Lecture-1 on 27.03 [Compatibility Mode]

8/13/2019 Lecture-1 on 27.03 [Compatibility Mode]

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Measurement Techniques-II

Basic Electrical Measurementsand Sensing Devices


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Block Diagram of Measurement

and Display

TransducerInputcircuit

Signalconditioner

Transmission

Processing

PhysicalProperty

Electrical

Signal

Display


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Analog and Digital MetersBasic electrical meters and measurement

of-voltage

rr n

power

frequency

Basic electronic instruments and measurements of

PMMC

Moving Iron

Dynamometers

DVM

CRO


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DC Meters Analog Meters

Digital Meters

Permanent-magnet moving-coil mechanism (PMMC)

DC ammeters DC voltmeters


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PMMC


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Torque and Deflection


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Coil Arrangement

Bobbin

Jewel Bearing


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Coil Arrangement


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dArsonaval movement


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Meter using dArsonaval movement


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Basic DC Ammeter circuit

1

sh sh m m

m m m m msh

sh sh

m

I R I R

I R I R RRI I I n

IWhere nI

=

= = =

=


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Internal and external Shunts


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Multirange ammeter

Simple arrangement Ayrton Shunt arrangement


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Ammeter Sensitivity Ammeter sensitivity is determined by the

amount of current required by the metercoil to produce full-scale deflection of the

.

The smaller the amount of current requiredto produce this deflection, the greater the

sensitivity of the meter.


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Ammeter loading effect


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Ammeter safety Precautions Always connect an ammeter in series with

the circuit under test. Always start with the highest range on an

Observe the proper circuit polarity toprevent the meter from being damaged.

Never use a dc ammeter to measurealternating current.


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Try yourself Ex. A 1mA meter movement with an internal

resistance of 50is to be used in a 0 to1A, 0to 5A and 0 to 10 A ranges in the

.of the required shunt resistances.


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Try yourself Ex. An ammeter has a PMMC instrument

with a coil resistance of Rm=99 andFSD current of 0.1mA. Shunt resistance

.passing through the ammeter at (a) FSD(b) 0.5 FSD (c) 0.25FSD


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Basic DC Voltmeter circuit

( )in m s m

ins m

m

V I R R

VR R

I

= +

=


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Multirange voltmeter


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Voltmeter Sensitivity The sensitivity of a voltmeter is given in ohms

per volt.

Sensitivity, Rm RsSVin

+=

The sensitivity of a 1mA movement is thereciprocal of 1 mA, or 1k per volt

fsd

mSVolt Amp I

= = =


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Loading Effect


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Voltmeter safety precautions Always connect a voltmeter across a

circuit Always begin with the highest range of a

.

In dc voltmeters, observe the proper circuitpolarity to prevent damage to the meter.

Never use dc voltmeter to measure acvoltage


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Try yourself In the circuit shown in fig. the voltage across the

resistor of value 25kis to be measured first byusing a voltmeter of sensitivity of 1k/V, andthen with a voltmeter of sensitivity of 20k/V.

Calculate the reading of the voltmeter in eachcase and the % error in the measurement.

Let the True value of the voltage across the

25kresistor =25V


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Ohmmeter An ohmmeter is an instrument used to

measure resistance and check thecontinuity of electrical circuits and

.


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Series type ohmmeter


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Shunt type ohmmeter


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Ohmmeter safety precautions Be certain that the circuit is de-energized and

discharged before connecting an ohmmeter Do not apply power to the circuit while

measuring resistance

When finished using the ohmmeter, switch it tothe OFF position

Always adjust the ohmmeter for zero reading

after changing a range and before makingresistance measurement.


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Multimeter safety precautions


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Electrodynamic Meter Movements


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Electrodynamic Meter Movements Deflecting torque

Spring controlled torque

Damping

Calibration Features


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Electrodynamic Meter Movements Construction of a Voltmeter

Construction of an Ammeter

Construction of a Wattmeter

Construction of a Watt-hourmeter (Energy)


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Moving Iron (Vane) Meter

Movement


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Moving Iron (Vane) Meter

Movement Deflecting torque

Spring controlled torque

Damping

Features


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Digital Voltmeters (DVM)

Documents

Lecture-1 on 27.03 [Compatibility Mode]