan R: Platinium, 13%
Rhodium/Platinium etc
THERMOCOUPLE

• Wide Range
• Fast Response
• Active
• Inexpensive
THERMOCOUPLE -CAPABILITIES

• Non-Linear
• Accuracy
• Often between 0.5 and 2.2ºC, depending on TC type
• Noise
• Long leads can attract electrical signals
• Low strength signal from thermocouple
• Thermal shunting
• Heating of wire mass can affect measurements by absorbing
energy
• Corrosion
• High alkali or water environments can modify calibration
THERMOCOUPLE –LIMITATIONS

• Thermocouples are most suitable for measuring over a large
temperature range.
• They are less suitable for applications where smaller
temperature differences need to be measured with high
accuracy, for example the range 0–100 °C with 0.1 °C accuracy.
For such applications, Thermistors and RTD’s are more suitable.
THERMOCOUPLE –APPLICATIONS

• Thermistor is a combination of the words THERMal & resISTORS
• Thermistor is a temperature-sensing element composed of
semiconductor materials (typically a mix of metal oxides-
ceramic or polymer) that exhibits/gives a large change in
resistance in response to a small change in temperature .
• Thermistors usually have negative temperature coefficients
(NTC), the resistance of the thermistor decreases as the
temperature increases
• Positive temperature coefficient (PTC) thermistors also exist,
they make good current limiting devices.
• Thermistor operating range can be -45 °C to + 150°C
THERMISTOR

THERMISTOR
•Normally, thermistors have a
negative temperature coefficient,
i.e., the resistance decreases as
the temperature increases,
according to:
•This relationship is illustrated in
following Figure.

• Thermistors are generally accepted to be the most
advantageous sensor in many applications including
temperature measurement and control.
THERMISTOR
Leads, coated Disc Thermistor

• The relationship between resistance and temperature is linear,
then:
∆𝑹 = 𝑲∆𝑻
Whereas,
∆𝑹 = change in resistance
∆𝑻= change in temperature
K = temperature co-efficient of resistance
• Thermistors can be classified into two types depending on the sign of
k.
• If k is negative, resistance decreases with increasing temperature,
device is called negative temperature coefficient (NTC) thermistor.
• If k is positive, resistance increases with increasing temperature,
device is called a positive temperature coefficient (PTC) thermistor,
also known as Resistance Temperature Detector (RTD)
THERMISTOR TYPES

• Accuracy: Thermistors are one of the most accurate types of
temperature sensors. Typical accuracy of ±0.2°C.
• Range: Thermistors are fairly limited in their temperature range,
working only over a nominal range up to 150°C .
• Stability: Thermistors are chemically stable and not significantly
affected by aging.
• Sensitivity: Thermistors are very sensitive (up to 100 times more
than RTDs and 1000 times more than thermocouples) and can
detect very small changes in temperature.
• Speed: Thermistors are very fast, and due to their speed, they
are used for precision temperature control when very small
temperature differences must be detected quickly.
THERMISTOR CHARACTERISTICS

THERMISTOR-APPLICATIONS

• RTDs stands for Resistance Temperature Detectors
• RTDs are temperature sensors that exploit the predictable
change in electrical resistance of some materials with changing
temperature.
• RTDs are manufactured from metals whose resistance increases
with temperature.
• Within a limited temperature range, this resistivity increases
linearly with temperature:
Rt = R0[1 + α t − t0 ]
where:
Rt= resistance at temperature t
R0= resistance at a standard temperature t
α= temperature coefficient of resistance (°C−1
)
RTDs

• The “alpha” (α) constant is known as the temperature coefficient of
resistance and symbolizes the resistance change factor per degree of
temperature change.
• Just as all materials have a certain specific resistance (at 20o C), they
also change resistance according to temperature by certain amounts.
• For pure metals, this coefficient is a positive number, meaning that
resistance increases with increasing temperature.
• For the elements carbon, silicon, and germanium, this coefficient is a
negative number, meaning that resistance decreases with increasing
temperature.
• For some metal alloys, the temperature coefficient of resistance is very
close to zero, meaning that the resistance hardly changes at all with
variations in temperature (a good property if you want to build a
precision resistor out of metal wire!).
RTDs

TEMPERATURE COEFFICIENTS OF
RESISTANCE, AT 20 DEGREES C
50
Material Element/Alloy "alpha" per degree Celsius
Nickel Element 0.005866
Iron Element 0.005671
Molybdenum Element 0.004579
Tungsten Element 0.004403
Aluminum Element 0.004308
Copper Element 0.004041
Silver Element 0.003819
Platinum Element 0.003729
Gold Element 0.003715
Zinc Element 0.003847
Steel* Alloy 0.003
Nichrome Alloy 0.00017
Nichrome V Alloy 0.00013
Manganin Alloy +/-0.000015
Constantan Alloy 0.000074
* = Steel alloy at 99.5 percent iron, 0.5 percent carbon
* = Steel alloy at 99.5 percent iron, 0.5 percent carbon
The following table gives the temperature coefficients of resistance for
several common metals, both pure and alloy:

• Usually, they are provided in encapsulated probes
• They have an external indicator, controller or transmitter, or
enclosed inside other devices where they measure temperature
as a part of the device's function (i.e. temperature controller,
precision thermostat... )
RTDs
Platinum Wire RTD
Thin Film RTD

• RTDs are made up of platinum, nickel, iron or copper wound
around an insulator.
• Temperature range: wide temperature range about -196°C to
482°C.
• Accuracy: Industrial RTDs are very accurate, can be as high as
±0.1°C.
• The ultra high accurate version of RTD is known as Standard
Platinum Resistance Thermometers (SPRTs) having accuracy
at ±0.0001 °C.
• Good accuracy (better than thermocouples)
• Good interchangeability (exchange doesn’t disturb operation)
• Long-term stability
RTDs

• They are the most popular RTD type
• Nearly linear over a wide range of temperatures
• Small enough to have response times of a fraction of a
second
• They are among the most precise temperature sensors
available with resolution and measurement
uncertainties of ±0.0001°C or better.
RTDs- PRTS( platinum RTDs)

54
RTDs Resistance vs Temperature Graph

• Cost: thermocouples are cheapest by far, followed by RTDs
• Accuracy: RTDs or thermistors
• Sensitivity: thermistors
• Speed: thermistors
• Stability at high temperatures: not thermistors
• Size: thermocouples and thermistors can be made quite small
• Temperature range: thermocouples have the highest range,
followed by RTDs
Choice Between RTDs, Thermocouples, Thermisters

• A wide variety of instruments are available for measuring the
level of liquids. Some of these can also be used to measure the
levels of solids that are in the form of powders or small particles.
• In some applications, only a rough indication of level is needed,
and simple devices such as dipsticks or float systems are
adequate.
• However, in other cases where high accuracy is demanded,
other types of instrument must be used.
3. TRANSDUCERS FOR LEVEL MEASUREMENTS

• Dipsticks offer a simple means of measuring level approximately.
The ordinary dipstick is the cheapest device available.
• This consists of a metal bar on which a scale is etched, as shown
in following Figure (a).
• The bar is fixed at a known position in the liquid-containing
vessel.
• A level measurement is made by removing the instrument from
the vessel and reading off how far up the scale the liquid has
wetted. As a human operator is required to remove and read the
dipstick, this method can only be used in relatively small and
shallow vessels.
DIPSTICKS

• This essentially mechanical systems of measurement is
popular in many applications, but the maintenance
requirements are always high.
DIPSTICKS
Figure: Dipsticks: (a) simple dipstick; (b) optical dipstick.

• The optical dipstick, illustrated in following Figure (b), is an
alternative form that allows a reading to be obtained without
removing the dipstick from the vessel,
• It is applicable to larger, deeper tanks.
• Light from a source is reflected from a mirror, passes round the
chamfered end of the dipstick, and enters a light detector after
reflection by a second mirror. When the chamfered end comes
into contact with the liquid, its internal reflection properties are
altered, and light no longer enters the detector.
• By using a suitable mechanical drive system to move the
instrument up and down and measure its position, the liquid
level can be monitored.
OPTICAL DIPSTICKS

• Float systems, whereby the position of a float on the surface of a
liquid is measured by means of a suitable transducer, have a
typical measurement inaccuracy of ±1%.
• This method is also simple cheap and widely used.
• The system using a potentiometer, shown in Figure, is very
common, and is well known for its application to monitoring the
level in motor vehicle fuel tanks.
FLOAT SYSTEMS
Figure: Float system

 Ultrasonic level measurement is one of several non-contact
techniques available.
 The principle of the ultrasonic level gauge is that energy
from an ultrasonic source above the liquid is reflected back
from the liquid surface into an ultrasonic energy detector,
as illustrated in Figure.
61
ULTRASONIC LEVEL GAUGE

 Measurement of
the time of flight
allows the liquid
level to be
calculated.
62
ULTRASONIC LEVEL GAUGE

• The rate at which fluid flows through a closed pipe can be
quantified by either measuring
• the mass flow rate or
• the volume flow rate
• Of these alternatives, mass flow measurement is more accurate,
since mass, unlike volume, is invariant (doesn’t change under
different pressure conditions).
• In the case of the flow of solids, the choice is simpler, since only
mass flow measurement is appropriate.
4. TRANSDUCERS FOR FLOW MEASUREMENTS

• The method used to measure mass flow rate is largely
determined by whether the measured quantity is in a solid,
liquid or gaseous state.
• The main techniques available are
• Conveyor-based methods
• These methods are concerned with measurement of the
flow of solids
• Coriolis flowmeter
• The Coriolis flowmeter is primarily used to measure the
mass flow rate of liquids.
• Thermal mass flow measurement
• Thermal mass flowmeters are primarily used to measure
the flow rate of gases.
MASS FLOW RATE

• These methods are concerned with measurement of the flow of
solids that are in the form of small particles.
• Such particles are usually produced by crushing or grinding
procedures in process industries, and the particles are usually
transported by some form of conveyor.
• This mode of transport allows the mass flow rate to be
calculated in terms of the mass of material on a given length of
conveyor multiplied by the speed of the conveyor.
• Following figure shows a typical measurement system.
MASS FLOW RATE- CONVEYOR-BASED METHODS

• A load cell measures the mass M of material distributed over a
length L of the conveyor. If the conveyor velocity is v, the mass
flow rate, Q, is given by:
MASS FLOW RATE- CONVEYOR-BASED METHODS
Figure: Conveyor-based mass flow rate measurement

• Volume flow rate is an appropriate way of quantifying the flow
of all materials that are in a gaseous, liquid or semi-liquid slurry
form (where solid particles are suspended in a liquid host),
although measurement accuracy is inferior to mass flow
measurement.
• Materials in these forms are carried in pipes, and various
instruments can be used to measure the volume flow rate.
VOLUME FLOW RATE

 Differential pressure meters involve the insertion of some device into
a fluid-carrying pipe that causes an obstruction and creates a pressure
difference on either side of the device.
 Such meters are sometimes known as obstruction-type meters or
flow-restriction meters. One of the devices used to obstruct the flow
include the orifice plate, as illustrated in Figure.
Differential pressure meters
Figure: orifice plate

 When such a restriction is placed in a pipe, the velocity of
the fluid through the restriction increases and the pressure
decreases.
 The volume flow rate is then proportional to the square root
of the pressure difference across the obstruction. A
differential pressure transducer is then used to find the
pressure difference.
70
Orifice Plate
P1 P2

 The orifice plate is a metal disc with a concentric hole
in it, which is inserted into the pipe carrying the flowing
fluid.
 Orifice plates are simple, cheap and available in a wide
range of sizes. They account for almost 50% of the
instruments used in industry for measuring volume
flow rate.
 One limitation of the orifice plate is that its inaccuracy
is typically at least ±2% and may approach ±5%. Also,
the permanent pressure loss caused in the measured
fluid flow is between 50% and 90%.
71

 The ultrasonic technique of volume flow rate measurement
is a non-invasive method.
 It is particularly useful for measuring the flow of corrosive
fluids and slurries (a semi-liquid mixture).
 Besides its high reliability and low maintenance
requirements, a further advantage of an ultrasonic
flowmeter is that the instrument can be clamped externally
onto existing pipework rather than being inserted as an
integral part of the flow line.
 As the procedure of breaking into a pipeline to insert a
flowmeter can be as expensive as the cost of the flowmeter
itself, the ultrasonic flowmeter has enormous cost
advantages.
72
Ultrasonic Flowmeter

 Its clamp-on mode of operation has significant safety advantages
in avoiding the possibility of personnel installing flowmeters
encountering hazardous fluids such as poisonous, radioactive,
flammable or explosive ones.
 There is no permanent pressure loss like in differential pressure
flow meters. Also, any contamination of the fluid being
measured (e.g. food substances and drugs) is avoided.
73
Ultrasonic Flowmeter

 The transit-time ultrasonic flowmeter is an instrument designed
for measuring the volume flow rate in clean liquids or gases.
 It consists of a pair of ultrasonic transducers mounted along an axis
aligned at an angle with respect to the fluid-flow axis, as shown in
Figure.
 Each transducer consists of a transmitter receiver pair, with the
transmitter emitting ultrasonic energy which travels across to the
receiver on the opposite side of the pipe.
74
Transit-time ultrasonic flowmeter

75
Transit-time ultrasonic flowmeter
Ultrasonic element
Ultrasonic element
Flow
Figure: Transit-time ultrasonic flowmeter
• Fluid flowing in the pipe causes a
time difference between the transit
times of the beams travelling
upstream and downstream.
• When there is no flow the
upstream and downstream transit
times are same.
• When there is flow then ultrasonic
signals are accelerated in the
direction of flow and decelerated
against the flow.
• As a result signals take less time in
the direction of flow and more time
against the direction of flow.
Therefore, the time difference is
directly proportional to the flow
velocity. So calculation of time
difference gives flow velocity.

19IN Lec 3.pptx

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