EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

D.C potentiometers, D.C (Wheat stone, Kelvin and Kelvin Double bridge) & A.C bridges (Maxwell,
Anderson and Schering bridges), transformer ratio bridges, self-balancing bridges. Interference &
screening – Multiple earth and earth loops – Electrostatic and electromagnetic Interference –
Grounding techniques.

UNIT-3

COMPARISON METHODS OF
MEASUREMENTS

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EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

DC Potentiometer
An instrument that precisely measures an electromotive force (emf) or a voltage by
opposing to it a known potential drop established by passing a definite current through a resistor
of known characteristics. (A three-terminal resistive voltage divider is sometimes also
called a potentiometer.) There are two ways of accomplishing this balance: the current I may be
held at a fixed value and the resistance R across which the IR drop is opposed to the unknown may
be varied, current may be varied across a fixed resistance to achieve the needed IR drop.
The essential features of a general-purpose constant-current instrument are shown in
the illustration. The value of the current is first fixed to match an IR drop to the emf of a reference
standard cell. With the standard-cell dial set to read the emf of the reference cell, and the
galvanometer (balance detector) in position G1, the resistance of the supply branch of the circuit
is adjusted until the IR drop in 10 steps of the coarse dial plus the set portion of the standard-cell
dial balances the known reference emf, indicated by a null reading of the galvanometer. This
adjustment permits the potentiometer to be read directly in volts. Then, with the galvanometer in
Position G2, the coarse, intermediate, and slide-wire dials are adjusted until the galvanometer
again reads null. If the potentiometer current has not changed, the emf of the unknown can be
read directly from the dial settings.
There is usually a switching arrangement so that the galvanometer can be quickly
shifted between positions 1 and 2 to check that the current.

Figure 3.1 DC Potentiometer

Circuit diagram of a general-purpose constant-current potentiometer, showing essential

features Potentiometer techniques may also be used for current measurement, the unknown current
being sent through a known resistance and the IR drop opposed by balancing it at the voltage
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terminals of the potentiometer. Here, of course, internal heating and consequent resistance change
of the current-carrying resistor (shunt) may be a critical factor in measurement accuracy; and the
shunt design may require attention to dissipation of heat resulting from its I2R power consumption.
Potentiometer techniques have been extended to alternating-voltage measurements,
but generally at a reduced accuracy level (usually 0.1% or so). Current is set on an ammeter
which must have the same response on ac as on dc, where it may be calibrated with a potentiometer
and shunt combination. Balance in opposing an unknown voltage is achieved in one of two ways:
a slide-wire and phase-adjustable supply; separate in-phase and quadrature adjustments on
Slide wires supplied from sources that have a 90° phase difference. Such potentiometers have
limited use in magnetic testing.
An instrument that precisely measures an electromotive force (emf) or a voltage by
opposing to it a known potential drop established by passing a definite current through a resistor
of known characteristics. (A three-terminal resistive voltage divider is sometimes also
called a potentiometer.) There are two ways of accomplishing this balance the current I may be
held at a fixed value and the resistance R across which the IR drop is opposed to the unknown may
be varied; current may be varied across a fixed resistance to achieve the needed IR drop.
The essential features of a general-purpose constant-current instrument are shown in the
illustration.
The value of the current is first fixed to match an IR drop to the emf of a reference
standard cell. With the standard-cell dial set to read the emf of the reference cell, and the
galvanometer (balance detector) in position G1, the resistance of the supply branch of the circuit
is adjusted until the IR drop in 10 steps of the coarse dial plus the set portion of the standard-cell
dial balances the known reference emf, indicated by a null reading of the galvanometer. This
adjustment permits the potentiometer to be read directly in volts. Then, with the galvanometer in
position G2, the coarse, intermediate, and slide-wire dials are adjusted until the galvanometer
again reads null. If the potentiometer current has not changed, the emf of the unknown can be
read directly from the dial settings. There is usually a switching arrangement so that the
galvanometer can be quickly shifted between positions 1 and 2 to check that the current has not
drifted from its set value.
Potentiometer techniques may also be used for current measurement, the unknown current
being sent through a known resistance and the IR drop opposed by balancing it at the voltage
terminals of the potentiometer. Here, of course, internal heating and consequent resistance change
of the current-carrying resistor (shunt) may be a critical factor in measurement accuracy; and the

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EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

shunt design may require attention to dissipation of heat resulting from its I2R power consumption.
Potentiometer techniques have been extended to alternating-voltage measurements, but generally
at a reduced accuracy level (usually 0.1% or so). Current is set on an ammeter which must have
the same response on ac as on dc, where it may be calibrated with a potentiometer and shunt
combination.
Balance in opposing an unknown voltage is achieved in one of two ways: a slide-wire
and phase-adjustable supply; separate in-phase and quadrature adjustments on slide wires
supplied from sources that have a 90° phase difference. Such potentiometers have limited use in
magnetic testing an electrical measuring device used in determining the electromotive force (emf)
or voltage by means of the compensation method. When used with calibrated standard
resistors, a potentiometer can be employed to measure current, power, and other electrical
quantities; when used with the appropriate measuring transducer, it can be used to gauge various
nonelectrical quantities, such as temperature, pressure, and the composition of gases.
A distinction is made between DC and AC potentiometers. In DC potentiometers, the
voltage being measured is compared to the emf of a standard cell. Since at the instant of
compensation the current in the circuit of the voltage being measured equals zero, measurements
can be made without reductions in this voltage. For this type of potentiometer, accuracy can
exceed 0.01 percent. DC potentiometers are categorized as either high-resistance, with a slide-wire
resistance ranging from 104 to 105 ohms (Ω) and a current ranging from 10-1 to 10-9 amperes (A),
or low-resistance, with a slide-wire resistance below 2 × 103 ohms and a current ranging from 10-1 to
10-3 A.
The higher resistance class can measure up to 2 volts (V) and is used in testing highly
accurate apparatus. The low-resistance class is used in measuring voltage up to 100 mV. To
measure higher voltages, up to 600 V, and to test voltmeters, voltage dividers are connected to
potentiometers. Here the voltage drop across one of the resistances of the voltage divider is
compensated; this constitutes a known fraction of the total voltage being measured.
BRIDGES
Bridge circuits are mainly used to measure unknown quantities such as resistance,
inductance, capacitance, Impedance and admittance. Bridge circuit consists of 4 resistance arms
forming a closed circuit with dc source of current applied to two opposite junctions and a current
detector connected to the other two junctions.

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EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

Types of bridges
 A.C Bridges
 D.C Bridges
DC Bridges
Wheatstone bridge
A Wheatstone bridge is an electrical circuit invented by Samuel Hunter Christie in 1833
and improved and popularized by Sir Charles Wheatstone in 1843. It is used to measure an
unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes
the unknown component. Its operation is similar to the original potentiometer.

Operation

Figure 3.2 Wheatstone Bridge

In the figure 3.2, Rx is the unknown resistance to be measured; R1, R2 and R3 are resistors of
known resistance and the resistance of R2 is adjustable. If the ratio of the two resistances in the
known leg(R2 / R1) is equal to the ratio of the two in the unknown leg (Rx / R3), then the voltage
between the two midpoints (B and D) will be zero and no current will flow through the
galvanometer Vg. If the bridge is unbalanced, the direction of the current indicates whether R2 is
too high or too low. R2 is varied until there is no current through the galvanometer, which then reads
zero. Detecting zero current with a galvanometer can be done to extremely high accuracy.
Therefore, if R1, R2 and R3 are known to high precision, then Rx can be measured to high
precision. Very small changes in Rx disrupt the balance and are readily detected.

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EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

At the point of balance, the ratio of R2 / R1 = Rx / R3

𝐼1 − 𝐼2 − 𝐼𝑔 = 0
𝐼3 − 𝐼𝑥 + 𝐼𝑔 = 0
Alternatively, if R1, R2, and R3 are known, but R2 is not adjustable, the voltage difference
across or current flow through the meter can be used to calculate the value of Rx, using
Kirchhoff's circuit laws (also known as Kirchhoff's rules). This setup is frequently used
in strain gauge and resistance thermometer measurements as it is usually faster to read a voltage
level off a meter than to adjust a resistance to zero the voltage.
Derivation
First, Kirchhoff's first rule is used to find the currents in junctions B and D:
Then, Kirchhoff's second rule is used for finding the voltage in the loops ABD and BCD:
𝑅2 . 𝐼2 . 𝐼3 . 𝑅3
𝑅𝑥 =
𝑅1 . 𝐼1 . 𝐼𝑥
𝑅3 . 𝑅2
𝑅𝑥 =
𝑅1
The bridge is balanced and Ig = 0, so the second set of equations can be rewritten as:
(𝐼3 . 𝑅3 ) − (𝐼𝑔 . 𝑅𝑔 ) − (𝐼1 . 𝑅1 ) = 0
Then, the equations are divided and rearranged, giving:
(𝐼3 . 𝑅3 ) − (𝐼𝑔 . 𝑅𝑔 ) − (𝐼1 . 𝑅1 ) = 0
(𝐼𝑥 . 𝑅𝑥 ) − (𝐼2 . 𝑅2 ) + (𝐼𝑔 . 𝑅𝑔 ) = 0
From the first rule, I3 = Ix and I1 = I2. The desired value of Rx is now known to be given as:
𝐼3 . 𝑅3 = 𝐼1 . 𝑅1
𝐼𝑥 . 𝑅𝑥 = 𝐼2 . 𝑅2
If all four resistor values and the supply voltage (VS) are known, and the resistance of
the galvanometer is high enough that Ig is negligible, the voltage across the bridge (VG) can be found
by working out the voltage from each potential divider and subtracting one from the other. The
equation for this is:
𝑅𝑥 𝑅2
𝑉𝐺 = 𝑉𝑆 − 𝑉
𝑅3 + 𝑅𝑥 𝑅1 + 𝑅2 𝑆
This can be simplified to:
𝑅𝑥 𝑅2
𝑉𝐺 = ( − )𝑉
𝑅3 + 𝑅𝑥 𝑅1 + 𝑅2 𝑆
with an explosimeter. The Kelvin bridge was specially adapted from the Wheatstone bridge for
measuring very low resistances. In many cases, the significance of measuring the unknown
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EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

resistance is related to measuring the impact of some physical phenomenon - such as force,
temperature, pressure, etc. which thereby allows the use of Wheatstone bridge in measuring
those elements indirectly.
KELVIN BRIDGE

Figure 3.3 Kelvin Bridge

A Kelvin bridge (also called a Kelvin double bridge and some countries Thomson bridge)
is a measuring instrument invented by William Thomson, 1st Baron Kelvin. It is used to measure
an unknown electrical resistance below 1 Ω. Its operation is similar to the Wheatstone
bridge except for presence of additional resistors. These additional low value resistors and the
internal configuration of the bridge are arranged to substantially reduce measurement errors
introduced by voltage drops in the high current (low resistance) arm of the bridge
Accuracy
There are some commercial the devices reaching accuracies of 2% for resistance
ranges from 0.000001 to 25 Ω. Often, ohmmeters include Kelvin bridges, amongst other
measuring instruments, in order to obtain large measure ranges, for example, the Valhalla 4100
ATC Low-Range Ohmmeter.
The instruments for measuring sub-ohm values are often referred to as low-
resistance ohmmeters, milli-ohmmeters, micro-ohmmeters, etc
Principle of operation
The measurement is made by adjusting some resistors in the bridge, and the balance
is achieved when: in 1865 and further improved by Alan Bulletin in about 1926.

Resistance R should be as low as possible (much lower than the measured value)
and for that reason is usually made as a short thick rod of solid copper.
If the condition R3R4’ =R3’R 4 met (and value of R is low), then the last component
in the equation can be neglected and it can be assumed that:

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𝑅3 (𝑹𝟑 𝑹′𝟒 )−(𝑹′𝟑 .𝑹𝟒 )

𝑅𝑥 =𝑅2 . +R.
𝑅4 𝑹𝟒 .(𝐑+𝑹′ 𝟑 +𝑹′ 𝟒 )
AC BRIDGES
Schering Bridge
A Schering bridge is a bridge circuit used for measuring an unknown electrical
capacitance and its dissipation factor. The dissipation factor of a capacitor is the the ratio of its
resistance to its capacitive reactance. The Schering Bridge is basically a four-arm alternating-
current (AC) bridge circuit whose measurement depends on balancing the loads on its arms.
Figure3.4 below shows a diagram of the Schering Bridge.

Figure 3.4 Schering Bridge

In the Schering Bridge above, the resistance values of resistors R1 and R2 are known,
while the resistance value of resistor R3 is unknown. The capacitance values of C1 and C2
are also known, while the capacitance of C3 is the value being measured. To measure R3 and
C3, the values of C2 and R2 are fixed, while the values of R1 and C1 are adjusted until the
current through the ammeter between points A and B becomes zero. This happens when
the voltages at points A and B are equal, in which case the bridge is said to
be 'balanced'.
When the bridge is balanced, Z1/C2 = R2/Z3, where Z1 is the impedance of R1 in
parallel with C1 and Z3 is the impedance of R3 in series with C3. In an AC circuit that has
a capacitor, the capacitor contributes a capacitive reactance to the impedance. The
capacitive reactance of a capacitor C is 1/2πfC.
As such, Z1 = R1/[2πfC1((1/2πfC1) + R1)] = R1/(1 + 2πfC1R1) while Z3 = 1/2πfC3 + R3.
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Thus, when the bridge is balanced:

2πfC2R1/(1+2πfC1R1) = R2/(1/2πfC3 + R3); or
2πfC2(1/2πfC3 + R3) = (R2/R1)(1+2πfC1R1); or
C2/C3 + 2πfC2R3 = R2/R1 + 2πfC1R2.
When the bridge is balanced, the negative and positive reactive components are equal and
cancel out, so
2πfC2R3 = 2πfC1R2 or
R3 = C1R2 / C2.
Similarly, when the bridge is balanced, the purely resistive components are equal, so
C2/C3 = R2/R1 or
C3 = R1C2 / R2.

Maxwell's Bridge

Figure 3.5 Maxwell’s Bridge

The Maxwell bridge is used to measure unknown inductance in terms of calibrated resistance
and capacitance. Calibration-grade inductors are more difficult to manufacture than capacitors
of similar precision, and so the use of a simple "symmetrical" inductance bridge is not always
practical. Because the phase shifts of inductors and capacitors are exactly opposite each other, a
capacitive impedance can balance out an inductive impedance if they are located in opposite legs
of a bridge, as they are here.
Another advantage of using a Maxwell bridge to measure inductance rather than a
symmetrical inductance bridge is the elimination of measurement error due to mutual inductance
between two inductors. Magnetic fields can be difficult to shield, and even a small amount of
coupling between coils in a bridge can introduce substantial errors in certain conditions. With no

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EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

second inductor to react with in the Maxwell bridge, this problem is eliminated.
Anderson Bridge
It is a modified version of Maxwell’s inductance capacitance bridge. In this method, the
self inductance is measured in terms of a standard capacitance. It is applicable for precise
measurement of self-inductance over a wide range of values.

Figure 3.6 Anderson’s Bridge

Let 𝐿𝑋 be the selfinductance to be measured

𝑅𝑋 be the resistance of self-inductor.

𝑟1 be the resistance connected in series with selfinductor .

r,𝑅 2 ,𝑅3 ,𝑅4 be the known non-inductive resistances,and C be the fixed standard
capacitance

At balance

I1 = I3 and I2 = IC

1
𝐼1 𝑅3 = 𝐼𝐶 × 𝑗𝜔𝑐

𝐼𝐶 = 𝐼1 j𝜔c𝑅3

Writing the other balance equations

𝐼1 (𝑟1 + 𝑅𝑋 + 𝑗𝜔𝐿𝑋 ) = 𝐼2 𝑅2 + 𝐼𝑒

Sub the value of 𝐼𝐶 In the above equation,

𝐼1 (𝑟 + 𝑅𝑋 + 𝑗𝜔𝐿1 − 𝑗𝜔𝑐𝑅3 r)=𝐼2 𝑟4

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𝑅2𝑅3
From the above equations,𝑅𝑋 = −𝑟1
𝑅4

𝑅
𝐿𝑋 = C𝑅3 [𝑟(𝑅4 + 𝑅2 ) + 𝑅2 𝑅4 ]
4

Advantages
 A fixed capacitor can be used instead of a variable capacitors in the case of
Maxwell’s bridge.
 This bridge may be used for accurate determination of capacitance in terms of
inductance.
 It is much easier to obtain balance in case of Anderson’s bridge than in Maxwell’s
bridge for low Q coils.
Disadvantages
 The Anderson’s bridge is more complex than its prototype Maxwell’s bridge.
 An additional junction point increases the difficulty of shielding the bridge.

TRANSFORMER RATIO BRIDGES

The transformer ratio bridges are popular one. These bridges are mainly used for wide
range of applications. The transformer ratio bridges are replacing the conventional arc. bridges.
This bridge consists of voltage transformer whose performance approaches that of an
ideal transformer. But the ideal transformer has no resistance, no core loss and no leakage flux.
The ratio transformer consists of number tapings in order to obtain voltage division.
Voltage across the windings of the transformer is
V=4.44 f фm N volt
Where
N=number of turns
F=frequency in hertz
фm=maximum value of flux in wb
For a given value of фm and f, V=K1N
Fig shows an auto transformer with tapings. V is the input voltage to the winding. Here,
we assume that the auto transformer is ideal one, the division of input voltage V into output
voltages V1 and V2.

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EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

Different values of V1 and V2 may be had by changing the sliding contact on the tapings.
The construction of ideal transform we is impossible. But the ideals of zero winding
resistance, zero core loss and perfect coupling can be closely achieved by using special design
similar to those for instrument transformers.

Figure 3.8 Transformer ratio Bridge

The material used for construction of core should be such that it gives the very less core
losses at the desired operating frequency. The magnetizing current is minimised by using a
Toroidal core.
The main advantage of toroidal core is minimum leakage reactance. It gives almost
perfect magnetic coupling. The leakage reactance can be reduced by using a special type of
construction for the winding as shown in figure 3.9.

Figure 3.9 Multiconductor Rope

This type of winding takes the form of multi-conductor rope. By using multi-conductor
rope, we can get a decade of voltage division, and ten wires with successive seeks of turns
connected in series and a tapping is taken from each joint.
The resistance of the windings can be minimized by copper wire of heavy cross-section.
Fig shows a 4-decode ratio transformer. This type of arrangement gives a ratio error of less than
1 part in 104.

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EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

Figure 3.10 4-Decode Ratio Transformer

Applications
 It can be used for measurement of resistance, capacitance and inductance in
comparison with standard resistance, standard capacitance and standard inductance
respectively.
 It can be used for amplifier gain and phase shift.
 It is also used for measurement of transformer ratios.
Features
 It can be used only in a.c.
 It gives very small ratio errors.
 This ratio transformer has high input impedance and low output impedance. Due to
this, loading effect is very small.
 The frequency range is from 50 Hz to 50 Khz.

Measurement of resistance
In the unknown resistance R is in comparison with a standard resistance R s. The position
of the wiper is adjusted till the detector D shows null position. Current through the unknown
resistance
I1=V1/R= K1 N1/R
Current through the standard resistance I 2=V2/Rs= K1 N2/Rs
Under balance conditions, the current through the detector D is zero ieI 1=I2
Hence K1 N1/R = K1 N2/Rs
Or R= N1/ N2 Rs
Measurement of Capacitance
The measurement of capacitance by using ratio transformer. Here, unknown capacitance
C is measured in comparison with a standard capacitance Cs. A resistance R is connected across

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EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

the unknown capacitor C. This resistance represents the loss of the capacitor. Under balance
position, the magnitude and phase of the currents passing through detector should be the same; a
variable standard resistance is connected in parallel with the standard capacitor.
At balance position,
C = N2/ N1 Cs
R = N1/ N2 Rs
Dissipation factor D
D = 1/ωCR=1/ ωCsRs
Measurement of phase angle
Figure 3.11 shows the measurement of phase angle by using ratio transformer.

Figure 3.11 Ratio Transformer

Here, R and C are used. By varying the capacitance we can get phase shift. The value of
resistance should be larger in order that there are no loading effects on the ratio transformer. The
detector D is indicated zero by adjusting the capacitor value.
Phase angle ф = tan-1(-ωRC)
The magnitude of in-phase component is
Ф1 = (N2/N1+ N2)cos2 ф
SELF BALANCING BRIDGE
The term self balancing is used to describe bridges which are automatically re-balanced.
Here, we can discuss some self balancing bridges. These are
1. Self balancing bridge by using bolometer
2. Self balancing bridge as a resistance to current converter.
3. Self balancing bridge for fluid flow rate measurement.
Self balancing bridge by using bolometer
Figure 3.12 shows self balancing bridge by using bolometer. Here, the bolometer is used
as the coupling network between the output and input of a high gain frequency elective audio

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EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

amplifier. Bolometer measurements are based on the dissipation of the RF power in a small
temperature sensitive resistive element, called a Bolometer.

Figure 3.12 Bolometer

Here, the feedback is in proper phase to produce sustained oscillations of such amplitude
as well maintain the resistance of the bolometer at a fixed value which nearly balances the
bridge. When the supply is switched ON, the bridge is unbalanced. The gain of the amplifier is
large, so that the oscillations are allowed to build up until the bridge is balanced. By increasing
the gain of the amplifier, the bridge should be accurately balanced.
The test radio frequency is now dissipated into the bolometer element, which causes an
unbalance in the bridge circuit. The audio frequency output voltage automatically adjusts itself to
restore the bolometer resistance to its original value. The amount by which the AF power level in
the bolometer is reduced equals the applied RF power. Here, the voltmeter reads the AF voltage.
Most of the DC and AC bridges that can be balanced by adjusting the element utilize the
servo meter to balance the bridge circuit. The system thus serves as resistance-to-displacement
converter. The scale is calibrated in terms of the quantities to which the resistance transducer of
the bridge network responds. Where an output element related to the resistance of a transducer in
the bridge is desired, electronic self-balancing is adopted.

INTERFERENCE AND SCREENING

Interference is one of the most serious as well as most common problems in audio
electronics. We encounter interference when it produces effects like noise, hiss, hum
or cross-talk. If a radio engineer faces such problems, good theoretical knowledge as
well as experience is required to overcome them.
However, it should be considered, that interference is always present. All technical
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remedies only aim at reducing the effect of interference to such a degree, that it is neither audible
nor disturbing. This is mainly achieved by different ways of screening.
This paper will explain the technical background of interference and provides some
common rules and hints which may help you to reduce the problems.
Types of Interference
Theoretically, the effects and mechanism of a single interference can well be calculated.
But in practice, the complex coupling systems between pieces of equipment prevent precise
prediction of interference. The following picture shows the different types of interference
coupling.
If we consider all possible coupling paths in the diagram above we will find 10 different
paths. This means a variety of 1024 different combinations. It should be noted, that not only the
number of paths, but also their intensity is important.
Symmetrical and Asymmetrical Interference
Having a closer look at the interference of cable, we find that high frequency-interference
currents cause measurable levels on signal (audio) lines and on supply lines.
Through interference, asymmetrical signals are produced in respect to the ground. The
asymmetrical interference current flows along the two wires of the symmetrical line to the sink
and via the ground back to the source. These interference signals are cancelled at the
symmetrical input.

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EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

Galvanic Coupling of Interference

Galvanic coupling of interference occurs if the source and the sink of interference are
coupled by a conductive path. As can be seen from the equivalent circuit diagram, the source
impedance of the interference consists of the resistance RC and the inductance LC of the
conductor, which are common to the two parts of the circuit. From these elements the interference
source voltage can be calculated.
Capacitive Coupling of Interference
The capacitive coupling of interference occurs due to any capacitance between the source
and sink of interference.
Principle of capacitive coupling of interference
The current in the interference sink can be calculated as The interference voltage in the sink is
proportional to its impedance. Systems of high impedance are therefore more sensitive to
interference than those of low impedance. The coupled interference current depends on the rate of
change of the interference and on the coupling capacitance CC.
Inductive Coupling of Interference
Inductive coupling of interference occurs if the interference sink is in the magnetic field of
the interference source (e.g. coils, cables, etc.)
The interference voltage induced by inductive coupling is
 increasing the distance between conductors
 mounting conductors close to conductive surfaces
 using short conductors
 avoiding parallel conductors
 screening
 using twisted cable
Note that by the same means the capacitive as well as the inductive coupling of interference will
be reduced.
Interference by Radiation
Interference by electromagnetic radiation becomes important at cable lengths greater than
1/7 of the wavelength of the signals. At frequencies beyond 30Mhz, most of the interference
occurs by e.m. radiation
Interference by Electrostatic Charge
Charged persons and objects can store electrical charges of up to several micro- Coulombs,

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EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

which means voltages of some 10kV in respect to ground. Dry air, artificial fabrics and friction
favour these conditions. When touching grounded equipment, an instantaneous discharge produces
arcing with short, high current pulses and associated strong changes of the e.m. field.
Screening
When considering the effect of electrical and magnetic fields, we have to distinguish
between low and high frequencies. At high frequencies the skin effect plays an important roll for
the screening. The penetration describes the depth from the surface of the conductor, where the
current density has decayed to 37% compared to the surface of the conductor.
The interference and never fully prevent it. This means there will never be a system which
is 100% safe from interference. Because the efforts and the cost will rise with the degree of
reduction of interference, a compromise has to be found between the effort and the result. The
requirement for the reduction of interference will depend on:

- The strength of the interference source

- The sensitivity of the interference sink
- The problems caused by interference
-The costs of the equipment
MULTIPLE EARTH AND EARTH LOOPS
Earth loop forms a distinct part of the guarding system of electrical equipments.
An example of such an earth loop formed between a grounded transducer and a grounded
measuring instrument which are connected by relatively long cables; R1 and R2 represent the
cable resistances.

Figure 3.13 Earth Loop

Different methods to obtain ground connection

 If one rod does not give a sufficiently low ground spread resistance, a system of
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rods connected by a copper ground bus can be used.

 In some cases, the iron rods of a reinforced and concrete structure are welded
together and connected to special conducting rods to give the required low ground spread
resistance.
 A metallic underground water piping system also provides good contact with ground
and is the best solution for grounding of measurements.
Earth current
 Considerable earth currents may be expected as a result of the discharge of
current from power systems to ground. It is very difficult to evaluate these currents and no
satisfactory methods of measurement has been developed.
 Most of the data available are average values taken over a period of time
because of the complexity of parameters such as soil resistivity, soil moisture, depth of the
electrodes, weather conditions etc.
ELECTROMAGNETIC INTERFERENCE
Definition
If the parameter to be measured is at the place at which a measurement is to be displayed
or used for control purposes is at some distance from the point of measurement, then it can lead to
various problems. The main one is electrical noise or interference being superimposed on the
measurement signal. This is called electromagnetic interference.
Sources Of Electromagnetic Interference
Sources of noise and interference include
 Ac power circuits, solenoids switching fluorescent lightning, radio frequency
transmitters.
 Welding equipment.
 Inductive or capacitive coupling.
 By having earths of slightly different potentials.
Effects of electromagnetic interference
Electromagnetic interference often affects instrument signals, particularly when operating
with very sensitive instruments which are close to equipment that produces a log of electrical
noise.
Example: Some of the instruments which are close to the measuring instrument include
thyristor drives for AC motors which produce high-frequency spikes. Drives for DC motors also
produce noise, but at lower frequencies than AC drives, as do solid state relays and other
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EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

equipment with high inductance or capacitance, such as induction heating.

Electromagnetic compatibility
The electromagnetic compatibility regulations are designed to eliminate radio frequency
interference emissions from electrical machines and to ensure that these machines are immune to
such radiation from external sources.
Limiting Interference Problems
To limit interference problems, the following steps are being taken:
 Signal and power cables are routed as far as possible from one another.
 Sensors such as load beams are electrically insulated from other equipment.
 Shielding is used where necessary and appropriate earthing is done.
 Low pass filters are used to reduce high frequency noise in signals and the
signals themselves are amplified as near to the sensor as possible to reduce the
signal-to-noise ratio on the cabling.
 Sometimes the problem itself can be reduced by using, for example DC ,
instead of AC motors and drives.
 ‘Molybdenum’ metal is useful for mechanical shielding of electrically ‘noisy’
components.
ELECTROSTATIC INTERFERENCE AND SCREENING
The Basics of Balancing
Balanced connections in an audio system are designed to reject both external noise, from
power wiring etc., and also internal cross talk from adjacent signal cables. The basic principle of
balanced interconnection is to get the signal you want by subtraction, using a three wire
connection. In Many cases, one signal wire (the hot or in-phase) senses the actual output of the
sending unit, while the other (the cold or phase-inverted)senses the unit’s output –socket ground,
and the difference between them gives the wanted signal. Are in theory completely cancelled by
the subtraction. In real life the subtraction falls short of perfection, as the gains via the hot and
cold inputs will not be precisely the same, and the degree of discrimination Actually achieved is
called the Common-Mode Rejection Ratio, or CMRR.
Screening
While two wires carry the signal , the third is the ground wire which has the dual of both
joining the grounds of the interconnected equipment, and electrostaticallly screening the two
signal wires by being in some way wrapped around them. The ‘wrapping around’ can mean.

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EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

 A lapped screen with wires laid parallel to the central signal conductor. The screening
converge is not perfect, and can be badly degraded as it tends to open up on the outside of
cable bends.
 A braided screen around the central signal wires. This is more expensive, but opens up
less on bends. Screening is not 100%, but certainly better than screen.
 An overlapping foil screen, with the ground wire running down the inside of the foil and
in electrical contact with it. This is usually the most effective as the foil cannot open up
on the outside of bends, and should give perfect electrostatic screening, However, the
higher resistance of aluminium foil compared with Copper braid means that RF screening
may be worse.
Advantages of Balancing
 It discriminates against noise and crosstalk.
 Balanced interconnect aloes 6 dB more signal level on the line.
Electrical Noise
Noise gets into signal cables in three major ways:
Electrostatic coupling
An interfering signal with significant voltage amplitude couples directly to the inner
signal line, through stray capacitance.

Figure3.14 Electrostatic Coupling

The situation is shown in Figure 3.14 with C.C representing the stray capacitance between
imperfectly-screened conductors: this will be a fraction of a Pf in most circumstances. This
coupling can be serious in studio installations with unrelated signals going down the same
ducting.
The two main lines of defence against electrostatic coupling are effective screening

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EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

and low impedance drive.

An overlapping foil screen provides complete protection. Driving the line from a
low impedance, of the order of 100 Ohms or less, means that the interesting signal, having passed
through a very small capacitance, is a very small current and cannot develop much voltage across
such a low impedance. For the best effectiveness the impedance must remain low up to as high a
frequency as possible: this can be a problem as op[-amps invariably have a frequency, and this
makes the output impedance side with frequency possible.
This can be a problem as op-amps invariably have a feedback factor that begins to
fall from a low, and possibly sub-audio frequency, and this makes the output impedance rise with
frequency. From the point of view of electrostatic screening alone, the screen does not need tube
grounded at some point. Electrostatic coupling falls off with the square of distance. Rearranging
the cable-run away from the source of interference is more effective than trying to rely on very
good common-mode rejection.

Figure 3.15 Magnetic Coupling

An EMF Vm is induced in both signal conductors and the screen, and according to
some writers, the screen current must be allowed to flow freely or its magnetic field will not
cancel out the field acting on the signal conductors, and therefore the screen should be grounded
at both ends, to form a circuit. In practice on the common-mode rejection of the balanced system,
to cancel out the hopefully equal voltages Vm induced in the two signal wires. The need to ground
both ends for magnetic rejection is not a restriction, as it will emerge that there are other good
reasons why the screens should be grounded at both ends of a cable.

In critical situations the equality of these voltage is maximised by minimising the loop area
between the two signal wires, usually by twisting them tightly together. In practice most audio
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cables have parallel rather than twisted signal conductors, and this seems adequate most of the
time. Magnetic coupling falls off with the square of distance, so rearranging the cable-run away
from the source of magnetic field is usually all that is required. It is unusual for it to present
serious difficulties in a domestic environment.
 Ground voltages coupled in through the common ground impedance
This is the root of most ground loop problems. In the equipment safety ground
causes a loop ABCD; the more existence of a loop in itself does no harm, but it is
invariably immersed in a 50 Hz magnetic field that will induce mains frequency current
plus odd harmonics into it.
This current produces a voltage drop down the non- negligible ground-wire
resistance, and this once again effectively appears as a voltage source in each of the to
signal lines. Since the CMRR is finite , a proportion of this voltage will appear to be
differential signal, and will be reproduced as such.

GROUNDING TECHNIQUES

Grounding (or)Earthling
This is one of the simplest but most efficient methods to reduce interference.
Grounding can be used for three different purposes:
 Protection Ground
Provides protection for the operators from dangerous voltages. Widely used on
mains-operated equipment.
 Function Ground
The ground is used as a conductive path for signals.
Example: in asymmetrical cables screen, which is one conductor for the signal, is
connected to the ground.
 Screening Ground
Used to provide a neutral electrical path for the interference, to prevent that the
interfering voltages or currents from entering the circuit.
In this chapter we will only consider the third aspect. Grounding of equipment is often
required for the cases 1 or 2 anyhow, so that the screening ground is available "free
of charge".
Sometimes the grounding potential, provided by the mains connection, is very
"polluted". This means that the ground potential itself already carries an interfering signal.
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EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

This is especially likely if there are big power consumers in the neighbourhood or even in the
same building. Using such a ground might do more harm than good. The quality of the ground
line can be tested by measuring it with a storage scope against some other ground connection,
e.g. a metal water pipe or some metal parts of the construction. Never use the Neutral (N) of
the mains as ground. It might contain strong interference, Because it carries the load current of
all electrical consumers.
The grounding can be done by single-point grounding or by multi-point grounding. Each
method has advantages which depend on the frequency range of the signal frequencies. All parts
to be grounded are connected to one central point. This results in no "ground loops" being
produced. This means the grounding conductors do not form any closed conductive path in
which magnetic interference could induce currents. Furthermore, conductive lines between
the equipment are avoided, which could produce galvanic coupling of interference. Central
grounding requires consistent arrangement of the grounding circuit and requires
insulation of the individual parts of the circuit. This is sometimes very difficult to achieve. A
system using the single-point grounding.

Multi-Point Grounding
In multi-point grounding all parts are connected to ground at as many points as possible.
This requires that the ground potential itself is as widely spread as possible.
In practice, all conductive parts of the chassis, the cases, the shielding, the room and the
installation are included in the network. The interconnection of these parts should be done at as
many point possible.

Screening
When considering the effect of electrical and magnetic fields, we have to distinguish
between low and high frequencies. At high frequencies the skin effect plays an important roll for
the screening. The penetration describes the depth from the surface of the conductor, where the
current density has decayed to 37% compared to the surface of the conductor. the interference and
never fully prevent it. This means there will never be a system which is 100% safe from
interference. Because the efforts and the cost will rise with the degree of reduction of
interference, a compromise has to be found between the effort and the result.
The requirement for the reduction of interference will depend on:
 the strength of the interference source
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EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

 the sensitivity of the interference sink

 the problems caused by interference
 the costs of the equipment
We will discuss ways of preventing interference, their effect, and the main aspects for
the optimum efficiency of each method.

 Receiving / Inspection / Storage

The Package Compact Substation is shipped from the factory ready for installation on
site. It has been submitted to all normal routine tests before being shipped, and it is not required
to do any voltage testing before putting it into service, provided the substation has not
sustained any damage during transportation. Immediately upon receipt of the Package Compact
Substation, examine them to determine if any damage or loss was sustained during transit. If
abuse or rough handling is evident, file a damage claim with carrier and promptly notify
the nearest ABB office. ABB ELECTRICAL INDUSTRIES CO. LTD. is not responsible
for damage of goods after delivery to the carrier; however, we will lend assistance if notified of
claims.
 Personnel Safety
The first and most important requirements are the protection against contact with
live parts during normal service as well as maintenance or modifications. This is the reason why
all live parts have been metal enclosed, so that when the parts are live and the Package Compact
Substation doors are open, no one can be able to touch them. Also, it is safe in case any short-
circuiting or sparking occurs at the bus bars.
 Ventilation
Transformer compartment has been provided with sand trap louvers, to prevent ingress
of sand and that proper air circulation should take place.

 Earthling
Proper earthling bus bar has been provided.

 Handling
Lifting lugs has been provided on top of four corners of the housing for lifting the DPS
by crane and chains as a single unit, otherwise this can be done by a forklift of sufficient
capacity, but the lifting fork must be positioned under the transformer portion. Schering Bridge is
independent of frequency.

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EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

3.11 QUESTION BANK

PART – A
1. Draw Maxwell’s AC bridge and give the balance equation In terms of resistance.
2. Explain any two technical parameters to be consider in grounding
3. Give some applications of Whetstone’s bridge.
4. What is a potentiometer?
5. List the applications of dc and ac potentiometer.
6. Differentiate the principle of dc potentiometer and Ac potentiometer.
7. What is meant by transformer ratio bridge
8. What are the features of ratio transformer? List its applications.
9. What is meant by electromagnetic interference?
10. List the sources of electromagnetic interference.
11. What are the ways of minimizing the electro magnetic interference?
12. Define electromagnetic compatibility.(EMC)
13. What are the main causes of group loop currents?
14. What are the limitations of single point grounding method?
15. What is the necessity of grounding and state is advantages.
16. What is meant by ground loop? How it is created? 17. What are the sources of errors
in bridge measurement?
18. Define standardization.
19. Give the relationship between the bridge balance equation of DC bridge and AC bridge.
20. What does a bridge circuit consists of?

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EE8403 UNIT-3 COMPARATIVE METHODS OF MEASUREMENTS

PART – B
1. a) Explain in detail about the laboratory type DC potentiometer.( 1 2 )
b) Give the applications of AC potentiometers. (4)
2. a)Describe about the multiple earth and earth loops.(8)
b) Explain the different techniques of grounding. (8)
3. Explain voltage sensitive self balancing bridge, and derive the bridge sensitivity of
voltage sensitive bridge with fundamentals.( 1 6 )
4. a) With fundamentals distinguish between DC and AC Potentiometers, and give any
two specific applications for each.(8)
b) Discuss the advantages and limitations of electromagnetic Interference in measurements.(8)
5. a) Explain Kelvin’s double bridge method for the measurement of low resistance.(8)
b) Explain how inductance in measured by using Maxwell’s bridge. (8)
6. a)Explain the working principle of Anderson’s bridge and also derive its balance
equations. (8)
b) Explain the working principle of Schering bridge and also derive its balance equations. (8)

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