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Unit V
Bridges & Measurement Of
Physical Parameters
-BY
GVNSK SRAVYA
ASST. PROFESSOR
ECE DEPT.
Contents
Bridges:
 Wheat stone Bridge,
 Kelvin Bridge, and
 Maxwell Bridge
Measurement of Physical Parameters:
 Flow measurement
 Displacement meters
 Liquid level measurement
 Temperature Measurement
 Measurement of Humidity and Moisture
 Velocity Measurement
 Force Measurement
 Pressure- High pressure
 Vacuum level Measurement
 Data acquisition systems
2
Bridges
 Bridge circuit is an instrument to measure resistance, capacitance,
inductance and impedance.
 In simplest form it consists of four arms network.
 They use Comparison measurement technique and null- indication
principle.
 This means the indication is independent of the calibration of the
indicating device or any characteristics of it.
 Unknown components value is calculated when the bridge is
balanced.
3
Advantages of Bridge
Circuits
 Balance equation is independent of the magnitude of the input
voltage or the source impedance.
 High measurement accuracy since measurement is based on
comparison.
 Accuracy depends on components values and not on the
characteristics of the null detector.
4
Contd…
 Balance equation is independent of the sensitivity of the
null detector.
 Balance condition transparent to the interchange of source and
detector positions.
 Can be used in the control circuits.
5
Types of Bridges
 Two types of bridges are used in the measurement based on the
voltage signal with which those can be operated.
• DC Bridges- Wheatstone Bridge, Kelvin Bridge
• AC Bridges- Maxwell Bridge
6
DC Bridges
 If the bridge circuit can be operated with only DC voltage signal,
then it is a DC bridge circuit or simply DC bridge.
 DC bridges are used to measure the value of unknown resistance.
 The circuit diagram of DC bridge looks like as shown in below
figure.
7
Contd…
 The DC bridge has four arms and each arm consists of a resistor.
 Among which, two resistors have fixed resistance values, one resistor is a
variable resistor and the other one has an unknown resistance value.
 The above DC bridge circuit can be excited with a DC voltage source by
placing it in one diagonal.
 The galvanometer is placed in other diagonal of DC bridge. It shows some
deflection as long as the bridge is unbalanced.
8
Contd…
 Vary the resistance value of variable resistor until the galvanometer shows
null (zero) deflection.
 Now, the above DC bridge is said to be a balanced one. So, we can find the
value of unknown resistance by using nodal equations.
 DC bridges can be operated with only DC voltage signal.
 DC bridges are useful for measuring the value of unknown resistance, which
is present in the bridge.
 Wheatstone’s Bridge & Kelvin Bridge are examples of DC bridge.
9
Wheatstone Bridge
 Wheatstone’s bridge is a simple DC bridge, which is mainly having
four arms. These four arms form a rhombus or square shape and
each arm consists of one resistor.
 To find the value of unknown resistance, we need the galvanometer
and DC voltage source. Hence, one of these two are placed in one
diagonal of Wheatstone’s bridge and the other one is placed in
another diagonal of Wheatstone’s bridge.
 It measures resistance from 1 ohm to 1megaohm.
10
Contd…
 Wheatstone’s bridge is used to measure the value of medium resistance.
The circuit diagram of Wheatstone’s bridge is shown in below figure.
11
Contd…
 R3 is a standard resistor and R4 is unknown resistor which is to be
measured.
 Bridge can be balanced by varying the value of R3 resistor.
 The above bridge circuit is balanced when no current flows through
the diagonal arm, DB. That means, there is no deflection in the
galvanometer, when the bridge is balanced.
12
Contd…
 The bridge will be balanced, when the following two conditions are
satisfied.
13
Contd…
 As galvanometer current is zero,
14
Contd…
15
Sensitivity of Wheatstone
Bridge
 When the bridge is in unbalanced condition, current flows through the
galvanometer causing deflection of the pointer.
 The amount of deflection is the function of sensitivity of the galvanometer.
 The sensitivity can be expressed as amount of deflection per unit
current(mm/µA, radians/µA or degrees/µA).
 Greater the sensitivity of the galvanometer, greater its deflection.
16
Unbalanced Wheatstone bridge
 By using Thevenin's theorem, the current through the galvanometer
can be determined.
17
Unbalanced Wheatstone bridge
18
Unbalanced Wheatstone bridge
19
Unbalanced Wheatstone bridge
20
Applications
1. It is a DC bridge which is used to measure the resistance from 1ohm to
low mega ohm.
2. A Wheatstone bridge may be used to measure the dc resistance of various
types of wire, either for the purpose of quality control of the wire itself, or
of some assembly in which it is used.
3. For example, the resistance of motor windings, transformers, solenoids,
and relay coils can be measured.
4. Wheatstone Bridge Circuit is also used extensively by telephone
companies and others to locate cable faults.
5. The fault may be two lines shorted together, or a single line shorted to
ground.
21
Limitations
1. For low resistance measurement, the resistance of the leads and contacts
becomes significant and introduces an error. This can be eliminated by
Kelvin’s Double bridge.
2. For high resistance measurements, the resistance presented by the bridge
becomes so large that the galvanometer is insensitive to imbalance.
Therefore, a power supply has to replace the battery and a dc VTVM
replaces the galvanometer. In the case of high resistance measurements in
mega ohms, the Wheatstone's bridge cannot be used.
3. Another difficulty in Wheatstone Bridge Circuit is the change in
resistance of the bridge arms due to the heating effect of current through
the resistance. The rise in temperature causes a change in the value of the
resistance, and excessive current may cause a permanent change in value.
22
Problems
1. Wheatstone bridge consists of the following parameters,
R1=10K ohm, R2=15K ohm and R3=40K ohm. Find
the unknown resistance Rx.
Solution: R1R4 = R2R3
R1Rx=R2R3
Rx= R2R3/R1 = (15K X 40K)/10K
= 60K ohms
23
Problems
2. An unbalanced Wheatstone bridge
consists of the following parameters,
R1=1K ohm, R2=2.5K ohm,
R3=3.5K ohm, R4 = 10K ohm and
Rg=300 ohms and DC source is of
6V. Find the current through the
galvanometer.
24
Problems
Solution:
Eth = 0.312V
Rth = 2.778K ohms
Ig= 42.88 micro amperes
25
Kelvin’s Bridge
 When the resistance to be measured is of the order of magnitude of
bridge contact and lead resistance, a modified form of Wheatstone’s
bridge, the Kelvins Bridge theory is employed.
 Kelvins Bridge theory is a modification of Wheatstone’s bridge and is
used to measure values of resistance below 1 Ω.
 In low resistance measurement, the resistance of the leads connecting
the unknown resistance to the terminal of the bridge circuit may affect
the measurement.
26
Contd… 27
Contd…
 Ry represents the resistance of the connecting leads from R3 to
Rx (unknown resistance).
 The galvanometer can be connected either to point c or to point a.
 When it is connected to point a, the resistance Ry, of the connecting lead
is added to the unknown resistance Rx, resulting in too high indication
for Rx.
 When the connection is made to point c, R3, is added to the bridge arm
R3 and resulting measurement of Rx is lower than the actual value,
because now the actual value of R3 is higher than its nominal value by
the resistance Ry.
28
Contd…
 If the galvanometer is connected to point b, in between points c and a, in
such a way that the ratio of the resistance from c to b and that from a to
b equals the ratio of resistances R1 and R2, then
29
and the usual balance equations for the bridge give the relationship.
Contd… 30
Contd… 31
Contd… 32
Contd… 33
 Equation (11.8) is the usual Wheatstone’s balance equation and it
indicates that the effect of the resistance of the connecting leads
from point a to point c has been eliminated by connecting the
galvanometer to an intermediate position, b.
 The above principle forms the basis of the construction of Kelvin’s
Double Bridge, popularly known as Kelvin’s Bridge. It is a Double
bridge because it incorporates a second set of ratio arms. Figure
11.11 shows a schematic diagram of Kelvin’s double bridge.
Kelvins double bridge 34
Contd… 35
 The second set of arms, a and b, connect the galvanometer to a point
c at the appropriate potential between m and n connection, i.e. Ry.
The ratio of the resistances of arms a and b is the same as the ratio of
R1 and R2. The galvanometer indication is zero when the potentials
at k and c are equal.
Contd… 36
Contd… 37
Contd… 38
 This is the usual equation for Kelvins Bridge theory. It indicates that
the resistance of the connecting lead has no effect on the
measurement, provided that the ratios of the resistances of the two
sets of ratio arms are equal. In a typical Kelvins Bridge theory the
range of a resistance covered is 1 — 0.00001 Ω (10 μ ohm) with an
accuracy of ± 0.05% to ± 0.2%.
Advantages 39
 It can measure the resistance value in the range of 0.1 µohms to 1.0
ohm.
 Power consumption is less
 Simple in construction
 Sensitivity is high.
Disadvantages 40
 For knowing whether the bridge is balanced or not, the sensitive
galvanometer is used.
 To obtain good sensitivity of the device, a high current is required.
 Manual adjustments are to made periodically when required.
Problems 41
 In the figure below the ratio of Ra to Rb is 1000 ohm, R1 is 5 ohm
and R1 is 0.5R2, What is the value of Rx.
Problems 42
Problems 43
Problems 44
Problems 45
AC Bridges 46
 An AC Bridge, In its simplest form consists of 4 arms, a source of
excitation and a balance detector. Each arm consists of impedance.
 The source is an ac supply which supplies ac voltages at required
frequency.
 For low frequencies, powerline itself can act as source of supply.
 For high frequencies, the electronic oscillators are used as source. A typical
oscillator has a range of 50 Hz to 125kHz with a power output of around 7
W.
 Commonly used balanced detectors are head phones, tunable amplifiers or
vibration galvanometers.
Contd… 47
 The headphones are used as detectors at frequencies of 250Hz. to 3
to 4KHz.
 For single frequency a tuned detector is most sensitive detector at
frequency range is from 10Hz to 100Hz.
 For low audio frequencies vibration galvanometers are used at 5Hz
to 1000Hz, commonly used below 200Hz.
Contd… 48
 Impedances at AF or RF are commonly determined by means of an
ac Wheatstone bridge.
 This bridge is similar to a dc bridge, except that the bridge arms are
impedances.
 The bridge is excited by an ac source rather than dc and
the galvanometer is replaced by a detector, such as a pair of
headphones, for detecting ac.
AC Bridge 49
Contd… 50
 When the bridge is balanced, where Z1, Z2, Z3 and Z4 are
the impedances of the arms, and are vector complex quantities that
possess phase angles.
 It is thus necessary to adjust both the magnitude and phase angles of
the impedance arms to achieve balance, i.e. the bridge must be
balanced for both the reactance and the resistive component.
Maxwell’s Bridge 51
 It is an Maxwell Inductance Capacitance bridge developed by
Maxwell.
 It measures an unknown inductance in terms of a known
capacitance.
Contd… 52
Contd… 53
 The use of standard arm offers the advantage of compactness and
easy shielding. The capacitor is almost a loss-less component.
 One arm has a resistance R1 in parallel with C1, and hence it is
easier to write the balance equation using the admittance of arm 1
instead of the impedance.
Contd… 54
 The general equation for bridge balance is
Contd… 55
Contd… 56
 Maxwell Bridge Theory is limited to the measurement of low Q
values (1 — 10). The measurement is independent of the excitation
frequency. The scale of the resistance can be calibrated to
read inductance directly.
Maxwell Bridge Advantages and
Disadvantages
57
Advantages
 The frequency does not appear in the final expression of both
equations, hence it is independent of frequency.
 Maxwell’s inductor capacitance bridge is very useful for the wide
range of measurement of inductor at audio frequencies.
Contd…
58
 The bridge is particularly suited for inductances measurements,
since comparison with a capacitor is more ideal than with
another inductance.
 Commercial bridges measure from 1 — 1000 H, with ± 2% error. (If
the Q is very large, R1 becomes excessively large and it is
impractical to obtain a satisfactory variable standard resistance in
the range of values required).
Contd…
59
Disadvantages
1. The variable standard capacitor is very expensive.
2. Inductance cannot be measured over a wide range, since Q is
proportional to inductance.
3. The bridge is limited to measurement of low quality coils (1 < Q <
10) and it is also unsuitable for low value of Q (i.e. Q < 1) from this
we conclude that a Maxwell bridge is used suitable only for medium
Q coils.
Problems
60
 In Maxwell’s Bridge, as shown in the figure below, Determine the
values of the resistance R1 and inductance L1 of a coil after the
bridge is balanced with capacitance of 0.5 µF.
Contd…
61
 Solution:
R1=(R2R3)/R4
R1= 375 Ω.
L1= C1R2R3
L1 = 0.5µ x 2000 x 750 = 75mH
Contd…
62
 A maxwell bridge is used to measure inductive impedance. The
bridge constants at balance are C1=0.01 µF, R1=470KΩ, R2=5.1KΩ
and R3=100KΩ. Find the series equialent of the unknown
impedance.
Contd…
63
Measurement of Physical Parameters
64
 Flow measurement
 Displacement meters
 Liquid level measurement
 Temperature Measurement
 Measurement of Humidity and Moisture
 Velocity Measurement
 Force Measurement
 Pressure- High pressure
 Vacuum level Measurement
 Data acquisition systems
Flow Measurement
65
 In many process industries, measurement of flow is an important
aspect.
 Depending on the quantity flowing such as Solid, Liquid and
gaseous appropriate method of flow rate measurement is used.
 Incase of solids, rate of mass flow rate is measured while in case of
liquids and gases flow interms of volume flow rate is measured.
Contd…
66
Mass Flow Rate
Contd…
67
 The volume flow rate is used for finding flow of materials in liquids,
gases and semi liquid forms.
 Devices used for measurement are of 2 types
1. Mechanical type flow meters
2. Electrical type flow meters
Contd…
68
Different instruments are used for measurement of flow rate.
1. Head type flowmeters (based on differential pressure measurement)
2. Variable area meters (Rotameters)
3. Mechanical Flow meters (Turbine flow meter)
4. Electromagnetic Flow meter
5. Anemometer
6. Ultrasonic Flow meter
7. Vertex Flow meter
Variable Area Flowmeter(Rotameter)
69
 It gives visual indication of flow.
Advantages
1. Reliable and inexpensive.
2. It gives direct visual indication on linear scale.
3. It can measure flow rate in the range of 1m3/sec to 0.1m3/sec.
4. Accuracy is 0.5 to 3% of full scale range.
Disadvantages
1. It must be mounted vertically.
2. Bubbles in the fluid results in oscillations of float.
Turbine Flowmeter
70
Contd…
71
Advantages
1. Reliable & less rugged.
2. High accuracy.
3. Lighter.
Disadvantages
1. Large permanent pressure losses as bearing wears out.
Applications
1. Used in oil industries, for low viscosity and high flow measurements.
Electromagnetic Flowmeter
 Volume flow rates of
electrically conductive fluids
can be measured.
 According to Faraday’s law
of electromagnetic induction,
voltage V induced across
length l of flowing fluid with
velocity v in the magnetic
flux density B is given by
 V=B.l.v
72
Contd…
73
Advantages
1. Simple construction & rugged with no moving parts.
2. No obstruction in flow is required.
3. Output is independent of temperature, viscosity & Pressure.
4. Good accuracy & highly reliable.
5. Bidirectional flow measurements are possible.
6. It can measure flow in any size of tube because magnetic fields can be
generated easily.
Contd…
74
Disadvantages
1. Meter is expensive.
2. Conductivity of the fluid should not be less than 10µΩ/m.
3. The output signal is of very low level usually micro volts, therefore,
high amplification is required.
4. The system requires to generate magnetic field, which is bulky and
requires more floor space.
Liquid Level Measurement
75
 In modern manufacturing plants, various methods are used to measure
levels of liquids, solvents chemicals required for processing and
sometimes solids which are in the form of powders or small particles.
 It is used in industries not only for monitoring but also for measuring
liquid contents in tanks, reservoirs etc.,
Contd…
 Two methods are used for measurement
 Direct method
• Dip stick
• Bob and tape method
• Hook Gauge
• Float Gauge
 Indirect method
• Hydraulic method
• Mechanical method
• Pneumatic method
• Electrical method
• Ultrasonic method
• Nucleonic method
76
Ultrasonic Method for Level
Measurement
 It is used to measure liquid
level in a tank.
 It is based on principle of
sound echo.
 This principle is also used for
measurement of thickness of a
given material.
77
Contd…
78
Advantages
1. This is a non disturbance technique i.e., it does not involve placing sensor in
the material.
2. Installation is easy.
3. It is used for liquids as well as solids measurements.
Disadvantages
1. System is costly.
2. The electronic circuit required for detection, amplification, synchronization
makes it a complex circuit.
Pneumatic Method
 This method Is also called
Bubbler method or Purge
method.
 It is suitable for liquid
measurements and also for
corrosive liquids, liquids
containing solids.
79
Temperature Measurement
80
Based on the principle of operation of thermal expansion, different
temperature measurement instruments are used.
1. Bimetallic Strip
2. Liquid in gas thermometer
3. Pressure thermometer
Bimetallic Strip 81
Flat type Bimetallic Strip
Curved Bimetallic strip
Spiral Bimetallic Strip
Liquid in gas thermometer 82
 It is used to measure temperature from -200 degree centigrade to 1000
degree centigrade range.
Velocity Measurement 83
 Velocity transducers are used for measurement of Linear velocity as well as Angular
velocity.
Linear Velocity Measurement
 Based on the electromagnetic induction mechanical vibrations are converted in to
alternating voltages. Hence, called electromagnetic transducers.
 There are 2 types of EM transducers
1. Moving Coil type
2. Moving Magnet type
Moving Magnet type Velocity
Transducer
Advantages
1. There are no mechanical surfaces or
contacts, hence the maintenance
required is negligible.
2. Robust and less expensive.
3. Output voltage is linearly
proportional to the velocity.
Disadvantages
1. Limited frequency response.
2. The stray magnetic fields affects the
performance of these transducers.
84
Angular Velocity Measurement
 The main disadvantage of linear velocity measurement is that for large
distance travel it is very difficult to detect with fixed reference.
 In such cases, Angular velocity transducers are used by converting
linear velocity in to angular velocity.
 The measurement of speed can be done with the help of
Tachogenerator.
85
Contd…
 Tachogenerators are of 2 types
1. Electrical type tachogenerator
2. Mechanical type tachogenerator
 Electrical type are preferred over mechanical type.
86
Electrical type of Tachogenerator
DC Tachogenerator
87
Contd…
Advantages
1. Output voltage is small enough to measure it with DC Voltmeters.
2. Polarity of output directly indicates direction of rotation.
Disadvantages
1. Because of variation in contact resistances, considerable error is introduced in the output
voltage. Hence, the maintenance of commutator is required.
2. Non linearity in output occurs because of distortions in permanent magnetic field due to
large armature currents. Hence, input resistor should be very high as compared to output
resistor of generator.
88
AC Tachogenerator
 It is used to measure speed in only one direction.
 The emf induced in quadrature coil is directly proportional to the rotor
speed and is in phase with applied voltage to the reference coil.
89
Contd…
Advantages
1. Output can be calibrated in terms of amplitude and frequency of
induced voltage.
2. Commutator contact resistance problems are eliminated as the coil is
wounded on the stator.
Disadvantages
1. For low speed rotation, frequency of induced voltage is very low thus
ripples in output increases.
2. At high speeds frequency is very high, coil impedance increases. This
effects linearity of output.
90
Digital methods of measurement of
angular velocity
 Electro mechanical methods are used for angular velocity measurement
effectively for speeds of 10,000rpm or less.
 For higher speeds, electromechanical transducers are not suitable.
 The main advantage of digital method is that there is no physical
contact between shaft whose speed is to be measured and measuring
device, thus loading of shaft is avoided.
91
Contd…
 There are two methods
1. Photo Electric type Tachometer
2. Inductive type Tachometer
92
Photo Electric Tachometer
Advantages
1. Output is Pulsated output, i.e., output is in digital form.
So no additional ADC is required.
2. Amplitude of pulses is constant, hence complex
electronic circuit is not required.
Disadvantages
1. Life time of light source is drawback, typical life time
of light source is 50,000 to 60,000 hours. Hence
necessary to replace light source.
2. Accuracy depends on error represented by one pulse.
93
Inductive type Tachometer
 It is also called as Toothed rotor
variable reluctance tachometer.
Advantages
1. Simple and rugged in construction.
2. Maintenance free tachometer.
3. Calibration of tachometer is simple.
4. The transmission of output of tachometer is
easy.
94
Speed = n = pulses per second/number of
teeth
n= P/N rps
Measurement of Moisture
 There are two types of moisture measurement techniques.
1. Laboratory Techniques
2. Industrial Techniques
95
Laboratory Techniques for Moisture
Measurement
 It gives better accurate measurement, but requires longer time for
measurement.
 Different methods of Moisture measurement are
1. Water Separation Method
2. Gravimetric Methods
3. Phase Change Method
4. Equilibrium Relative Humidity Measurement
96
Industrial Techniques for Moisture
Measurement
 These techniques are based on the change of physical property of the
material with moisture.
 Different methods of Moisture measurement are
1. Electrical Methods
2. Neutron Moderation
97
Measurement of Humidity
 The amount of water vapour in surrounding air is called Humidity.
 Three separate terms are used to represent the Humidity.
 Absolute Humidity HA It is defined as mass of water vapour in unit
volume of a moist air.
 Relative Humidity HR It is defined as ratio of moisture content of the gas
to the maximum moisture of gas at a temperature i.e., saturated gas.
 Generally it is expressed in percentage. The relative humidity of dry air is
zero, while that of air with all moisture is 100%.
98
Contd…
 Specific Humidity Hs It is defined as mass of water in a unit mass
of a moist air.
 Dew Point It is defined as the saturation temperature of the mixture
at the corresponding vapour pressure. It is expressed in degree
centigrade.
99
Contd…
 The instruments used for measurement of relative humidity are
called HYGROMETERS.
 Some of the important types of Hygrometers are
1. Electrical Hygrometer
2. Psychrometer (Wet and dry bulb Hygrometer)
3. Hair Hygrometer
4. Dew Point Meter
100
Electrical Hygrometer – Resistive
Hygrometer (Humistor)
101
Contd… 102
Advantages
1. Simple in construction.
2. High response time.
3. High sensitivity.
4. Digital readout is possible.
5. 1% variation in relative humidity can be detected.
Disadvantages
1. Constant temperature environment is necessary.
2. Cannot measure extreme changes in humidity as scale becomes non linear.
3. Hygrometer gets damaged in 100% humid environment.
Displacement Measurement 103
 Displacement is classified as
1. Translational Displacement
2. Rotational Displacement
 Some of the translational displacement transducers are
1. Resistive potentiometers
2. Strain gauges
3. LVDT
4. Variable inductance transducers
Some of the angular displacement transducers are
1. Capacitive transducers
2. Synchros
Translational Displacement Transducers 104
 Resistive potential Displacement Transducer
Rotational Displacement Transducers 105
 Incremental shaft encoder for Clockwise and anti clockwise direction
Force Measurement 106
 According to Newton’s Law, F= m.a
 Different methods are used for the measurement of Force.
1. Force measurement using Accelerometer
2. Force measurement using Vibrating wire sensor
3. Force measurement using Load cell
Force Measurement using Vibrating
Wire Sensor
 The resonant frequency is
given by
107
Force Measurement using Load Cell 108
 Some force measuring devices using Load cell are
1. Cantilever Beam type load cell
2. Column type load cell
Cantilever Beam type load cell 109
Column type load cell 110
Pressure Measurement 111
 Pressure is a non electrical, physical quantity defined as force acting
per unit area measured at a given point over a surface.
 Classification of Pressure
Pressure Measurement by U tube
Manometer
 P – Pref = ρ. g. h
 P- pressure field
 Pref – reference pressure
source
 g- gravitational constant
 h- difference in heights of
liquid levels in 2 columns
112
Pressure Measurement by Well type
Manometer
 Pressure measurement using well type manometer is given as,
113
High Pressure Measurement
 When the pressure exceeds 10,000 psi high pressure measurement
techniques are used.
 Bulk modulus resistance pressure gauge is used for high pressure
measurement.
114
Bulk modulus resistance pressure
gauge
115
Vaccum Measurement 116
 Pressure below atmospheric pressure is defined as Vacuum pressure.
 Vaccum ranges are
Contd… 117
 Different instruments used for vaccum measurement are
1. Mc Leod Gauge
2. Thermal Conductive Gauge
1. Pirani Gauge
2. Thermo couple Gauge
3. Ionization Gauge
Pirani Gauge
Advantages
1. Easy to operate
2. Simple design
Limitations
1. Scale is non linear
2. Calibration of pirani gauge is
dependent on nature of gas.
118
Data Acquisition Systems (DAS)
 The system used for data processing, data conversion, data
transmission, data storage is called DAS.
 DAS can be classified as follows
1. Analog DAS
2. Digital DAS
119
Data Acquisition Systems (DAS)
 A schematic block diagram of a General of DAS is shown below.
120

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EMI Unit 5 Bridges and Measurement of Physical Parameters

  • 1. Unit V Bridges & Measurement Of Physical Parameters -BY GVNSK SRAVYA ASST. PROFESSOR ECE DEPT.
  • 2. Contents Bridges:  Wheat stone Bridge,  Kelvin Bridge, and  Maxwell Bridge Measurement of Physical Parameters:  Flow measurement  Displacement meters  Liquid level measurement  Temperature Measurement  Measurement of Humidity and Moisture  Velocity Measurement  Force Measurement  Pressure- High pressure  Vacuum level Measurement  Data acquisition systems 2
  • 3. Bridges  Bridge circuit is an instrument to measure resistance, capacitance, inductance and impedance.  In simplest form it consists of four arms network.  They use Comparison measurement technique and null- indication principle.  This means the indication is independent of the calibration of the indicating device or any characteristics of it.  Unknown components value is calculated when the bridge is balanced. 3
  • 4. Advantages of Bridge Circuits  Balance equation is independent of the magnitude of the input voltage or the source impedance.  High measurement accuracy since measurement is based on comparison.  Accuracy depends on components values and not on the characteristics of the null detector. 4
  • 5. Contd…  Balance equation is independent of the sensitivity of the null detector.  Balance condition transparent to the interchange of source and detector positions.  Can be used in the control circuits. 5
  • 6. Types of Bridges  Two types of bridges are used in the measurement based on the voltage signal with which those can be operated. • DC Bridges- Wheatstone Bridge, Kelvin Bridge • AC Bridges- Maxwell Bridge 6
  • 7. DC Bridges  If the bridge circuit can be operated with only DC voltage signal, then it is a DC bridge circuit or simply DC bridge.  DC bridges are used to measure the value of unknown resistance.  The circuit diagram of DC bridge looks like as shown in below figure. 7
  • 8. Contd…  The DC bridge has four arms and each arm consists of a resistor.  Among which, two resistors have fixed resistance values, one resistor is a variable resistor and the other one has an unknown resistance value.  The above DC bridge circuit can be excited with a DC voltage source by placing it in one diagonal.  The galvanometer is placed in other diagonal of DC bridge. It shows some deflection as long as the bridge is unbalanced. 8
  • 9. Contd…  Vary the resistance value of variable resistor until the galvanometer shows null (zero) deflection.  Now, the above DC bridge is said to be a balanced one. So, we can find the value of unknown resistance by using nodal equations.  DC bridges can be operated with only DC voltage signal.  DC bridges are useful for measuring the value of unknown resistance, which is present in the bridge.  Wheatstone’s Bridge & Kelvin Bridge are examples of DC bridge. 9
  • 10. Wheatstone Bridge  Wheatstone’s bridge is a simple DC bridge, which is mainly having four arms. These four arms form a rhombus or square shape and each arm consists of one resistor.  To find the value of unknown resistance, we need the galvanometer and DC voltage source. Hence, one of these two are placed in one diagonal of Wheatstone’s bridge and the other one is placed in another diagonal of Wheatstone’s bridge.  It measures resistance from 1 ohm to 1megaohm. 10
  • 11. Contd…  Wheatstone’s bridge is used to measure the value of medium resistance. The circuit diagram of Wheatstone’s bridge is shown in below figure. 11
  • 12. Contd…  R3 is a standard resistor and R4 is unknown resistor which is to be measured.  Bridge can be balanced by varying the value of R3 resistor.  The above bridge circuit is balanced when no current flows through the diagonal arm, DB. That means, there is no deflection in the galvanometer, when the bridge is balanced. 12
  • 13. Contd…  The bridge will be balanced, when the following two conditions are satisfied. 13
  • 14. Contd…  As galvanometer current is zero, 14
  • 16. Sensitivity of Wheatstone Bridge  When the bridge is in unbalanced condition, current flows through the galvanometer causing deflection of the pointer.  The amount of deflection is the function of sensitivity of the galvanometer.  The sensitivity can be expressed as amount of deflection per unit current(mm/µA, radians/µA or degrees/µA).  Greater the sensitivity of the galvanometer, greater its deflection. 16
  • 17. Unbalanced Wheatstone bridge  By using Thevenin's theorem, the current through the galvanometer can be determined. 17
  • 21. Applications 1. It is a DC bridge which is used to measure the resistance from 1ohm to low mega ohm. 2. A Wheatstone bridge may be used to measure the dc resistance of various types of wire, either for the purpose of quality control of the wire itself, or of some assembly in which it is used. 3. For example, the resistance of motor windings, transformers, solenoids, and relay coils can be measured. 4. Wheatstone Bridge Circuit is also used extensively by telephone companies and others to locate cable faults. 5. The fault may be two lines shorted together, or a single line shorted to ground. 21
  • 22. Limitations 1. For low resistance measurement, the resistance of the leads and contacts becomes significant and introduces an error. This can be eliminated by Kelvin’s Double bridge. 2. For high resistance measurements, the resistance presented by the bridge becomes so large that the galvanometer is insensitive to imbalance. Therefore, a power supply has to replace the battery and a dc VTVM replaces the galvanometer. In the case of high resistance measurements in mega ohms, the Wheatstone's bridge cannot be used. 3. Another difficulty in Wheatstone Bridge Circuit is the change in resistance of the bridge arms due to the heating effect of current through the resistance. The rise in temperature causes a change in the value of the resistance, and excessive current may cause a permanent change in value. 22
  • 23. Problems 1. Wheatstone bridge consists of the following parameters, R1=10K ohm, R2=15K ohm and R3=40K ohm. Find the unknown resistance Rx. Solution: R1R4 = R2R3 R1Rx=R2R3 Rx= R2R3/R1 = (15K X 40K)/10K = 60K ohms 23
  • 24. Problems 2. An unbalanced Wheatstone bridge consists of the following parameters, R1=1K ohm, R2=2.5K ohm, R3=3.5K ohm, R4 = 10K ohm and Rg=300 ohms and DC source is of 6V. Find the current through the galvanometer. 24
  • 25. Problems Solution: Eth = 0.312V Rth = 2.778K ohms Ig= 42.88 micro amperes 25
  • 26. Kelvin’s Bridge  When the resistance to be measured is of the order of magnitude of bridge contact and lead resistance, a modified form of Wheatstone’s bridge, the Kelvins Bridge theory is employed.  Kelvins Bridge theory is a modification of Wheatstone’s bridge and is used to measure values of resistance below 1 Ω.  In low resistance measurement, the resistance of the leads connecting the unknown resistance to the terminal of the bridge circuit may affect the measurement. 26
  • 28. Contd…  Ry represents the resistance of the connecting leads from R3 to Rx (unknown resistance).  The galvanometer can be connected either to point c or to point a.  When it is connected to point a, the resistance Ry, of the connecting lead is added to the unknown resistance Rx, resulting in too high indication for Rx.  When the connection is made to point c, R3, is added to the bridge arm R3 and resulting measurement of Rx is lower than the actual value, because now the actual value of R3 is higher than its nominal value by the resistance Ry. 28
  • 29. Contd…  If the galvanometer is connected to point b, in between points c and a, in such a way that the ratio of the resistance from c to b and that from a to b equals the ratio of resistances R1 and R2, then 29 and the usual balance equations for the bridge give the relationship.
  • 33. Contd… 33  Equation (11.8) is the usual Wheatstone’s balance equation and it indicates that the effect of the resistance of the connecting leads from point a to point c has been eliminated by connecting the galvanometer to an intermediate position, b.  The above principle forms the basis of the construction of Kelvin’s Double Bridge, popularly known as Kelvin’s Bridge. It is a Double bridge because it incorporates a second set of ratio arms. Figure 11.11 shows a schematic diagram of Kelvin’s double bridge.
  • 35. Contd… 35  The second set of arms, a and b, connect the galvanometer to a point c at the appropriate potential between m and n connection, i.e. Ry. The ratio of the resistances of arms a and b is the same as the ratio of R1 and R2. The galvanometer indication is zero when the potentials at k and c are equal.
  • 38. Contd… 38  This is the usual equation for Kelvins Bridge theory. It indicates that the resistance of the connecting lead has no effect on the measurement, provided that the ratios of the resistances of the two sets of ratio arms are equal. In a typical Kelvins Bridge theory the range of a resistance covered is 1 — 0.00001 Ω (10 μ ohm) with an accuracy of ± 0.05% to ± 0.2%.
  • 39. Advantages 39  It can measure the resistance value in the range of 0.1 µohms to 1.0 ohm.  Power consumption is less  Simple in construction  Sensitivity is high.
  • 40. Disadvantages 40  For knowing whether the bridge is balanced or not, the sensitive galvanometer is used.  To obtain good sensitivity of the device, a high current is required.  Manual adjustments are to made periodically when required.
  • 41. Problems 41  In the figure below the ratio of Ra to Rb is 1000 ohm, R1 is 5 ohm and R1 is 0.5R2, What is the value of Rx.
  • 46. AC Bridges 46  An AC Bridge, In its simplest form consists of 4 arms, a source of excitation and a balance detector. Each arm consists of impedance.  The source is an ac supply which supplies ac voltages at required frequency.  For low frequencies, powerline itself can act as source of supply.  For high frequencies, the electronic oscillators are used as source. A typical oscillator has a range of 50 Hz to 125kHz with a power output of around 7 W.  Commonly used balanced detectors are head phones, tunable amplifiers or vibration galvanometers.
  • 47. Contd… 47  The headphones are used as detectors at frequencies of 250Hz. to 3 to 4KHz.  For single frequency a tuned detector is most sensitive detector at frequency range is from 10Hz to 100Hz.  For low audio frequencies vibration galvanometers are used at 5Hz to 1000Hz, commonly used below 200Hz.
  • 48. Contd… 48  Impedances at AF or RF are commonly determined by means of an ac Wheatstone bridge.  This bridge is similar to a dc bridge, except that the bridge arms are impedances.  The bridge is excited by an ac source rather than dc and the galvanometer is replaced by a detector, such as a pair of headphones, for detecting ac.
  • 50. Contd… 50  When the bridge is balanced, where Z1, Z2, Z3 and Z4 are the impedances of the arms, and are vector complex quantities that possess phase angles.  It is thus necessary to adjust both the magnitude and phase angles of the impedance arms to achieve balance, i.e. the bridge must be balanced for both the reactance and the resistive component.
  • 51. Maxwell’s Bridge 51  It is an Maxwell Inductance Capacitance bridge developed by Maxwell.  It measures an unknown inductance in terms of a known capacitance.
  • 53. Contd… 53  The use of standard arm offers the advantage of compactness and easy shielding. The capacitor is almost a loss-less component.  One arm has a resistance R1 in parallel with C1, and hence it is easier to write the balance equation using the admittance of arm 1 instead of the impedance.
  • 54. Contd… 54  The general equation for bridge balance is
  • 56. Contd… 56  Maxwell Bridge Theory is limited to the measurement of low Q values (1 — 10). The measurement is independent of the excitation frequency. The scale of the resistance can be calibrated to read inductance directly.
  • 57. Maxwell Bridge Advantages and Disadvantages 57 Advantages  The frequency does not appear in the final expression of both equations, hence it is independent of frequency.  Maxwell’s inductor capacitance bridge is very useful for the wide range of measurement of inductor at audio frequencies.
  • 58. Contd… 58  The bridge is particularly suited for inductances measurements, since comparison with a capacitor is more ideal than with another inductance.  Commercial bridges measure from 1 — 1000 H, with ± 2% error. (If the Q is very large, R1 becomes excessively large and it is impractical to obtain a satisfactory variable standard resistance in the range of values required).
  • 59. Contd… 59 Disadvantages 1. The variable standard capacitor is very expensive. 2. Inductance cannot be measured over a wide range, since Q is proportional to inductance. 3. The bridge is limited to measurement of low quality coils (1 < Q < 10) and it is also unsuitable for low value of Q (i.e. Q < 1) from this we conclude that a Maxwell bridge is used suitable only for medium Q coils.
  • 60. Problems 60  In Maxwell’s Bridge, as shown in the figure below, Determine the values of the resistance R1 and inductance L1 of a coil after the bridge is balanced with capacitance of 0.5 µF.
  • 61. Contd… 61  Solution: R1=(R2R3)/R4 R1= 375 Ω. L1= C1R2R3 L1 = 0.5µ x 2000 x 750 = 75mH
  • 62. Contd… 62  A maxwell bridge is used to measure inductive impedance. The bridge constants at balance are C1=0.01 µF, R1=470KΩ, R2=5.1KΩ and R3=100KΩ. Find the series equialent of the unknown impedance.
  • 64. Measurement of Physical Parameters 64  Flow measurement  Displacement meters  Liquid level measurement  Temperature Measurement  Measurement of Humidity and Moisture  Velocity Measurement  Force Measurement  Pressure- High pressure  Vacuum level Measurement  Data acquisition systems
  • 65. Flow Measurement 65  In many process industries, measurement of flow is an important aspect.  Depending on the quantity flowing such as Solid, Liquid and gaseous appropriate method of flow rate measurement is used.  Incase of solids, rate of mass flow rate is measured while in case of liquids and gases flow interms of volume flow rate is measured.
  • 67. Contd… 67  The volume flow rate is used for finding flow of materials in liquids, gases and semi liquid forms.  Devices used for measurement are of 2 types 1. Mechanical type flow meters 2. Electrical type flow meters
  • 68. Contd… 68 Different instruments are used for measurement of flow rate. 1. Head type flowmeters (based on differential pressure measurement) 2. Variable area meters (Rotameters) 3. Mechanical Flow meters (Turbine flow meter) 4. Electromagnetic Flow meter 5. Anemometer 6. Ultrasonic Flow meter 7. Vertex Flow meter
  • 69. Variable Area Flowmeter(Rotameter) 69  It gives visual indication of flow. Advantages 1. Reliable and inexpensive. 2. It gives direct visual indication on linear scale. 3. It can measure flow rate in the range of 1m3/sec to 0.1m3/sec. 4. Accuracy is 0.5 to 3% of full scale range. Disadvantages 1. It must be mounted vertically. 2. Bubbles in the fluid results in oscillations of float.
  • 71. Contd… 71 Advantages 1. Reliable & less rugged. 2. High accuracy. 3. Lighter. Disadvantages 1. Large permanent pressure losses as bearing wears out. Applications 1. Used in oil industries, for low viscosity and high flow measurements.
  • 72. Electromagnetic Flowmeter  Volume flow rates of electrically conductive fluids can be measured.  According to Faraday’s law of electromagnetic induction, voltage V induced across length l of flowing fluid with velocity v in the magnetic flux density B is given by  V=B.l.v 72
  • 73. Contd… 73 Advantages 1. Simple construction & rugged with no moving parts. 2. No obstruction in flow is required. 3. Output is independent of temperature, viscosity & Pressure. 4. Good accuracy & highly reliable. 5. Bidirectional flow measurements are possible. 6. It can measure flow in any size of tube because magnetic fields can be generated easily.
  • 74. Contd… 74 Disadvantages 1. Meter is expensive. 2. Conductivity of the fluid should not be less than 10µΩ/m. 3. The output signal is of very low level usually micro volts, therefore, high amplification is required. 4. The system requires to generate magnetic field, which is bulky and requires more floor space.
  • 75. Liquid Level Measurement 75  In modern manufacturing plants, various methods are used to measure levels of liquids, solvents chemicals required for processing and sometimes solids which are in the form of powders or small particles.  It is used in industries not only for monitoring but also for measuring liquid contents in tanks, reservoirs etc.,
  • 76. Contd…  Two methods are used for measurement  Direct method • Dip stick • Bob and tape method • Hook Gauge • Float Gauge  Indirect method • Hydraulic method • Mechanical method • Pneumatic method • Electrical method • Ultrasonic method • Nucleonic method 76
  • 77. Ultrasonic Method for Level Measurement  It is used to measure liquid level in a tank.  It is based on principle of sound echo.  This principle is also used for measurement of thickness of a given material. 77
  • 78. Contd… 78 Advantages 1. This is a non disturbance technique i.e., it does not involve placing sensor in the material. 2. Installation is easy. 3. It is used for liquids as well as solids measurements. Disadvantages 1. System is costly. 2. The electronic circuit required for detection, amplification, synchronization makes it a complex circuit.
  • 79. Pneumatic Method  This method Is also called Bubbler method or Purge method.  It is suitable for liquid measurements and also for corrosive liquids, liquids containing solids. 79
  • 80. Temperature Measurement 80 Based on the principle of operation of thermal expansion, different temperature measurement instruments are used. 1. Bimetallic Strip 2. Liquid in gas thermometer 3. Pressure thermometer
  • 81. Bimetallic Strip 81 Flat type Bimetallic Strip Curved Bimetallic strip Spiral Bimetallic Strip
  • 82. Liquid in gas thermometer 82  It is used to measure temperature from -200 degree centigrade to 1000 degree centigrade range.
  • 83. Velocity Measurement 83  Velocity transducers are used for measurement of Linear velocity as well as Angular velocity. Linear Velocity Measurement  Based on the electromagnetic induction mechanical vibrations are converted in to alternating voltages. Hence, called electromagnetic transducers.  There are 2 types of EM transducers 1. Moving Coil type 2. Moving Magnet type
  • 84. Moving Magnet type Velocity Transducer Advantages 1. There are no mechanical surfaces or contacts, hence the maintenance required is negligible. 2. Robust and less expensive. 3. Output voltage is linearly proportional to the velocity. Disadvantages 1. Limited frequency response. 2. The stray magnetic fields affects the performance of these transducers. 84
  • 85. Angular Velocity Measurement  The main disadvantage of linear velocity measurement is that for large distance travel it is very difficult to detect with fixed reference.  In such cases, Angular velocity transducers are used by converting linear velocity in to angular velocity.  The measurement of speed can be done with the help of Tachogenerator. 85
  • 86. Contd…  Tachogenerators are of 2 types 1. Electrical type tachogenerator 2. Mechanical type tachogenerator  Electrical type are preferred over mechanical type. 86
  • 87. Electrical type of Tachogenerator DC Tachogenerator 87
  • 88. Contd… Advantages 1. Output voltage is small enough to measure it with DC Voltmeters. 2. Polarity of output directly indicates direction of rotation. Disadvantages 1. Because of variation in contact resistances, considerable error is introduced in the output voltage. Hence, the maintenance of commutator is required. 2. Non linearity in output occurs because of distortions in permanent magnetic field due to large armature currents. Hence, input resistor should be very high as compared to output resistor of generator. 88
  • 89. AC Tachogenerator  It is used to measure speed in only one direction.  The emf induced in quadrature coil is directly proportional to the rotor speed and is in phase with applied voltage to the reference coil. 89
  • 90. Contd… Advantages 1. Output can be calibrated in terms of amplitude and frequency of induced voltage. 2. Commutator contact resistance problems are eliminated as the coil is wounded on the stator. Disadvantages 1. For low speed rotation, frequency of induced voltage is very low thus ripples in output increases. 2. At high speeds frequency is very high, coil impedance increases. This effects linearity of output. 90
  • 91. Digital methods of measurement of angular velocity  Electro mechanical methods are used for angular velocity measurement effectively for speeds of 10,000rpm or less.  For higher speeds, electromechanical transducers are not suitable.  The main advantage of digital method is that there is no physical contact between shaft whose speed is to be measured and measuring device, thus loading of shaft is avoided. 91
  • 92. Contd…  There are two methods 1. Photo Electric type Tachometer 2. Inductive type Tachometer 92
  • 93. Photo Electric Tachometer Advantages 1. Output is Pulsated output, i.e., output is in digital form. So no additional ADC is required. 2. Amplitude of pulses is constant, hence complex electronic circuit is not required. Disadvantages 1. Life time of light source is drawback, typical life time of light source is 50,000 to 60,000 hours. Hence necessary to replace light source. 2. Accuracy depends on error represented by one pulse. 93
  • 94. Inductive type Tachometer  It is also called as Toothed rotor variable reluctance tachometer. Advantages 1. Simple and rugged in construction. 2. Maintenance free tachometer. 3. Calibration of tachometer is simple. 4. The transmission of output of tachometer is easy. 94 Speed = n = pulses per second/number of teeth n= P/N rps
  • 95. Measurement of Moisture  There are two types of moisture measurement techniques. 1. Laboratory Techniques 2. Industrial Techniques 95
  • 96. Laboratory Techniques for Moisture Measurement  It gives better accurate measurement, but requires longer time for measurement.  Different methods of Moisture measurement are 1. Water Separation Method 2. Gravimetric Methods 3. Phase Change Method 4. Equilibrium Relative Humidity Measurement 96
  • 97. Industrial Techniques for Moisture Measurement  These techniques are based on the change of physical property of the material with moisture.  Different methods of Moisture measurement are 1. Electrical Methods 2. Neutron Moderation 97
  • 98. Measurement of Humidity  The amount of water vapour in surrounding air is called Humidity.  Three separate terms are used to represent the Humidity.  Absolute Humidity HA It is defined as mass of water vapour in unit volume of a moist air.  Relative Humidity HR It is defined as ratio of moisture content of the gas to the maximum moisture of gas at a temperature i.e., saturated gas.  Generally it is expressed in percentage. The relative humidity of dry air is zero, while that of air with all moisture is 100%. 98
  • 99. Contd…  Specific Humidity Hs It is defined as mass of water in a unit mass of a moist air.  Dew Point It is defined as the saturation temperature of the mixture at the corresponding vapour pressure. It is expressed in degree centigrade. 99
  • 100. Contd…  The instruments used for measurement of relative humidity are called HYGROMETERS.  Some of the important types of Hygrometers are 1. Electrical Hygrometer 2. Psychrometer (Wet and dry bulb Hygrometer) 3. Hair Hygrometer 4. Dew Point Meter 100
  • 101. Electrical Hygrometer – Resistive Hygrometer (Humistor) 101
  • 102. Contd… 102 Advantages 1. Simple in construction. 2. High response time. 3. High sensitivity. 4. Digital readout is possible. 5. 1% variation in relative humidity can be detected. Disadvantages 1. Constant temperature environment is necessary. 2. Cannot measure extreme changes in humidity as scale becomes non linear. 3. Hygrometer gets damaged in 100% humid environment.
  • 103. Displacement Measurement 103  Displacement is classified as 1. Translational Displacement 2. Rotational Displacement  Some of the translational displacement transducers are 1. Resistive potentiometers 2. Strain gauges 3. LVDT 4. Variable inductance transducers Some of the angular displacement transducers are 1. Capacitive transducers 2. Synchros
  • 104. Translational Displacement Transducers 104  Resistive potential Displacement Transducer
  • 105. Rotational Displacement Transducers 105  Incremental shaft encoder for Clockwise and anti clockwise direction
  • 106. Force Measurement 106  According to Newton’s Law, F= m.a  Different methods are used for the measurement of Force. 1. Force measurement using Accelerometer 2. Force measurement using Vibrating wire sensor 3. Force measurement using Load cell
  • 107. Force Measurement using Vibrating Wire Sensor  The resonant frequency is given by 107
  • 108. Force Measurement using Load Cell 108  Some force measuring devices using Load cell are 1. Cantilever Beam type load cell 2. Column type load cell
  • 109. Cantilever Beam type load cell 109
  • 110. Column type load cell 110
  • 111. Pressure Measurement 111  Pressure is a non electrical, physical quantity defined as force acting per unit area measured at a given point over a surface.  Classification of Pressure
  • 112. Pressure Measurement by U tube Manometer  P – Pref = ρ. g. h  P- pressure field  Pref – reference pressure source  g- gravitational constant  h- difference in heights of liquid levels in 2 columns 112
  • 113. Pressure Measurement by Well type Manometer  Pressure measurement using well type manometer is given as, 113
  • 114. High Pressure Measurement  When the pressure exceeds 10,000 psi high pressure measurement techniques are used.  Bulk modulus resistance pressure gauge is used for high pressure measurement. 114
  • 115. Bulk modulus resistance pressure gauge 115
  • 116. Vaccum Measurement 116  Pressure below atmospheric pressure is defined as Vacuum pressure.  Vaccum ranges are
  • 117. Contd… 117  Different instruments used for vaccum measurement are 1. Mc Leod Gauge 2. Thermal Conductive Gauge 1. Pirani Gauge 2. Thermo couple Gauge 3. Ionization Gauge
  • 118. Pirani Gauge Advantages 1. Easy to operate 2. Simple design Limitations 1. Scale is non linear 2. Calibration of pirani gauge is dependent on nature of gas. 118
  • 119. Data Acquisition Systems (DAS)  The system used for data processing, data conversion, data transmission, data storage is called DAS.  DAS can be classified as follows 1. Analog DAS 2. Digital DAS 119
  • 120. Data Acquisition Systems (DAS)  A schematic block diagram of a General of DAS is shown below. 120