electrical measurement and instrumentation

7/30/2019 Electrical Measurement and Instrumentation

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Jawaharlal Nehru Engineering College

Laboratory Manual

ELECTRICAL MEASUREMENT AND INSTRUMENTATION

For

Second Year Students


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This manual is intended for the second year students of instrumentation in thEsubjectInstrumentation-1. Manual typically contains practical/Lab Sessions related MeasurementSystem fundamentals covering various aspects related the subject to enhanced understanding.

Students are advised to thoroughly go though this manual rather than only topics mentionedin the syllabus as practical aspects are the key to understanding and conceptual visualization of

theoretical aspects covered in the books.

Good Luck for your enjoyable laboratory sessions.


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DOs and DONT DOs in Laborary:

1. Do not handle any equipment before reading the instructions/Instruction manuals

2. Read carefully the power ratings of the equipment before it is switched on whether ratings 230V/50 Hz or 115V/60 Hz. For Indian equipments, the power ratings are normally 230V/50Hz. If youhave equipment with 115/60 Hz ratings, do not insert power plug, as our normal supply is 230V/50Hz, which will damage the equipment.

3. Observe type of sockets of equipment power to avoid mechanical damage

4. Do not forcefully place connectors to avoid the damage

5. Strictly observe the instructions given by the teacher/Lab Instructor

Instruction for Laboratory Teachers::

1. Submission related to whatever lab work has been completed should be done during the next labsession. The immediate arrangements for printouts related to submission on the day of practicalassignments.

2. Students should be taught for taking the printouts under the observation of lab teacher.

3. The promptness of submission should be encouraged by way of marking and evaluation patternsthat will benefit the sincere students.

Author JNEC EEP DEPT., Aurangabad


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SUBJECT INDEX

1. Study of LVDT Characteristics

2. Study of Wheatstone bridge.

3. Study of Kelvins Bridge.

4. Study of Cathode Ray Oscilloscope (CRO).

5. Study of Thermocouple (Temperature Measurement)


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EXPERIMENT NO.1

LVDT CHARACTERISTICS

Aim: To study the characteristics of LVDT using ANSHUMAN LVDT trainer kit.

Apparatus: ANSHUMAN LVDT trainer kit,

Theory:The basic structure of LVDT is a movable core of a permeable material & three coilas shown in fig. The inner core is a primary, which provides magnetic flux through itsexcitation by some AC source. The two secondary coils have voltages induced do toflux linkage with the primary. When the core is centrally located the voltageinduced in each secondary is same & when the core is displaced, the change in fluxlinkage causes one secondary voltage to increase & other to decrease. Thesecondary windings are generally connected in series opposite so that the voltageinduced in each are out of phase with the other. In this case, as shown in the fig. 2,the output voltage is zero when the core is centrally located & increases as the coreis moved in either direction in or out.

The voltage amplitude is linear with the core displacement over some range ofcore travel. Further more there is a phase shift as the core moves both to & fromthe central location.

Block diagram:


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Procedure:

1) Connect the 9- D-PIN type male connector of LVDT box to the PIN D- typefemale connector of LVDT signal conditioning circuit.2) Connect the main cords of the LVDT signal conditioning board in the main supplysocket.3) Rotate the micrometer screw such that the reading in micrometer is exactly 10mm. Because this is our null or zero position. The reading below the 10 mm isconsidered as +ve displacement reading above the 10 mm is considered as vedisplacement.For eg. , If the reading is 7 mm thenDisplacement =10 7= 3mm& If the reading is 14 mm, then, displacement =10 14= -4 mm.

4) Make the power ON.5) Now display should show 00.00, If not,If the error is above +/- 00.10 mm, then adjust the coil such that the displaymatches exactly with the reading i. e. 00.00.If the error is below +/- 00.10 mm, If the error is above +/- 00.10 mm, then adjustthe coil such that the display matches exactly with the reading i.e. 00.00.6) Now rotate the micrometer such that the reading on the micrometer isdisplacement = 10 0.0 = 10.00 mm7) Now the reading on display must be 10.00, if not8) Adjust the SPAN POT of signal conditioning circuit such that the reading indisplay must be 10.00. the display actually shows the core displacement.

9) Now again rotate the micrometer screw such that the reading on micrometer is10 mm. Now the display must show 00.00 mm.10) If not, repeat the steps 5 to 9 again.11) Now again rotate the micrometer so that the reading on micrometer is 20.00mm.

Displacement = (10.00 - 20.00)= - 10.00 mm12) Now check the display reading. In this case the reading on the display is around 10.00 mm.13) Now LVDT trainer kit is ready for the experiment14) Now again rotate the micrometer screw such that the displacement of core iszero.

15) Now move the core of the LVDT IN THE +VE & -ve direction with respect to thenull position & observe the readings on the display & enter your readings in theobservation table.

Observation Table:


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Least count of micrometer: value of smallest division on main scaleTotal no of divisions

= 0.01 mm

Sr.

No.

Core displacement Display reading Secondary Voltage

Graph:Core displacement Vs display reading Core Displacement Vs SecondaryVoltage

Conclusion:Distance on display depends upon core position in +ve & -ve direction.


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EXPERIMENT NO. 2

Study of Wheatstone bridge.

OBJECTIVE: To measure the resistance value using Wheatstones Bridge.

EXPERIMENTAL SETUP: Wheat stone bridge, galvanometer, D.C. power supply, unknownresistance, connecting leads etc.

THEORY:

INTRODUCTION:Bridge circuits are extensively used for measuring components values such as R, L and C.

Since the bridge circuit merely compares the value of tan unknown component with that of anaccurately known component (a standard), its measurement accuracy can be very high.

The Wheatstone bridge is used for accurate measurement of resistance.

BRIDGE CIRCUIT DETAILS:The source of emf and switch is connected to points A and B, while a sensitive current

indicating meter, the galvanometer, is connected to points C and D. The galvanometer is a sensitivemicroammeter, with a zero center scale. When there is no current through the meter, thegalvanometer pointer resets at 0, i.e. mid scale. Current in one direction causes the pointer todeflect on one side and current in the opposite direction to the other side.

When SW1 is closed, current flows and divides into the two arms at point A, i.e. I1 and I2.The bridge is balanced when there is no current through the galvanometer, or when the potentialdifference at points C and D is equal, i.e. the potential across the galvanometer is zero.

To obtain the bridge balance equation, we have from the fig.

I1R1 = I2R2 ------------------------ (1)For the galvanometer current to be zero, the following condition should be satisfied.

I1 = I3 = E -----------------(2)R1 + R3

I2 = I4 = E --------------------(3)R2 + R4

Substituting in eq. (1)

R4 = R2 R3R1

This is the equation for the bridge to be balanced.


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Balance Equation: R4 = R2 R3R1

SENSITIVITY OF WHETSTONE BRIDGE:

Sensitivity is deflection per unit current.

Sensitivity = deflectionUnit current.

Where, S=linear or angular per micro-AS=mm/micro-AS=radians/micro-A

Therefore, total deflection is given byD=S*I

Where I=current in amperes (micro-A)


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THEVENINS EQUIVALENT FOR UNBALANCED WHEATSTONES BRIDGE:

Thevenins equivalent voltage is found by disconnecting the galvanometer from the bridgecircuit, as shown in the above figure, and determining the open circuit voltage between terminals aand b.

Applying the voltage divider equation, the voltage at point a, can be determined as follows

Ea = E R3 and at point b, Eb = E R4R1 + R3 R2 + R4

Therefore, the voltage between a and b is the difference between Ea and Eb , Which representsThevenins equivalent voltage.

Eth = Eab = Ea Eb = E R3 - E R4

R1 + R3 R2 + R4

Therefore Eab = E R3 - R4R1 + R3 R2 + R4

Thevenins equivalent resistance can be determined by replacing the voltage source E with itsinternal impedance or otherwise short-circuit and calculating the resistance looking into terminals a


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and b. Since the internal resistance looking into terminals a and b. Since the internal resistance isassumed to be very low, we treat it as 0W. Thevenins equivalent resistance circuit is shown below.

The equivalent resistance of the circuit is R1R3 in series with R2R4i.e. R1R3 + R2R4 .

Therefore, Thevenins equivalent circuit is given in figure below. If the galvanometer is connectedacross the terminals a and b of fig 2 or its Thevenins equivalent fig 4 it will experience the samedeflection at the output of the bridge.


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The magnitude of current is limited by both Thevenins equivalent resistance and any

resistance connected between point a and b. The resistance between a and b consists only of

the galvanometer resistance Rg. The deflection current in the galvanometer is therefore given

by

Ig = Eth

Rth + Rg

LIMITATIONS:

For low resistance measurement, the resistance of the leads and contacts becomes

significant and introduces an error. This can be eliminated by Kelvins Double Bridge.

Another difficulty in Wheatstones bridge 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 resistance, and excessive current may cause a permanent change in

value.

APPLICATIONS:


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The Wheatstones bridge can 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. For example, the resistance of motor windings, transformers, solenoids, and relay

coils can be measured.

Wheatstones bridge is also used extensively by telephone companies and others to

locate cable faults. The fault may be two lines shorted together, or a single line shorted to

ground.

CONCLUSION:

REVIEW QUESTIONS AND ANSWERS:1. Compare the measuring accuracy of a Wheatstones bridge with the accuracy of an ordinary

ohmmeter?2. Define the term null as it applies to bridge measurement.3. What are different types of null detector used in bridge measurement?4. A wheatstones bridge cannot be used for precision measurements. Give reasons.


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Experiment No.3

To Study Kelvins Bridge.

OBJECTIVE: To measure the resistance value using Kelvins Bridge.

EXPERIMENTAL SETUP: Kelvins bridge, galvanometer, D.C. power supply, unknown resistance,connecting leads etc.

THEORY:

When the resistance to be measured is of the order of magnitude of bridge contactand lead resistance, a modified form of Wheatstone's bridge, the Kelvin bridge is employed.Kelvin's bridge is a modification of Wheat- stone's bridge and is used to measure

values of resistance below1W. In low resistance measurement, the resistance of the leads connecting the unknownresistance to the terminals of the bridge circuit may affect the measurement.

Consider the circuit in Fig.1, where Ry represents the resistance of the connectingleads from R3 to Rx (unknown resistance). The galvanometer can be connected either to pointc or to point a. When it is connected to point a, the resistance Ry, of the connecting lead isadded to the unknown resistance R x' resulting in too an high indication for Rx. When the


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connection is made to point c, Ry is added to the bridge arm R3 and resulting measurement ofRx is lower than the actual value, because now the actual value of R3 is higher than itsnominal value by the resistance Ry. If the galvanometer is connected to point b , in betweenpoints c and a, in such a way that the ratio of the resistance from c to b and that from a tob equals the ratio of resistances R 1 and R2,

Equation (3) is the usual Wheatstone's balance equation and it indicates that theeffect of the resistance of the connecting leads from point a to point c has been eliminatedby 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 setof ratio arms. Figure 2 shows a schematic diagram of Kelvin's double bridge.

The second set of arms, a and b , connect the galvanometer to a point c at theappropriate potential between m and n connection, i.e. Ry. The ratio of the resistances ofthe arms a and b is the same as the ratio of R1 and R2. The galvanometer indication is zerowhen the potentials at k and c are equal.


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This is the usual equation for Kelvin's bridge. It indicates that the resistance of theconnecting lead Ry, has no effect on the measurement, provided that the ratios of the twosets of arms are equal. In a typical Kelvin's bridge the 'range of a resistance covered is 1-0.0001W (10 mohm) with an accuracy of 0.05% to 0.2%.PRACTICAL KELVINS DOUBLE BRIDGE:

Figure No.3 shows a commercial Kelvins bridge capable of measuring resistance from10 - 0.00001W.

Contact potential drops in the circuit may cause large errors. This effect is reduced

by varying a standard resistance consisting of nine steps of 0.001W each, plus a calibratedmanaging bar of 0.0011Wwith a sliding contact. When both contacts are switched to selectthe suitable value of standard resistance, the voltage drop between the ratio arm connectionpoints is changed, but the total resistance around the battery circuit is unchanged.

This arrangement places any contact resistance in series with the relatively highresistance value of the ratio arms, rendering the contact resistance effect negligible. Theratio R1 / R2 is selected (as given in Fig.No.3) such that a relatively large part of thestandard resistance is used and hence RX is determined to the largest possible number ofsignificant figures. Therefore, measurement a.c. current improves.


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CONCLUSION:

REVIEW QUESTIONS AND ANSWERS:1. Compare the measuring accuracy of a Kelvins bridge with the accuracy of an ordinary

ohmmeter?

2. What are the advantages of Kelvins bridge over Wheatstones bridge?


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EXPERIMENT NO.4

Study of Cathode Ray Oscilloscope (CRO).

OBJECTIVE: To study construction, front Panel of CRO. To measure the Voltage, Current,

Frequency, Time period of the Input waveform. To study lissajous pattern.

FIG: FUNCTIONAL BLOCK OF A SIMPLE CRO

TYPICAL SPECIFICATIONS:

VERTICAL DEFLECTION:

Bandwidth (-3dB): d.c. to 20MHz ( 2Hz to 20KHz on a.c.)Sensitivity: 2mV/cm to 10V/cmAccuracy: 3%Input Impedance: 1MW/28pf approx.Input Coupling: DC-GND-ACInput Protection: 400V d.c. or pk a.c.

HORIZONTAL DEFLECTION:Timebase: 0.5ms/cm to 0.2ms/cm, 18 rangesAccuracy: 3%

ADDITIONAL FACILITIES:Calibrator: 1V, 2% squarewave at approx. 1KHz.

Ramp Output: Approx. 3.5V ramp from 5KW.SUPPLY:220/240V 10%45 TO 65 Hz approx. 40VA.


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Fig No. 2: - Simple CRO

APPLICATIONS OF OSCILLOSCOPE:I. Measurement of Voltage:

The most direct voltage measurement made with the help of an oscilloscope is the

peak to peak (p-p) value. The rms value of the voltage can then be easily calculatedfrom the p-p value.

To measure the voltage from the CRT display, one must observe the setting of thevertical attenuator expressed in V/div and the peak to peak deflection of beam, i.e. thenumber of divisions. The peak value of voltage is then computed as follows.

Vp-p = (volts/div) (no. Of div)


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Fig No. 3: - Sine Waveform

II. Period and Frequency Measurement:

The period and frequency of periodic signals are easily measured with anoscilloscope. The waveform must be displayed such that a complete cycle isdisplayed on the CRT screen. Accuracy is generally improved if a signal cycledisplayed fills as much of the horizontal distance across the screen as possible.

The period is calculated as follows.

T = (time/div) (No. of div/cycle)The frequency is then calculated as f = 1/T

III. Measurement of Frequency by Lissajous Method:

This particular pattern results when sine waves are applied simultaneously toboth pairs of the deflection plates. If one frequency is an integral multiple(harmonic) of the other, the pattern will be stationary, and is called a lissajousfigure.

In this method of measurement a standard frequency is applied to one set ofdeflection plates of the CRT tube while the unknown frequency (of approximatelythe same amplitude) is simultaneously applied to the other set of plates. However,the unknown frequency is presented to the vertical plates and the known frequency

(standard) to the horizontal plates. The resulting patterns depend on the integraland phase relationship between the two frequencies. (The horizontal signal isdesignated as fh and the vertical signal as fv.)


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Fig No. 4: - Basic circuit for frequency Measurements withLissajous Pattern for Integral Frequencies

Measurement Procedure:Set up the oscilloscope and switch off the internal sweep (change to Ext). Switch off

sync control. Connect the signal source as given in Fig. 4. Set the horizontal and verticalgain control for the desired width and height of the pattern. Keep frequency fv constantand vary frequency fh , noting that the pattern spins in alternate directions and changesshape. The pattern stands still whenever fv and fh are in an integral ratio ( either even orodd). The fv = fh pattern stands still and is a single circle or ellipse. When fv = 2fh , atwo loop horizontal pattern is obtained as shown in Fig. 5.

To determine the frequency from any Lissajous figure, count the number of horizontalloops in the pattern, divide it by the number of vertical loops and multiply this quantity byfh , (known or standard frequency).

In Fig.5 (g), there is one horizontal loop and 3 vertical loops, giving a fraction of 1/3.The unknown frequency fv is therefore 1/3 fh. An accurately calibrated, variablefrequency oscillator will supply the horizontal search frequency for frequencymeasurement. For the case where the two frequencies are equal and in phase, the patternappears as a straight line at an angle of 45 with the horizontal. As the phase betweenthe two alternating signals changes, the pattern changes cyclically, i.e. an ellipse (at 45with the horizontal) when the phase difference is p/4, a circle when the phase differenceis p/2 and an ellipse (at 135 with horizontal) when the phase difference is 3p/4, and astraight line pattern (at 135 with the horizontal) when the phase difference is pradians.

Unknown


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Fig No. 5: - Lissajous Pattern for Integral Frequencies

As the phase angle between the two signals changes from p to 2p radians, the patternchanges correspondingly through the ellipse-circle-ellipse cycle to a straight line. Hencethe two frequencies, as well as the phase displacement can be compared using Lissajousfigures techniques.

When the two frequencies being compared are not equal, but are fractionally related,a more complex stationary pattern results, whose form is dependent on the frequencyratio and the relative phase between the two signals as in fig 6.


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REVIEW QUESTIONS AND ANSWERS:1. What precautions are taken before the CRO is plugged in for operation?2. Differentiate between Dual Trace and Dual Beam Oscilloscope?3. Is there any control for beam rotation in CRO? Explain.4. How the oscilloscope is calibrated to ensure correct sensitivity?

5. How capacitors measurement is performed using CRO?


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Experiment No.5

Temperature Measurement Using Thermocouple.

Aim: - To Plot the characteristic of Thermocouple.

Experimental Setup: - Thermocouple:

i. Copper Constantanii. Iron - Constantan

Electric Heater Water Container Multimeter Thermometer Temp. Measurement Trainer.

Theory: -Devices that measure temperature on the basis of thermoelectric principle is called

Thermocouples.

Thermoelectric Effect: -Different metals have different electrical & thermal transport properties. When a

temperature differential is maintained across a given metal, the vibration of atoms & motion ofelectrons is affected so that a difference in potential exists across material.

This potential difference is related to the fact that electrons in the hotter end of thematerial have more thermal energy than those in the cooler end & thus tend to drift toward thecooler end. This drift varies for different metals at the same temperature because of theirdifference in thermal conductivities. If the circuit is closed by connecting the ends throughanother conductor, a current is found to flow in the closed loop.

Seeback Effect: -The current flows in the closed circuit made up of two different metals if the

junctions of the two metals are kept at different temperatures. In each lead, the concentration ofvalance electrons is proportional to the temperature, and at the point of contact, the electrondiffer through the boundary layer between the two leads, resulting in one lead becoming positiveand the other becoming negative. Thus emf generated is proportional to the temp. difference in apredictable manner. This phenomenon is called as Seeback effect.


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T2 emf T1

Fig. [a] Seeback Effect = T2

T2 ( qa - qb ) dTWhere ,

= emf produced in volts.T1, T2 = junction temp. in K.qa , qb = Thermal transport constant of two metals.

Therefore emf produced is proportional to the difference in temp. and further, to the differencein the metallic thermal transport constant. However in practice it is found that two constants Qa &

Qb are nearly independent of temperature and that on approximate linear relationship exists as = a ( T2-T1 )

Where,a = constant in volts/kT1, T2 = Junction tempreture in K.

Practicle Thermocouple :-To provide an output that is definite with respect to the

temperature to be measured , an arrangement such as shown in figure below is used.Tr

A C

B CTr

Fig. [b] practical measurement with a thermocouple system employs a third wire type &extension wires to carry the emf to the measurement device.The above fig.[b] shows that the measurement Junction Tm is exposed to the environment whosetemp. is to be measured. This junction is formed of metals A & B. Two other junctions then are

formed to a common metal C, which then connects to the measurement apparatus. The referencejunctions are held at a common, known temperature Tr, the reference junction temp.To use theThermocouple to measure a temperature, the reference junction temperature must be known andthe reference junction must be a held at constant temperature.


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Graph: -^

o/pVoltage

>Temp. ( c)

Result: -

Precaution: -While connecting the thermocouple to the input terminals, observe the polarity.


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