paper ei(2) by pankaj sir

8/10/2019 PAPER EI(2) by Pankaj Sir

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Jainam Solved Paper Electronic Instrumentation

It is the smallest change in input which an electronic instrument

is able to detect. Thus, it is the full scale value of the lowest

voltage range multiplied by the resolution of the meter.

Sensitivity,

fs

S=

Where,

f

( s)min

=Lowest full-scale value of digital meter

R= Resolution in decimal

c)ACCURACY:

The accuracy of an instrument is a measure of how close the

output reading of the instrument is to the correct value. In the

accuracy specifications, the following two quantities are included.

A percentage of range.

A percentage of reading.

It is an important system design rule that instruments are

chosen such that their range is appropriate to the spread of

values being measured, in order that the best possible accuracy is

maintained in instrument readings.

d)AVERAGE/TRUE ROOT MEAN SQUARE:

The standard deviation of an infinite number of data is the Square

root of the sum of all the individual deviations squared divided by

the number of readings.

It may be expressed as

= d 1

2+d22+d32 +dn2

n

The standard deviation is also known as root mean square

deviation, and is the most important factor in the statisticalanalysis of measurement data. Reduction in this quantity

effectively means improvement in measurement.

Jainam Publication TMPage 2


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e)CREST FACTOR:

The crest factor or peak-to-average ratio (PAR) is a measurement of

awaveform, calculated from thepeak amplitudeof the waveform

divided by theRMSvalue of the waveform [1]:

The purpose of the crest factor calculation is to give an analyst a

quick idea of how much impacting is occurring in a waveform.

Impacting is often associated with roller bearing wear, cavitation

and gear tooth wear.

f)FORM FACTOR:

Form factor is defined as the ratio of root mean square value to

the average value of the unit.

In the case of a sinusoidal wave ie, an analogue wave, the form

factor is approximately 1.11. In the case of a square wave i.e., a

digital wave, the RMS and the average values are equal; therefore,

the form factor is 1.

Q. 2: What do you understand by the following terms:

i) Normal mode rejection ratio

ii) Common mode rejection ratio

iii)Effective common rejection ratio

iv)Zero or offset frequency response (CSVTU April- May 2009)

Ans:

i) NORMAL MODE REJECTION RATIO:

http://en.wikipedia.org/wiki/Waveformhttp://en.wikipedia.org/wiki/Peak_amplitudehttp://en.wikipedia.org/wiki/Root_mean_squarehttp://en.wikipedia.org/wiki/Crest_factor#cite_note-0http://en.wikipedia.org/wiki/Peak_amplitudehttp://en.wikipedia.org/wiki/Root_mean_squarehttp://en.wikipedia.org/wiki/Crest_factor#cite_note-0http://en.wikipedia.org/wiki/Waveform


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Normal-mode rejection ratio (NMRR) describes the ability of the

DMM to reject an ACnormal-mode signal, usually at power line

frequencies. NMRR is given by the following formula:

NMRR = 20*log(Vin/Verror)

WhereVerroris the value returned by the DMM for an applied AC

normal-mode voltageVin.

NMRR is useful for measurement systems that can eliminate

signals at a given frequency or over a range of frequencies. NMRR,

which is often used to indicate the capability of the instrument to

reject powerline noise of 50 or 60 Hz, is valid only at the specified

frequency and is useful when making DC measurements.

ii) COMMON MODE REJECTION RATIO:

Thecommon-mode rejection ratio(CMRR) of adifferential

amplifier(or other device) is the tendency of the devices to reject

the input signals common to both input leads. A high CMRR is

important in applications where the signal of interest is

represented by a small voltage fluctuation superimposed on a

(possibly large) voltage offset, or when relevant information is

contained in the voltage difference between two signals.

The CMRR is a measure of how well the device rejects a common-

mode signal.

Its simply the ratio of the differential gainAvover the common-

mode gainAcm.

http://zone.ni.com/reference/en-XX/help/370384N-01/dmm/normal_mode/http://en.wikipedia.org/wiki/Differential_amplifierhttp://en.wikipedia.org/wiki/Differential_amplifierhttp://zone.ni.com/reference/en-XX/help/370384N-01/dmm/normal_mode/http://en.wikipedia.org/wiki/Differential_amplifierhttp://en.wikipedia.org/wiki/Differential_amplifier


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iii) EFFECTIVE COMMON REJECTION RATIO:

Effective common-mode rejection ratio (ECMRR) is the sum of

CMRR and NMRR at a given frequency and is only valid for DCmeasurements. It is the effective rejection on a given noise signal

that is applied to both input leads because it is rejected first by

the CMRR capability of the instrument and then again by its

NMRR capability. This specification is useful at powerline

frequencies, particularly for laboratory and manufacturing floor

environments. An equivalent equation to represent ECMRR is as

follows:

ECMRR = 20*log10(VCM/Verror)

whereVerroris the value returned by the digital multimeter in

response to an applied common mode voltageVCM.

iv) ZERO OR OFFSET FREQUENCY RESPONSE:

Proportional control action is characterized by a permanent

residual error in the operating point of the controlled variablewhen a change in load occurs. This error is called the OFFSET.

The offset can be reduced by selecting higher value of controller

gain (KP) corresponding to narrow bandwidth.

Fig.1. Offset Error

Figure 1, shows how offset error occurs in a proportional

control action. Let the system error be zero at nominal load withcontroller output P = 50% corresponding.



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If, however, a transient error occurs, the system tends to adjust

the controller output so that the point A (corresponding to zero

error) is reached again. But for this to happen there must be

change in the load system. This changed controller load produces

a new value (P new) of the characteristic output, which gives rise

to point B on the linear characteristic of the proportionalcontroller. The permanent small difference between the percentage

error values corresponding to point B and A is called the offset

error of the fcontrol.

Q. 3: - Explain digital voltmeters?

(CSVTU Nov-Dec 2010, Nov-Dec 2009, April-May 2009, Nov-Dec

2008, Nov-Dec 2007)

Ans:

DIGITAL VOLTMETERS (DVMS)

The digital voltmeter (DVM) displays measurements of ac or

dc voltages as discrete numerals instead of a pointer deflection ona continuous scale as in analog instruments. It is aversatile and

accurate instrument that is employed in many laboratory

measurement applications.

Digital voltmeters (DVMs) are measuring instruments that

convert analog voltage signals into a digital or numeric readout.

This digital readout can be displayed on the front panel and also

used as an electrical digital output signal. Any DVM is capable of

measuring analog dc voltages. However, with appropriates signal

conditioners preceding the input of the DVM, quantities such as

ac voltages, ohms, dc and ac current, temperature, and pressure

can be measured. The common element in all these signal

conditioners is the dc voltage, which is proportional to the level of

the unknown quantity being measured. This dc output is then

measured by the DVM.

It is a versatile and accurate instrument that is employed

in many laboratory measurement applications. Because of



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development and perfection of IC modules, the size, power

requirements and costofthe digital voltmeter has been drastically

reduced and, therefore, DVMs can actively compete with

conventional analog instruments, both in price and portability.

The block diagram of a simple digital voltmeteris shown inFig.1.

The unknownvoltagesignal is fed to the pulse generator

which generates a pulse whose width is directlyproportionaltothe input unknown voltage. The output of the pulse generator is

applied to one leg of anANDgate. The input signal to the other leg

of the AND gate is a train of pulses. The outputof theAND gate is,

thus, a positive trigger train of durationt seconds and the inverter

convertsit intoa negative trigger train. The counter, counts the

number of triggers intsecondswhich mproportional to the

voltage under measurement. Thus, the counter can be calibrated

to indicate voltage in volts directly.

Thus, we see that the DVM described above is an ADC

which converts an analog signal into a train of pulses, the number

of which is proportional to the input voltage. So a digital voltmeter

can be made by using any one of the A/D conversion methods and

can be represented by a block diagram shown in Fig. 2. Thus the

DVMs can be classified on the basis of ADCs used.



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Fig. 2 shows the,"voltage-to-time conversion"using gated -dock

pulses.

At the start of the measuring cycle, a ramp voltage is

initiated; this voltage can be positive going or, negative goings the

negative-going ramp, shown in fig.2 is continuously compared

with the unknown input-voltage. At the instant that the ramp voltage equals the unknown

voltage, a coincidence circuit, or comparator generates a pulse



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which opens a gate, the ramp voltage continues to decrease with

time until finally reaches 0 V and a second comparator generates

an output pulse which closes the gate. An oscillator generates

clock pulses which are allowed to pass through-the-gate to a

number of decades counting units (DCUs) which totalize the

number of pulses passed through the gate. Thedecimal numberdisplayed by the indicator tubes associated with the DCUs, is a

measure of the magnitude of the input voltage.

The sample-rate multivibrator (MV) determines the rate

at which the measurement cycles are initiated. The sample-rate

circuit provides an initiating pulse for the ramp generator to start

its next ramp voltage. At the same time, a reset pulse is generated

which returns all the DCUs to their zero state, removing the

display momentarily from the indicator tubes.

Advantages:

1)The ramp technique circuit is easy to design and its cost is

low.

2)The output pulse can be transmitted over long feeder lines.

However, the single ramp requires excellent characteristics

regarding linearity of the ramp and time measurement.

Disadvantage:

Large errors are possible when noise is superimposed on the

input signal. Input filters are usually required with this type

of converter.

2)DUAL SLOPE INTEGRATING TYPE DVM:

In ramp techniques, superimposed noise can cause large errors.

In the dual ramp technique, noise is averaged out by the positive

and negative ramps using the process of integration.

The basic Dual Slope Integrating Type DVM consist of Five basic

building blocks ,

1.Op-amp, used as an integrator;2.A level comparator;

3.A basic block, for generating timing pulses;



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This unknown time t is determined by counting timing pulsesfrom

the clock until the voltage across the capacitor reaches its basic

reference value (reference may be ground or any other basic

reference level ).

Then, from similar triangles of Figure shown above we have :

The count aftertwhich isproportional to the input voltage Vi is

displayed as the measured voltage.

This instrument can be used to measure currents, resistances andA.C. voltages by using appropriate signal conditioners.

Advantages:

i)The averaging characteristics and cancellation of errors that

usually limit the performance of ramp-type DVM are the

main advantages of such DVMs:

ii)The integration characteristics provide the average value of

the input signal during the period of first integration.

Consequently, disturbance, such as spurious noise pulses,

areminimized.

iii) Long-term drifts in the time constant, as may result from

temperature variations or aging, do not affect conversion

accuracy. The long-term alterations in clock frequency have

no effect.

iv) Higher accuracy and resolution.

v) Greater speed.

vi) No parallax.

vii)Reducedhuman error.

viii)Compatibility with other digital equipment for further

processing and recording.

Q. 5: Discuss digital frequency meters? (CSVTU April-May

2011, April-May 2010, Nov-Dec 2008)



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In the "A.C. voltage mode", the applied input is fed

through a calibrated, compensated attenuator, to a precision full-

wave rectifier circuit followed by a ripple reduction filter. The

resulting D.C. is fed to ADC and the subsequent display system.

For current measurements, the drop across an internal

calibrated shunt is measured, directly by the ADC in the "D.C.

current mode", and after A.C. to D.C. conversion in the "A.C.

current mode". This drop is often in the range of 200 mV

(corresponding to full scale).

Due to the lack of precision in the A.C.-D.C. conversions,

the accuracy in the A.C. range is in general of the order of 0.2 to

0.5%. In addition, the measurement range is often limited to

about 50 Hz at the lower frequency end due to the ripple in therectified signal becoming a non-negligible percentage of the

display and hence results in fluctuation of the displayed number.

At the higher frequency end, deterioration of the performance of

the AC/DC converter limits the accuracy.

The A.C. measurement, isoften average reading, r.m.s.

calibrated.



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And we can read the rpm of the rotating shaft directly. So,

the relation between the gate period and the number of pulses

produced by the pickup isG= 60/P.If we fix the gate period as

one second (G = 1 s), then the revolution pickup must be capable

of producing 60 pulses per revolution.

Figure shows a schematic diagram of a digital tachometer.

Q. 8: - Discuss digital pH meter? (CSVTU April-May 2010)

Ans: -

DIGITAL PH METER:

The measurement of hydrogen ion activity (pH) in a solution can

be accomplished with the help of a pH meter. For those unfamiliarwith the terminology, a very brief review is included.

pH is a quantative measure of acidity. If the pH is less

than 7, the solution is acidic (the lower the pH, the greater the

acidity). A neutral solution has a pH of 7 and alkaline (basic)

solutions have a pH greater than 7.

The pH unit is defined as

pH = - log (concentration of H+

)where H+is the hydrogen or hydronium ion.



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Now as the inputPxwhich has a phase difference with

respect to P0crosses zero (0) in the positive half cycle, the zero

detector is activated, causing its output to go high(1). This high

input in turn toggles the J-K flipflops-2, making its output goes

high. This output (Q) of flipflops-2 is connected to the clear input

of flipflops-l forcing the flipflops-l to reset. Hence the output offlipflops-l goes to zero (0). The AND gate is thus disabled, and the

counter stops counting.

The number of pulses counted while enabling and

disabling the AND gate is in direct proportion to the phase

difference, hence the display unit gives a direct readout of the

phase difference between the two inputs having the same

frequency. If the input signal frequency is f, then the clock

frequency must be 360times the input frequency for accurate

measurements.

Q. 10: - What do you mean by digital capacitance meter?

Ans:

DIGITAL CAPACITANCE METER:

Since the capacitance is linearly proportional to the time constant,

when a capacitor is charged by a constant current source and

discharged through a fixed resistance, we can use a 555 timer

along with some digital test equipment to measure capacitances.

One obvious way is to measure the time period of the

oscillations. By choosing the right size of charging resistance, we

can get a reading directly in microfarads ornanofarads.Unlike

many capacitance measuring schemes, this one easily handles

electrolytics up to the tens of thousands of microfarads.

A better way is to measure only the capacitor discharge

time, as shown in Figure below



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With this method, any leakage in the capacitor under test

will make the capacitor appear smaller in value than it actually is,

and is an effective indicator of how the test capacitor will behave

in most timing and bypass circuits.

In this circuit, the 555 timer is used as an astable

multivibrator. At the peak of the charging curve, a digital counter

is reset and a clock of 100 kHz pulses is turned on and routed tothe counter. When the discharge portion of the cycle is completed,

the display is updated and the value of the capacitor is readout.

By selecting the proper reference frequency and charging currents,

one can obtain a direct digital display of the value of the

capacitance.

Be sure to properly shield the leads and keep them short

for low capacity measurements, since the 50 Hz hum can causesome slight instability.

Q. 11: -Explain the advantages of electronic voltmeters over

conventional type voltmeters are regards(CSVTU Nov-Dec

2010)

(i) detection of low level signals

(ii) power consumption



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(iii) loading effects

(iv) frequency range.

Ans:

Inall electronics voltmeter circuits the principle involved is

that an indicator on a permanent magnet moving coil

instrument portional to the input voltage is obtained by means

of amplification in one or more stages with a high input

impedance.

Being costlier than electrical instrument but it has many

advantages over conventional ones, as discussed below:

1.High Sensitivity

2.Low Power Consumption

3.High Accuracy

4.High Frequency range

5.Low level signal detection

6.Less loading effect

7.High input impedance

Let us explain the advantage of electronic voltmeter with

example:

Analog instruments use PMMC movement for indication.

This movement cannot be constructed with a full scale sensitivity

of less than 50 A. Any measurement using a P'MMC movement

must draw a current of 50 A from the measured quantity for its

operation for full scale deflection if conventional voltmeters are

used. This would produce great loading effects especially in

electronic and communication circuits. Electronic voltmeters avoid

the loading errorsby supplying power required for measurement

by using external circuits like amplifiers. The amplifiers not only

supply power for the operation but make it possible for low level

signals, which produce a current less than 50 A for full scaledeflection, to be detected which otherwise cannot be detected in

the absence of amplifiers.



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iv) Hysteresis Error:

The maximum separation due to hysteresis between

upscale-going and downscale-going indications of a measured

variable. The delay between the action and reaction of a

measuring instrument. Hysteresis is the amount of error thatresults when this action occurs.

The maximum differences in outputs at any measured value

within the specific range when approaching the point first with

increasing and then with decreasing input may be termed as

Hysteresis.

v)Dynamic Error:

The Dynamic Error also called measurement error is the

difference between the true value of the quantity changing with

Time and the value indicated by the measurement system if no

static error is assumed.

vi) Cross Sensitivity:

It is a factor which is to be taken into account when we

measure mechanical quantity. It is a situation where the actual



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quantity is being measured is in one plane and another quantity

which is subjected to variations is on another plane.

Q: 13: - A 3 digit DVM has an accuracy specification of

0.05% of the reading 1 digit.

(i) What is the error in volt when reading is 2 V on its 10

V range?

(ii) What is the error in volt when reading is 5 V on its 10

V range?

(iii)What is the % error of reading, when the reading is

0.1 V on its 10 V range? 7(CSVTU Nov-Dec 2011, April-May 2011)

Ans:

1.Number of full digits on display, n=3

So, resolution, R=1

10n=

1

103 = 0.001

i.e. the meter cannot distinguish between values that differs

from each other by less than 0.001 of full scale.

For full scale range of 1 V, the resolution isfs R i.e.

1x0.001=0.001V

For full scale range of 10 V, the resolution is 10 x 0.001 = 0.01 V

2.The display for 2 V reading on 10 V scale of31

2 digital

meter would be 02.00

The digit in the least significant digit has a value offs R i.e.

5 x1

103 = 0.005 V

Total possible error =0.5

100 x reading + value of digit in LSD



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2)There are 6 digit faces in 5 display, so 13.46 would be

displayed as 13.4600 V.

3)Resolution on 1V range is 0.00001 * 1=0.00001

Any reading up to the 5thdecimal can be displayed.Hence 0.54867 will be displayed as 0.54867 V.

4)Resolution on 10V range is 0.00001 * 10=0.0001

Any reading up to the 4thdecimal can be displayed.

Hence 0.54867 will be displayed as 0.5486 V.

Q. 17: - What do you mean by analog and digital systems.

Write some advantages and limitations of digital over

analog system?

(CSVTU Nov-Dec 2011, Nov-Dec2009, April-May 2008)

Ans:

Analog System:

An analog system comprises devices that manipulate the

physical quantities represented in analog form. Example:

automobile speedometer.

Digital System:

A digital system is a combination of devices for manipulating

physical quantities or information represented in digital form.

Example: digital computer.

Advantages of digital system over analog system:

1)Easier to design.

2)Easy storage of information

3)Greater accuracy and precision.

4)Operation is programmable.



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5)Digital circuits are less affected by the noise.

Limitation of digital technique:

1)Cost is more than analog

2)Isolation problems are more.

3)Construction is complicated.

Examination, April May, 2012

Q.1. (a)Define the terms sensitivity& accuracy. 2

Ans: Refer Answer 1-(b) and(c).

(b)Compare the characteristic of ramp & dual slope type DVM.

7

Ans: Refer Answer 4.

(c)In the wheatstone bridge shown in figure, the values of

resistance of various arms are given. The galvanometer has a

current sensitivity of 10 mm/A & an internal resistance of100. Calculate the deflection of galvanometer & sensitivity of

bridge in terms of deflection per unit change in resistance. 7

Ans:

Resistance of unknown resistance in balance condition

R=(P/Q)*S

R=(1000/100)*200=2000

R=2005-2000=5

Thevenins source generator emf



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(b)Explain the operational principle of a dual slope integrating

type DVM with help of diagrams. If the DVM has maximum

range of 255 V, determine the time it will take to read the

unknown voltageVX=180V , using a clock oscillator of 10 kHz.

Ans: -

For dual slope integrating type DVM refer answer.

Remaining solution:

Maximum range of DVM Vmax= 255 V

Unknown voltage Vx= 180 V

Frequency of clock oscillator = 10 kHz

Time taken to read unknown voltage

T= {(Vmax- Vx)/2f Vmax= 4.68 sec

(c)What is the resolution of a31

2 digit display on 1V & 10V

ranges? A31

2 digital voltmeter has an accuracy specification

of 0.5 of reading 1 digit. What is the possible error in

volts, when instrument is reading 2.00 V on 5 V scale.

Ans: Refer answer 13.

(d)Explain the merits of digital systems over analog systems.

Also, discuss the limitations of digital techniques.

Ans:Refer answer 17.

Examination, April May, 2011Q.1.(a) What is resolution of 3 digit? 2Ans: Refer answer 13.

(b)Describe any one method of integrating type DVM with theiradvantage & disadvantage. 7


(c)Explain digital frequency meter. 7Ans: Refer answer 5.

(d)A 3 digit DVM has an accuracy specification of 0.05% of thereading1 digit.



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(i) What is the error in volt when reading is 5 V on its 10 V range?(ii) What is the % error of reading, when the reading is 0.1 V on its10 V range. 7


Examination, Nov. Dec., 2010

Q.I.What are the advantages of dual slope over ramp type DVM.2


Q. II.Explain the advantages of electronic voltmeters over

conventional type voltmeters are regards 7

(i) detection of low level signals

(ii) power consumption

(iii) loading effects

(iv) frequency range.

Ans:Refer answer 3.

Q. III.Explain the operating principle of a ramp type DVM. 7


Q. IV.A 4 digit voltmeter is used for voltage measurements.7

i) Find its resolution

ii) How would 12.98 V be displayed on a 10 V range.

iii)How would 0.6973 be displayed on 1 V and 10 V ranges.



Q.1.(a)Define resolution. Give the resolution of a 41

2 digit

display.2

Ans: Refer answer 1 and 14.

(b)Discuss dual slope integrating type digital voltmeters. 7


(c)Explain digital frequency meter. 7



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(d)Describe the digital phase meter. 7



Q.1. (a)A Wheatstone bridge requires a change of 7in the

unknown arm of the bridge to produce a change in deflection of

3 mm of the galvanometer. Determine the sensitivity. Also

determine the inverse sensitivity or scale factor. 2


(b)Describe the working principle and block diagram of Dual slopeintegrating type DVM. 7

Ans:Refer answer 4.

(c)State the advantages of a DVM (digital voltmeter) over an analog

meter. Also explain the basic principle of a digital voltmeter.7

Ans:Refer answer 3 and 17.

(d)A 41

2 digit voltmeter is used for voltage measurements: 7

(i) Find its resolution.

(ii) How would 0.6973 be displayed on 1 V and 10 V rages.

(iii)A moving coil voltmeter has uniform scale with 100 divisions.

The full scale reading is 200 V and 1/10thof scale division can be

estimated by the instrument. Determine resolution of instrument

in volts.



Q.1.(a)A 4 digit voltmeter is used for voltmeter measurement,

find its resolution. 2




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Q. I.Define Accuracy, Average value, Root mean square value &

Form factor. 2

Ans:Refer answer 1.

Q. II.What are the advantages of Digital instruments over Analog

instruments? What are the different types of Digital Voltmeters,

compare their performance. 7

Ans:Refer answer 17 and 3.

Q. III.Describe the arrangement of Digital Voltmeter based onDual Slope integration method. What are the advantages of

integrating type DVM? 7

Ans:Refer answer 4.

Q. IV. (a)Define Resolution and Sensitivity as related to digital

instruments. 2+5

(b)The lowest range on a 4 digit DVM is 10 mV full scale.

Determine:

(i) Sensitivity and resolution of meter

(ii) How would 0.6789 be displayed on 10 V and 100 V ranges.

Ans:Refer answer 1 for (a) and 14 for (b).


Q.1 (a)Define sensitivity and resolution of a digital meter.2

Ans:Refer answer 1.

(b)Describe the digital meters and its working in detail. 7

Ans:Refer answer 3.

(c)Describe the working principle and block diagram of a dual

slope integrating type digital voltmeter. 7



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Ans:Refer answer 4.

(d)A 5 digit voltmeter is used for voltage measurement:

(i) Find its resolution.

(ii) How would 13.46 V be displayed on a 10 V range?

(iii) How would 0.54867 be displayed on 1 V and 10 V ranges.7Ans:Refer answer 16.

Unit-II Transducers

Q. 1: - What do you mean by transducer?

(CSVTU April-May 2008, Nov-Dec 2007)

Ans:

A transducer is a device that converts oneform of energyto

another. Energy types include (but are not limited to)electrical,

mechanical,electromagnetic(includinglight),chemical,acoustic

orthermalenergy. Transducers are widely used in measuring

instruments.

Q. 2: -Write selection criteria of transducers?Or

Write down the factors which effect the choice of

transducers?

Ans:

The following is the summary of the factors influencing

the choice of a transducer for measurement of a physical quantity.

1.Operating Principle:The transducers are many a timesselected on the basis of operating principle used by them.

http://en.wikipedia.org/wiki/Form_of_energyhttp://en.wikipedia.org/wiki/Electricityhttp://en.wikipedia.org/wiki/Mechanicshttp://en.wikipedia.org/wiki/Electromagnetismhttp://en.wikipedia.org/wiki/Lighthttp://en.wikipedia.org/wiki/Chemistry#Energyhttp://en.wikipedia.org/wiki/Acousticshttp://en.wikipedia.org/wiki/Heathttp://en.wikipedia.org/wiki/Form_of_energyhttp://en.wikipedia.org/wiki/Electricityhttp://en.wikipedia.org/wiki/Mechanicshttp://en.wikipedia.org/wiki/Electromagnetismhttp://en.wikipedia.org/wiki/Lighthttp://en.wikipedia.org/wiki/Chemistry#Energyhttp://en.wikipedia.org/wiki/Acousticshttp://en.wikipedia.org/wiki/Heat


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11. Usage and Ruggedness:The ruggedness both of

mechanical and electrical intensities of transducer versus its

size and weight must be considered while selecting a suitable

transducer.

12. Electrical aspects:The electrical aspects that need

consideration while selecting a transducer include the length

and type of cable required.

13. Stability :The transducer should exhibit a high degree

of stability to be operative during its operation and storage

life.

14. Reliability:Reliability should be assured in case of

failure of transducer in order that the functioning of the

instrumentation system continues uninterrupted.

Q. 3: - Discuss transducers characteristics.

Ans:

The important characteristics of transducers are

1.Input characteristics

i) Type of Input and Operating Range

ii) Loading Effects

2.Transfer characteristics

(i) transfer function

(ii) error, and

(iii) response of transducer to environmental influences.

3.Output characteristics

i) type of electrical output,

ii) output impedance,

iii) useful range.

Input Characteristics

a)Type of Input and Operating Range:

The foremost consideration for the choice of a

transducer is the input quantity it is going to measure and its



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operating range. The type of input, which can be any physical

quantity, is generally determined in advance. However, the choice

of a particular transducer that depends upon the useful range of

input quantity ever which the transducer can be used. The useful

operating range of the transducer may be a decisive factor in

selection of a transducer for a particular application. The upper

limit is decided by the transducer capabilities while the lower limit

of range is normally determined by the transducer error or by the

unavoidable noise originating in the transducer.

b)Loading Effects:

Ideally a transducer should have no loading effect on

the input quantity being measured. In theory, it is impossible,

although in practice steps may be taken to reduce the loading

effects to negligible proportions.

Transfer Characteristics

The transfer characteristics of transducers require

attention of three separate elements,

Transfer Function:

Thetransfer functionof a transducer defines a

relationship between the input quantity and the output. The

transfer function is,

q0=f(qi)

Where,q0andqiare respectively output and input of the

transducer.

Thesensitivityof a transducer is defined as the differential

quotient,

S=dq0

dq1

In general, the sensitivity of transducers is not constant but is

dependent upon the upon quantityq1.



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The maximum separation due to hysteresis between

upscale-going and downscale-going indications of a measured

variable. The delay between the action and reaction of a

measuring instrument. Hysteresis is the amount of error that

results when this action occurs.

The maximum differences in outputs at any measured value

within the specific range when approaching the point first with

increasing and then with decreasing input may be termed as

Hysteresis.

(2) Dynamic errors are those which produce when input changewith respect to time.

(3,4) Environmental Response(due to noise and drift and due to

change in frequency):

The response of the transducer to environmental

influences is of a great importance. This is often given insufficient

attention when choosing the best transducer for a particularmeasurement. This gives rise to results that are not as accurate as

expected, or, worse, results that are accepted as more accurate

than they actually are. The performance of the transducer is fully

defined by its transfer function and errors, provided that the

transducer is in constant environments and not subject to any

disturbances like stray electromagnetic and electrostatic fields,

mechanical shocks and vibrations temperature changes, pressureand humidity changes, changes in supply voltage and improper

mechanical mountings. If transducers are subjected to the above



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environmental disturbances, which they are, precautions are

taken, so that changes in transfer function and resulting errors

there from do not occur.

Output Characteristics

The three conditions in the output characteristics which should

be considered are,

(i) type of electrical output,

(ii) output impedance,

(iii)useful range.

Type of Electrical Output:

The types of output which may be available from the

transducers may be a voltage, current, impedance or a time

function of these amplitudes. These output quantities may or may

not be acceptable to the latter stages of the instrumentation

system. They may have to be manipulatedi.e.their magnitudes

changed or they may have to be changed in their format by signalconditioning equipment so as to make them drive the subsequent

stages of instrumentation system.

Output Impedance:

The output impedance, Z0 of a transducer determines to

the extent the subsequent stages of instrumentation is loaded.

Ideally, the value of output impedance should be zero if no loadingeffects are there on the subsequent stage. However, the output

impedance, Zn, cannot made equal to zero and therefore, its value

should be kept as low as possible to minimize the loading effects.

The output impedance determines the amount of power

that can be transferred to the succeeding stages of the

instrumentation system for a given output signal level. If theoutput impedance is low compared to the forward impedance of

the system, the transducer has the characteristics of aconstant



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voltage source(provided the output of the transducer is a voltage),

while in case the forward impedance is high as compared with the

output impedance of transducer, it behaves asconstant current

source.

When the output impedance of the transducer is equalto that of the following stages of instrumentation system,matching

takes place and maximum power is transferred from the

transducer to the succeeding stages. However, it must be

understood that in case maximum power transfer takes place,

when the output resistance of transducer, is equal to the

resistance of the succeeding stages, the efficiency is only 50%.

Useful Output Range:

The output range of a transducer is limited at the lower

end by noise signals which may shroud the desired input signal.

The upper limit is set by the maximum useful input level. The

output range can be increased, in some cases, by the inclusion of

amplifier in the transducer. However, the inclusion of an amplifier

also increases the noise level and therefore in such situations the

amplifier may not be of any use at all.

Q. 4: - Classify transducers.

(CSVTU April-May 2011, Nov-Dec 2009, April-May 2009, April-

May 2008)

Ans:

The transducers can be classified as follows:

1.On the basis of transduction form used,

2.Primary and secondary transducers,

3.Passive and active transducers,

4.Analog and digital transducers,

5.Transducers and inverse transducers.

On the basis of Transduction:-

The transducers can be classified on the basis of

principle of transduction as:



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I. resistive

II. inductive

III. capacitive

It depends upon how they convert the input quantity into

resistance, inductance or capacitance respectively. They can beclassified as piezoelectric, thermoelectric, magneto restrictive,

electro kinetic and optical,

Primary and secondary transducers:-

When the input signal is directly sensed by the

transducer and physical phenomenon is converted into the

electrical form directly then such a transducer is called the

primary transducer. For example a thermistor used for the

measurement of temperature fall in this category. The thermistor

senses the temperature directly and causes the change in

resistance with the change in temperature.

When the input signal is sensed first by some detector or

sensor and then its output being of some form other than input

signal is given as input to a transducer for conversion into

electrical form, then such a transducer falls in the category of

secondary transducers. For example, in case of pressure

measurement, bourdon tube is a primary sensor which converts

pressure first into displacement, and then the displacement is

converted into an output voltage by an LVDT. In this case LVDT is

secondary transducer.

Passive and active transducers:-

Active transducers:

They are also known as self-generating type

transducers. The transducers develop their own voltage or

current. The energy required for production an output signal is

obtained from the physical phenomenon being measured.

Examples: Thermocouples and thermopiles, piezoelectric pick-up,

photovoltaic cell.



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Ans:

The resistance of a metal conductor is expressed by a

simple equation that involves a few physical quantities. The

relationship is given by

R =

L

where, R =Resistance, = Resistivity of conductor materials, Q-m,

L = Length of conductor, m, and

A =Cross-sectional area of the conductor, m2.

Any method of varying one of the quantities involvedin the above relationship can be the designed basis of anelectrical

resistance transducer.There are a number of ways in which

resistance can be changed by a physical phenomenon.

1.Thetranslational and rotational "potentiometers"which work on

the basis of change in the value of resistance with change in

length of the conductor can be used for measurement of

translational or rotary displacements.2."Strain gauges" work on the principle that the resistance of a

conductor or a semiconductor changes when strained.This

property can be used for measurement of displacement, force

and pressure.

3.The resistivity of materials changes with the change of

temperature thus causing a change of resistance. This property

may be used for measurement of"temperature".

4.In aresistance transduceran indication of measured physical

quantity is given-bya change in the resistance.It may be

classified(as discussed above) as follows :

Mechanically varied resistance Potentiometer

Thermal resistance change Resistance thermometers

Resistivity changeResistance strain gauge



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Q. 6: What are potentiometers, explain their working?

(CSVTU April-May 2010)

Ans:

Potentiometersconvert the linear motion or the angular motion

of a rotating shaft into changes in resistance.The device is avariable resistor whose resistance is varied by the movement of a

slider over a resistance element.

1.Translatory devices have strokes from 2.5 mm to 5 mm.

2.Rotational devices have full scale from 10 to 60 full turn.

3.Helipot Potentiometer is there which is combination of both

translatory type and rotational type.

The potentiometer shown in Figs. 1 is Linear motion

potentiometer and Fig 2 Rotary motion potentiometer is form a

part of the bridge circuit whose output voltage is changed by the

slider position is shown in fig 3.



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1.The slider is powered by the mechanical part on which the

linear displacement or angular measurement are to be made.

2.Due to arm movement, the slider moves over the resistance

element and thus shorts out a portion of the resistance.The

change in resistance in the, potentiometer is then an indication of

the amount of motion and the direction of movement is indicated

by whether the resistance is increasing or decreasing.The

unbalanced voltage is measured directly or fed into an amplifier

and recorded.

The output voltage under ideal condition is given by

e0= (resistance at output terminal/resistance at input terminal)x

input voltage

RP(! i

! t)

RP ei

(!i

!t)e i

SensitivityS

"utput

input=

e"

! i=

ei

!t

Q. 7: - How can we measure the temperature in industries?




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Ans:

Temperature is one of the most widely measured and controlled

variable in industry, as a lot of products during manufacturing

requires controlled temperature at various stages of processing.

A wide variety of temperature transducers and

temperature measurement systems have been developed for

different applications requirements. Most of the temperature

transducers are of Resistance Temperature Detectors (RTD),

Thermistors and Thermocouples. Of these RTD's and Thermistors

are passive devices whose resistance changes with temperature

hence need an electrical supply to give a voltage output. On the

other hand thermocouples are active transducers and are based

on the principle of generation of thermoelectricity, when two

dissimilar metals are connected together to form a junction called

the sensing junction, an emf is generated proportional to the

tempera

ture of the junction.

Q. 8: -Discuss the following:i) Resistance Temperature Detectors

ii) Thermistors

(CSVTU Nov-Dec 2010, April-May 2009)

Ans:

i) RTD:

The resistance of a conductor changes when its

temperature is changed. This property is utilized for measurement

of temperature.

The variation of resistanceR With temperatureT (K)

can be represented by the following relationship for most of the

metals as :

R = R0(1+1T+2T2++ nT

n+ ...)

Where, R0= resistance at temperatureT= 0 and 1, 2, n, are

constants.



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coefficient of temperature resistancei.e.their resistance decreases

with increase of temperature.

The negative temperature coefficient of resistance can

be as large as several percent per degree Celsius. This allows the

thermistors circuits to detect very small changes in temperature

which could not be observed with an RTD or a thermocouple.

Thermistors are widely used in applications which

involve measurements in the range of - 60C to 15C. The

resistance of thermistors ranges from 0.5 ohm to 0.75 M-ohm.

Thermistor is a highly sensitive device. The price to be paid off for

the high sensitivity is in terms of linearity. The thermistor exhibits

a highly non linear characteristic of resistance versus

temperature.

Resistance-Temperature Characteristics of Thermistors:

The mathematical expression for the relationship

between the resistance of a thermistor and absolute temperature

of thermistor is :

RT2= RT1exp [ (1

#2 -1

#1 )]

where

RT1=resistance of the thermistor at absolute temperature T1at K,

RT2= resistance of the thermistor at absolute temperatureT2at K

= a constant depending Upon the material of thermistor,

typically 3500 to 4500 K.

The resistance temperature characteristics of a typical thermistorare given in Fig. The resistance temperature characteristics of Fig.

show that a thermistor has a very high negative temperature co-

efficient of resistance, making it an ideal temperature transducer.



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The characteristics of thermistors are no doubt non-

linear but a linear approximation of the resistance-temperature

curve can be obtained over a small range of temperatures. Thus,

for a limited range of temperature, the resistance of a thermistor

varies as given by Equation:

R= R0(1+0).

Q. 9: - Describe strain gauge in detail. Find the expression

for gauge factor?

(CSVTU Nov-Dec 2011, April-May 2010, Nov-Dec 2009)

Ans:

The strain gauge is basically a device used for measuring

mechanical surface strain and is one of the most extensively used

electrical transducers. Its popularity stems from the fact that it

can detect and convert force or small mechanical displacements

into electrical signals. Many other quantities such as torque,

pressure, weight, and tension etc., which involve effects of force or

displacement, can also be measured by strain gauges.

Furthermore, if the mechanical displacements under

measurement have a time-varying form, such as vibration motion,

signals with frequencies of up to 100 kHz can be detected or

measured. The applications of strain gauges may be broadly

classified into two areas



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(i)Applications where the gauge measure strain as the primary

objective of measurement as in the case of stress analysis of

machines and structures.

(ii)Applications where measurement of strain is utilized in

transducers as a measure of another parameter such as load,

pressure, acceleration, or another force associated variable.

The working of strain gauge is based on the fact that when

stress is applied on the metal conductor its resistance changes

owing to change in length and X-sectional area of the conductor

(Fig.). Resistance of conductor under stress is also changed due to

change in resistivity of the conductor, this property called the

piezo-resistive effect. That is why strain gauges are also called thepiezo-resistive strain gauges.

If a conductor of length L, area of cross-section A is subjected to

axial tension, the resistance will change because of change in

length, area and resistivity of the material.

Resistance of an unstrained conductor is given by an expression

R= L /

Also area of the Wire A= (/4)D2=KD2

Then R= L /$ % &2

--------------------------------------------1

Let under strained conditions resistance of conductor be changed

by R because of change in length by L, cross-sectional area by

A and resistivity by p. These quantities can be related with each



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other by differentiating Resistance expression with respect to

stress i.e.,

dR=$ &

2( % dL+L% d)L(2$& % d&)

($ % &2)2 ----------------------------2

dR=1

$ % &2 ( % dL+L % d2L(d& /&) )

dR

R =dL

L +d

- 2d &

& --------------------------------------3

Also we know that possions ratio is defined as ratio of lateralstrain to longitudinal strain.

i.e. =-

d &

&

dL

L

-----------------------------------------4

Putting value of equation 4 in equation 3 we get

dR

R =dL

L +d

+ 2'

dL

L

dR

R =d

+ (1+2'

dL

L ----------------------------------------5

Dividing both side bydL

L we get

dR

R/

dL

L ) = 1+ 2 + (

d

/dL

L ) ------------------------------6

But we know that Gauge factor is defined as the ratio of per unit

change in resistance to the per unit change in length.

i.e. Gf=

dR

R/ dLL ) ----------------------------------------7



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Stress=

=1000 -g/*m2

Modulus of Elasticity = Y = 2106

-g /*m2

To Calculate:

1)% change in resistance

2)Poissons ratio

Solution:

Y =

stress

str(in=

1000

L

L

L

L =

1000

. =

1000

2106=5104

+f=2=

R

R

L

L

=

R

R

5104

RR

=10104=( R

R)=10106

(Ans)

+f=2=1+2/

/=0.5 (Ans)

Q. 12: - A strain gauge is bounded to a beam of 0.1 m long

and has a cross-sectional area 4 cm2. Youngs modulus for

steel is 207 GN/m2. The strain gauge has an unstrained

resistance of 240 and a gauge factor of 22. When a load

is applied, the resistance of gauge changes by 0.013 .

Calculate the change in length of the steel beam and the

amount of force applied to the beam.

(CSVTU Nov-Dec 2010)

Ans:



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The supply to bridge is of 6 volt and G= 2. The applied

stress is 300 MN/m2and Youngs Modulus of elasticity of

cantilever material is 60 gN/m2. Determine

(a)Strain

(b)Change in resistance

(c)The output voltage 7


Ans:

Given:

Resistance of strain gauges. R = 150

Fixed resistances. R = 150

Applied stress. P = 300 x 106N/m2

Modulus of elasticity, E = 60 x 109

N/m2

Calculation:

Strain

Change in resistance

The output voltage

Solution:



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One of the gauges, sayR1, is subjected to tensile stress and the

other to compressive stress soR1 increases while R3decreases.

1.Strain = P/) = (300 x 106N/m2)/(60 x 109N/m2) = 5xl0-

3

2.Change in the resistance of strain gauges,

R = G x x R = 2 x 5xl0 -3x 150 = 1.5

3.Output voltage when the detector connected across output

terminals is of infinite resistance

Vout=V

2G x =

6

2 x2 x 5xl0-3= 30 mV

4.Output voltage when the detector connected across output

terminals is of 600

VOL=[1 /((/Rm)+1)] Vout=

[1 /((150 /600)+1)] 30 =

24 mV

5.Detector current. I = VOL / Rm=24mV/600 =40 n A

6.Current through strain gauges = V/2R = 6/(2x150)= 20 mA

7.Change in resistance R2for restoration of balance

R2 =R4(R1+ R 1)

R3 R3 =150 (150+1.5)

1501.5 = 3.03

Q. 14: - Explain any inductive transducer?

(CSVTU April-May 2012, Nov-Dec 2011, April-May 2011, Nov-

Dec 2008, April-May 2008, Nov-Dec 2007)

Ans:

Linear-variable-differential transformer (LVDT)

LVDT is a passive inductive transducer and is commonly employed

to measure force (or weight, pressure and acceleration etc. which

depend on force) in terms of the amount and direction of

displacement of an object.



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3.Similarly, when the movable core moves towards coil S1, E1> E2

andV0= E1-E2and is inphase with E1.

4.Thus, from above discussion, we find that the magnitude ofV0

is afunction of the distance moved by the coreand itspolarity or

phaseindicates as to in which direction it has moved.5.If core is attached to moving object, themagnitudeofV0gives the

position of that object.

Advantages:



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1.It gives a high output and therefore many a times there is no

need for intermediate amplification devices.

2.The transducer possesses a high sensitivity as high as 40

V/mm.

3.It shows a low hysteresis and hence repeatability is excellent

under all conditions.

4.Most of the LVDTs consume a power of less than 1 W.

5.Less friction and less noise (due to absence of sliding contacts).

6.These transducers can usually tolerate a high degree of shock

and vibration without any adverse effects.

7.It can operate over a temperature range from -265C to 600C.

8.It is available in radiation-resistant design for operation in

nuclear reactors.

Disadvantages:

1.These transducers are sensitive to stray magnetic fields but

shielding is possible. This is done by providing magnetic shields

with longitudinal slots.

2.Relatively large displacements are required for appreciabledifferential output.

3.The receiving instrument must be selected to operate on A.C.

signals or demodulator network must be used if a D.C. output

is required.

4.Several times, the transducer performance is affected by

vibrations.

5.The dynamic response is limited mechanically by the mass ofcore and electrically by the frequency of applied voltage. The

frequency of the carrier should be at least ten times the highest

frequency component to be measured.

Applications:

1.Measurement of material thickness in hot strip or slab steel

mills.2.In accelerometers.



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Capacitance C =1

d =1r 10

d

10 =Free space permittivity

1r

=Relative Permittivity

1 =Permittivity of medium

A=Overlapping Area of Plate

d = Distance between two plates

The displacement is measured by measuring the change in

capacitance brought about by:

1.Change in area

2.Change in distance between the plates

Differential capacitor System:

The Differential capacitor System is used for the measurement of

the linear displacement.

In this system two capacitance C1 and C2 are taken and an

alternating voltage E is applied to both the capacitor.

The two capacitor are identical to each other and placed one over

the other as shown in figure1,at this time

C=C1 =C2 =1

d



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Advantages:

1.Require extremely small force for operation (hence very useful

for use in small systems).

2.Extremely sensitive.3.Require small power for operation,

4.High input impedance; therefore, loading effects are minimum.

5.Frequency response is good.

6.A resolution of the order of 2.5 x 10-3mm can be obtained.

7.Can be used for applications where stray magnetic fields render

the inductive transducers useless.

Disadvantages.

1.The metallic parts must be insulated from each other. The

frames must be earthed to reduce the effects of stray

capacitances.

2.They show non-linear behaviour several times on account of

edge effects;guard ringsmust be used to eliminate this effect.

3.The cable connecting the transducer to the measuring point is

also a source of error. The cable may be source of loading

resulting in loss of sensitivity. Also loading makes the low

frequency response poor.

Uses of the capacitive transducers.

1.To measureboth linear and angular displacements.

2.Tomeasure force and pressure.

3.Used as pressure transducers in all those cases where thedielectric constant of a medium changes with pressures.

4.To measure humidity in gases.

5.Used in conjunction with mechanical modifiers for

measurement ofvolume, density, weight, input leveletc.

Q. 17: - A capacitive transducer uses two quartz diaphragms

of area 600 mm2separated by a distance of 2.5 mm. A

pressure of 8 105N/m

2. When applied to the top diaphragm

causes a deflection of 0.5 mm. The capacitance is 400 10-



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12F when no pressure is applied to diaphragms. Determine

the value of capacitance after the application of pressure.


Ans:

Suppose0

1 and0

2 are respectively the values of capacitance

before and after application of pressure. Letd

1 andd2be the

values of distance between the diaphgrams for corresponding

pressure conditions.

01=

1

d1 and0

2=

1

d2

01

02=

d1

d2

If,d

1=2.5

thend

2=2.50.5=2.0mm

Value of capacitance after application of pressure

02=

4001012

2.5

2=5001012

Q. 18: - A capacitive transducer circuit used for

measurement of linear displacement. Suppose a flat

frequency response with an amplitude ratio within 5% is

required down to a frequency range of 20 Hz. What is the

minimum allowable value of time constant?

Calculate phase shift at this frequency. Area of plates is

300mm2& the distance between plates is 0.125 mm.

calculate the value of series resistance R. what is the

amplitude ratio at 5 Hz with the above time constant?


Ans:

For a flat response with 5%, the amplitude ratio is:



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M=1-0.05 = 0.95

M = 0.95 = 1/{1+(1/)2}1/2

So, = 24.2 * 10-3sec = 24.2 msec

Phase shift = (/2)-tan-1 = 18.20

Capacitance (C) = A/d = 21.24* 10-12

Therefore series resistance R = /C = 1140 M

Amplitude ratio at 5 Hz = 0.605

Q. 19: - A capacitance transducer of two parallel plates of

overlapping area of

5104

m2

is immersed in water. Thecapacitance C has been found to be 9.5 pF. Calculate the

separation d between the plates & the sensitivity, S. Given1r for water = 81;

10=8.854p/m %

(CSVTU Nov-Dec 2011)

Ans:

C = 0rA/d

On substituting the values we get the value of displacement as

d=37.75 mm

Sensitivity = dC/dd = - 0rA/d2= 0.025 * 10-8F/m

Q. 20: Discuss piezoelectric transducer and derive the

expression for output voltage? (CSVTU Nov-Dec 2009)

Ans:A piezoelectric material is one in which an electric potential

appears across certain surfaces of a crystal if the dimensions of

the crystal are changed by the application of a mechanical force.

This potential is produced by the displacement of charges. The

effect is reversible i.e. conversely, if a varying potential is applied to

the proper axis of the crystal, it will change the dimension of the

crystal thereby deforming it. This effect is known as piezoelectriceffect. Element exhibiting piezoelectric qualities are called as

electro resistive elements.



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The piezoelectric effect can be made to respond tomechanical deformations of the material in many different modes.

The modes can be thickness expansion, thickness shear and face

shear.

The piezoelectric effect is direction sensitive. The tensile

force produces a voltage of one polarity while a compressive force

produces a voltage of opposite polarity. The magnitude and

polarity of the induced surface charges are proportional to the

magnitude and direction of the applied force F.

A piezoelectric element generates a charge and this charge

appears as a voltage across the electrodes. i.e.

E=Q/C

We know that charge Q= d*F Coulamb (i)

Where, d= charge sensitivity of the crystal (C/N)

And F=force applied in N.

F= AE t/t

Where A= cross sectional area and E= Youngs modulus

Also, E=stress/strain = (F/A)/( t/t)

So, from equation (i) Q = dAE t/t



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The voltage output of the transducer under no load conditions, is

thereforeE0.Under conditions of load: -

Impedance of load ZL=RL

1+(23 0LRL )

Total impedance of circuit Zt=1

230p +RL

1+23 RL 0L

Voltage across the load ELwill be

EL=4L

4t )

0

EL=23 0pRL

1+23 (0L+0p)RL )

0

The Magnitude of voltage across the load is:

EL=

p+0L

02RL

2

1+32

3 0pRL

We know that E0=

d

0p

Then EL=

p+0L0

2RL2

1+32

3 0pRL



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EL=

p+0L0

2RL2

1+32

3 RL

At medium and High Frequencyp+0L

0

32

EL=0p

(0L+0p) )

0

Q. 22: - A Barium titanate pickup has the dimension of 5 mm

5 mm 1.25 mm. The force acting on it is 5 N. The

charge sensitivity of Barium titanate is 150 pc/N & its

permittivity is 12.5 10

9

F/m. If the modulus of elasticityof Barium titanate is 120 10

6N/m

2. Calculate strain,

charge & capacitance.

(CSVTU April-May 2012, Nov-Dec 2011, April-May 2011, April-

May 2010)

Ans:

Given:

1.Dimension of barium titanate = 5mm 5mm 1.25mm

2.Force F =5N

3.Charge Sensitivity d =150pC/N

4.Permittivity = 12.5109

F/m

5.Modulus of elasticity= 12 x 106N/m2

To Calculate:



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1.Strain

2.Charge

3.Capacitance

Solution:

Area of Plate (A) = 5mm 5mm = 25106

m2

Pressure (P) = F/A=5/ 25106

=0.2 MN/m2

Voltage sensitivity g = d/ = (1501012

)/12.510

9

)

= 12103

Vm/N

Voltage Generated Eo= g t P =12 x 10-3x 1.25 x 10-3x 0.2 x 106

=3 V

Strain= Stress/Youngs Modulus = (0.2103

)/1210

6

)

= 0.0167

Charge Q= d F = 1501012 )* 5 ) = 750 pC

Capacitance C= Q/Eo= (750 pC)/3 =250 pF

Q. 23: - A piezo electric transducer has a capacitance of

1000 pF and a change sensitivity of 40 10-3C/m. The

connecting cable has a capacitance of 300 pF while the

oscilloscope used for readout has a read-out input

resistance of 1 m with a parallel capacitance of 50 pF.

(i) What is the high frequency sensitivity (V/m) of the

entire measuring system?

(ii) What is the sensitivity (V/m) of the transducer alone?

(iii)What is the lowest frequency that can be measured

with 5% amplitude error by the entire system?



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(iv) What is the value of an external shunt capacitance

that can be connected in order to extend the range of

5% error down to 10 Hz? (CSVTU Nov-Dec 2008)

Ans:

Given:

1.A piezo electric transducer has a capacitance of 1000 pF

2.a change sensitivity of 4010-3C/m.

3.capacitance of 300 pF

4.input resistance of 1 m

5.a parallel capacitance of 50 pF.

Calculate:

(v) What is the high frequency sensitivity (V/m) of the entire

measuring system?

(vi) What is the sensitivity (V/m) of the transducer alone?

(vii)What is the lowest frequency that can be measured with

5% amplitude error by the entire system?(viii)What is the value of an external shunt capacitance that

can be connected in order to extend the range of 5% error

down to 10 Hz

Solution:

1.Charge sensitivity of transducer Kq = 4010

3 C/m

Capacitance of transducer Cp = 100010

12 F

Hence Sensitivity Of Transducer K = Kq/Cp = (4010

3 )/(1000

1012

)



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5.High Frequency sensitivity with external capacitance = 40

103

/48.380 = 827kV/m

Q. 24: - Explain photovoltaic cells?

Ans:

The photo-voltaic or solar cell, produces an electrical

current when connected to a load. Both silicon (Si) and selenium

(Se) types are known for these purposes.

Multiple unit silicon photo-voltaic devices may be used for

sensing light in applications such as reading punched cards in the

data processing industry.

Gold-doped germanium cells with controlled spectral

response characteristics act as photo-voltaic devices in the infra-

red region of the spectrum and may be used as infra-red

detectors.

The silicon solar cell converts the radiant energy of the

sun into electrical power. The solar cell consists of a thin slice of

single crystal P-type silicon, up to 2 cm2into which a very thin

(0.5 micron) layer of N-type material is diffused. The conversion

efficiency depends on the spectral content and intensity of

illumination



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Q. 25: - What do you mean by Hall Effect? ExplainHall Effect

transducers?

(CSVTU April-May 2011, Nov-Dec 2008)

Ans:

When a current carrying conductor is placed in a

magnetic field, a transverse effect is noted. This effect is called

Hall Effect (discovered by Hall in 1879). Hall found that: "When a

magnetic field is applied at right angles to the direction of electric

current an electric field is set up which is perpendicular to both

the direction of electric current and the applied magnetic field".

In other words:

"When any specimen carrying a current I is placed in the

transverse magnetic field B, then an electric field E is induced in

the specimen in the direction perpendicular to both I and B. The

phenomenon is known as Hall effect".

The principle of working of a Hall Effect transducer is

that if a strip of conducting material carries a current in the

presence of a transverse magnetic field, a difference of potential is

produced between the opposite edges of the conductor. The

magnitude of the voltage depends upon the current, the strength

of magnetic field and the property of the conductor called Hall

Effect. The Hall Effect is present in the metals and



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semiconductors in varying amounts, depending upon the

densities and motilities of carriers.

When the transverse magnetic field passes through the

strip, an output voltage across the output leads. This voltage is

proportional to the current and the field strength.

The output voltage ) H

$6 7 B

#

Where, KH= Hall Effect coefficient

T = thickness of strip

I = current

B = flux density

Thus, the voltage produced may be used for measurement

of either the current or the magnetic field strength B.

Hall effect transducers are the transducers in which Hall

effect is utilized to measure various electrical or non-electrical

quantities.

Commercial Hall effect transducers are made fromgermanium or other semiconductor materials.

Q. 26: - Write some applications of Hall Effect transducers?

Ans:

The following are the applications of Hall Effect transducers:

1. Displacement measurement:

Hall Effect transducer may be used to measure a linear

displacement or to locate a structural element is cases where it ispossible to change the magnetic field strength by variation in the

geometry of a magnetic structure.

2. Current measurement:

Hall Effect transducer can be used to measure current in a

conductor without interrupting the circuit and without making

electrical connection between the conductor circuit and the meter.

When a D.C. or A.C. current flows through the conductor,

it sets up a magnetic field around. This magnetic field is



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proportional to the current. A Hall effect transducer is insert it in

a slotted ferromagnetic tube which acts as a magnetic

concentrator. The voltage produce* at the output terminals is

proportional to the magnetic field strength and hence is

proportion to the current, flowing through the conductor.

3. Magnetic flux measurement:

The magnetic flux can be measured by using Hall Effect

transducer. Here, i semiconductor plate is inserted into the

magnetic field which is to be measured. Tin magnetic lines of force

are perpendicular to the semiconductor. The transducer gives an

output voltage which is proportional to the magnetic field intensity

(B).4. Fluid level measurement:

Hall Effect sensors can be used as position, displacement

and proximity sensors if object is being sensed with a small

permanent magnet. Such a sensor can be used to determine the

level of fuel in an automobile fuel tank.

Q. 27: - Describe semiconductor photo-diode?(CSVTU Nov-Dec 2010, Nov-Dec 2007)

Ans:

The photodiode incorporatesa P and an N type - layer. The system

has the electrical characteristics of a rectifier. Radiation directed

in the vicinity of the PN junction and causes a flow of current. Fig.

shows the circuit of a photodiode.



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The photodiode is reverse biased. The reverse biased

saturation current is dependent upon the intensity of the incident

light.

Fig. shows the typical characteristics of a photodiode.

The photodiode is very useful for applications where the

space is restricted. The effective area of a photodiode is about 0.2

mm2and it has a pinhead (serving as electrode) of a diameter of

0.5 mm.

The photocurrent versus light relationship is linear over

a wide range. In order to maintain the linearity the bias voltage

should be kept constant. From Fig. it is clear that the output

resistanceR = AV/AI,is very high and is of the order of tens of

M. The d.c. resistance,V/I,is the diode leakage resistance and

that too is very high. This d.c. resistance depends upon the lightintensity.

The frequency response of a photodiode is largely

dependent upon the intrinsic capacity which is typically 2 pF for a

reverse bias of - 10 V.

The cut off frequency is given byfc= 1/2RLCwhereC

is capacitance of photodiode and RLis the load resistance. The cut

off frequency is of the order of MHz. Even in dark there is alwaysleakage current of the photodiode and this current is known as

dark current:The dark current doubles about every 10C increase



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the photodiode is varied. The highest mode that can be

reproduced is thus dependent upon the response of the

photodiode as well as the film speed, the width of slit and the

definition of photographic emulsion.

4.Photodiodes can be used as detectors of modulated light in

optical communication systems.

5.The photodiodes can be used in switching circuits as they have

a fast response time. A switching circuit using a photodiode is

the transistor is normally 'ON* due to the bias resistor. When

the photodiode is illuminated, the base current is reduced

taming the transistor OFF.

Q. 28: Explain phototransistor? (CSVTU Nov-Dec 2007)

Ans:

A phototransistor is a normal transistor in which the

envelopeenclosing the junction is transparent to allow light to fall

on the base emitter junction . At any PN junction hole-electron

pairs are generated when light fall on the junction, so that any

light falling on the base-emitter junction, produces a current

which is amplified by transistor action, making the device way

sensitive.



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Illumination of the central region causes the release of

electron hole pairs. This lowers the barrier potential across both

junctions, causing an increase in the flow of electrons from the

left region into the centre region and on to the right region.The

sensitivity of a photo diode can be increased by as much as 100

times bv adding a junction, resulting in an NPN device.

The advantages of the phototransistor are: low power

consumption, small size, immediate operation on switching on,

low voltage operation and long life.

A phototransistor gives a high gain. These transistor digital

applications because of the small rise and fall times. The rise

time, which represents the response to dark-to-light irradiance

is about 1 microseconds and the fall time which represents

light-to-dark light irradiance is about 10 microseconds.

Q. 29: -What are frequency generating transducers?

Ans:

PRESSURE INDUCTIVE TRANSDUCER

A simple, arrangement, where in a change in the inductance of a

sensing element is produced by a pressure change, is given in Fig.

below



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angle measurement. All these devices work essentially on the

same principle that is of a rotating transformer. A Synchro

appears like an AC motor consisting of a rotor and a stator.

Synchro's are normally used in control system, but have

properties that can be used in instrumentation also.

A Synchro can be an angular position transducer working

on inductive principle, wherein a variable coupling between

primary and secondary winding is obtained by changing the

relative orientation of the windings.

Internally, most synchro's are similar in construction.

They have a rotor with one or three windings capable of revolvinginside a fixed stator. There are two common types of rotors, the

salient poleand thewound rotor.

The primary winding is a single phase winding wound on

a rotor made of laminations. The connection to the rotor windings

are made through precision slip rings.

The stator has a 3-phase winding with the windings of the

3-phase displaced by 120o. The Synchro may be viewed as a

variable coupling transformer. A Synchro is also called asSelsyn.



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Synchro systems consists of two or more interconnected

Synchro. They are grouped or connected together according to the

purpose to be used.

A Synchro system formed by interconnection of the

devices called the Synchro transmitter and Synchro controltransmitter is perhaps the most widely med error detector in

feedback control system. It measures and compares two angular

displacements and its output voltage is approximately linear with

angular displacement.

The conventional Synchro transmitter (TX) uses a salient

pole rotor with sleeved slot When an ac excitation voltage is

applied to the rotor, the resultant current produces a magnetic

field and by transformer action induces voltage in the stator coils.

The effective voltage induced in any stator coil depends upon die

angular position of the coil axis with respect to the rotor axis

(when the coil voltage is known, the induced voltage at any

angular displacement can be determined).

Q. 31: - Discuss the method for measurement of angularvelocity?

(CSVTU Nov-Dec 2009, April-May 2009)

Ans:

The measurement of angular velocity is more prominent than that

of linear velocity. In many cases the only way for measuring linear

velocity is by its conversion into an angular velocity.

The main problem with linear velocity measurement is in use of a

fixed reference and in detection in case of moving body travelling

over a long distance. The various devices used in measurement of

angular velocity are described below.

1.DC Generator Tachometer.

This generator consisting of a small armature which is coupled to

the shaft of the machine whose speed is to be measured.

It is an ordinary miniature de generator consisting of a smallarmature rotating in a constant magnetic field.



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and an aluminum disc facing the poles of the permanent magnet,

the disc being mounted on the shaft carrying the instrument

pointer.

3. Reluctance pulse pick-ups:This transducer is very suitable for the measurement of shaft

speed and liquid flow. It is based on the principle that if the field

of any magnet is varied momentarily by the motion of an external

magnetic body near it, a voltage pulse is generated at the coil of

the magnet because of the change in flux surrounding the coil.

The transducer consists of a permanent magnet on which a coil is

wound. The output voltage depends upon the rate of change of

magnetic flux and the number of turns. Flux depends upon theclearance between the pick up and the actuating medium, the rate

of movement and size of the actuating medium. The output voltage

is inversely proportional to the distance between the head of the

pick up and the actuating medium.

The pick up is actuated by the teeth of a gear or blades of

turbines. In rpm measurement, the pick up is placed near the

teeth of a gear. Th

paper ei(2) by pankaj sir

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