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AGNI COLLEGE OF TECHNOLOGY DEPARTMENT OF ECE
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1. What are the basic elements of a generalized measurement system? (Nov’ 2003 ECE).
The basic elements of a generalized measurement system are (i) Primary sensing unit (ii) Data conditioning unit (iii) Display unit.
2. Define the term ‘precision’. (Nov’ 2003 ECE).
A measure of the reproducibility of measurement i.e. given a fixed value of Variable, precision is a measure of the degree to which successive measurement differ from one another.
3. What are the classifications of errors in measurements? (Nov’ 2004 ECE).
The classifications of errors are
(i) Gross errors
(ii) Systematic errors
(iii) Schematic errors
(iv) Random errors
4. List any four static characteristics of a measuring system. (May’ 2004 ECE).
Some of the static characteristics of a measuring system are
(i) Accuracy,
(ii) Sensitivity
(iii) Drift
(iv) Hysteresis
(v) Dead zone
(vi) Linearity
5. Define the term ‘Accuracy’. (May’ 2005, Nov’ 2005 ECE).
Accuracy refers to the degree of closeness or conformity to the true value of the quantity under measurement
6. What are the two basic factors in specifying the dynamic performance of an instrumentation system? (Nov’ 2005 ECE).
The two basic factors in specifying the dynamic performance of an instrumentation system are
(i) Transient response.
(ii) Steady state response
7. Mention the different types of standards in measurement. (Nov’ 2005 ECE).
The different types of standards in measurement are
(i) International standards
(ii) Primary standards
(iii) Secondary standards
(iv) Working standards.
8. What is meant by ‘calibration’? (Nov’ 2005 ECE).
Calibration is the process of making an adjustment or making a scale so that the readings of an instrument agree with the accepted and the certified standard.
9. Write the two conditions to be satisfied to make an ac bridge balance. (May’ 2004/Nov’2006 ECE).
The galvanometer current is zero
I1=I3=E/ (R1+R3)
I2=I4=E/ (R2+R4)
R1R4=R2R3
10. How can time and frequency standards to be disseminated? (May’ 2006 ECE)
Time and frequency can be disseminated via radio or television transmission. It is also disseminated by low frequency navigation system called LORON-C.
11. Compare attraction and repulsion type instruments. (May’ 2006 ECE)
Attraction Type Instrument:
The coil is flat and has arrow slot like opening. The moving iron is a flat disc or a sector, eccentrically mounted. When the current flows through the coil, the magnetic field is produced and a moving iron moves from a weaker field to the stronger field inside it, or in other words the moving iron is attracted in.
Repulsion Type:
There are two vanes inside the coil, one fixed and the other movable. These are similarly magnetized when the current flows through the coil and there is a force of repulsion between the two vanes resulting in movement of the moving vane.
12. An experiment performed once gave a reading that showed a deviation of 10%. If it is repeated 65 times, what is the likely deviation? (May’ 2006 ECE)
Refer book
13. Differentiate Zero drift and span drift. (May’ 2006 ECE)
Zero drift is the deviation of the output from its zero value, when the variable to be measured is constant.
Span drift or sensitivity drift is the change in the indication along the upward scale.
14. Precision is a necessary but not a sufficient condition for accuracy. True or False-justify your answer. (Nov’ 2006 ECE)
True. Precision is a necessary but not a sufficient condition for accuracy. Because Accuracy refers to the degree of closeness or conformity to the true value of the quantity under measurement. Precision refers to the degree of agreement within a group of measurements or instruments.
15. What are the different types of standards of measurement? (May 2007)
There are different types of standards of measurement, classified by their function and application in the following categories:
i) International standards
ii) Primary standards
iii) Secondary standards
iv) Working standards
16. What is a transfer instrument? (May 2007)
Refer book
17. What is the need for measurement?(May 2007-R01)
Measurement enables a person to determine the value of an unknown quantity and is mainly needed for determining the value or magnitude of a quantity or variable.
18. Write the need for calibration? (May 2007-R01)
Calibration is needed for finding the errors and accuracy of instruments.
19. Define standards. Classify them.(Nov 2007)
A standard of measurement is a physical representation of unit of measurement. A unit is realized by reference to an arbitrary material standard or a natural phenomenon including physical and atomic constants.
i) International standards
ii) Primary standards
iii) Secondary standards
iv) Working standards
20. Bring out the difference between moving coil and moving iron instruments.(Nov 2007)
Moving iron instruments are generally used for measuring AC voltage and currents
Moving coil instruments are used for measuring DC quantities.
21. Define static error and how are they classified.(May 2008)
Systematic errors can also be sub divided into static or dynamic errors. Static errors are caused by limitations of the measuring device or the physical laws governing its behavior. The static error is introduced in a micrometer when excessive pressure is applied in torquing the shaft.
22. What is the difference between analog and digital instruments.(May 2008)
The major differences between analog and digital instruments are analog instruments are those which are based on some mechanical instruments like gears etc and digital instruments are those based on ICs etc. For eg. Analog get input and show output mechanically and digital instruments shows digital output
23. What is measurement and how is it classified?(May 2009)
Measurement is essentially the act or the result of a quantitative comparison between a given quantity and quantity of the same kind chosen as a standard or a unit.
24. Distinguish between active and passive transducers. (May 2009)
Active transducers are those which require electric current (a source of energy) for working, while passive transducers are those which does not need an external source. Passive transducers directly produce electric signals without an external energy source.
25. Differentiate accuracy from precision.(Nov 2009/May 2010)
Accuracy indicates how far the measured value is close to the true value while precision indicates how far a particular measured value is from the average of the number of reading taken. Thus precision is not related to true value of the parameter to be measured .
26. In a wheatstone bridge a change of 6Ω in the unknown arm of the bridge is required to produce a change in deflection of 2.4 mm of the galvanometer. Determine sensitivity and deflection factor. (Nov 2009)
Refer book
27. What is meant by measurement?
Measurement is an act or the result of comparison between the quantity and a predefined standard.
28. Mention the basic requirements of measurement.
•The standard used for comparison purpose must be accurately defined and should be commonly accepted.
•The apparatus used and the method adopted must be provable.
29. What are the 2 methods for measurement?
•Direct method and
•Indirect method.
30. Explain the function of measurement system.
The measurement system consists of a transducing element which converts the quantity to be measured in an analogous form. the analogous signal is then processed by some intermediate means and is then fed to the end device which presents the results of the measurement.
31. Define Instrument.
Instrument is defined as a device for determining the value or magnitude of a quantity or variable.
32. List the types of instruments.
The 3 types of instruments are
•Mechanical Instruments
•Electrical Instruments and
•Electronic Instruments
33. Classify instruments based on their functions.
Indicating instruments
Integrating instruments
Recording instruments
34. Give the applications of measurement systems.
•The instruments and measurement systems are used for
•Monitoring of processes and operations.
•Control of processes and operations.
•Experimental engineering analysis.
35. Why calibration of instrument is important?
The calibration of all instruments is important since it affords the opportunity to check the instrument against a known standard and subsequently to errors in accuracy.
36. Explain the calibration procedure.
Calibration procedure involves a comparison of the particular instrument with either.
•A primary standard
•A secondary standard with a higher accuracy than the instrument to be calibrated or an instrument of known accuracy.
37. Define Calibration.
It is the process by which comparing the instrument with a standard to correct the accuracy.
38. Name the different essential torques in indicating instruments.
Deflecting torque
Controlling torque
Damping torque
39. Name the types of instruments used for making voltmeter and ammeter.
PMMC type
Moving iron type
Dynamometer type
Hot wire type
Electrostatic type
Induction type.
State the advantages of PMMC instruments
Uniform scale.
No hysteresis loss
Very accurate
High efficiency.
41. State the disadvantages of PMMC instruments
Cannot be used for ac measurements
Some errors are caused by temperature variations.
42. State the applications of PMMC instruments
measurement of dc voltage and current
used in dc galvanometer.
43. How the range of instrument can be extended in PMMC instruments.
In ammeter by connecting a shunt resister
In voltmeter by connecting a series resister.
44. State the advantages of Dynamometer type instruments
Can be used for both dc and ac measurements.
Free from hysteresis and eddy current errors.
45. State the advantages of Moving iron type instruments
Less expensive
Can be used for both dc and ac
Reasonably accurate.
46.What are the basic elements of a generalized measurement system?
The basic elements of a measurement system are primary sensing element, variable conversion element, variable manipulation element, data transmission element and data presentation element.
47. Define the term precision.
Precision is the measure of the consistency or repeatability of a series of measurements.
Precision=1-|Xi-X/Xi|
Xi=the value of the ith measurements.
X=the average value of n measurements
48.What is the difference between accuracy and precision?
Accuracy : It is the closeness with which an instrument reading approaches the true value of the variable being measured.
Precision: It is the measure of reproducibility of the measurements.
49. Define resolution of an instrument.
Resolution of an instrument is the smallest change in the measured quantity that will produce a detectable change in the instrument reading.
50. Define span.
Span is defined as the algebraic difference the highest and lowest scale values of an instrument.
51.What is random error?
Random error is an error due to unknown causes and observed when the magnitude and polarity of a measurement fluctuates in a unpredictable manner.
52. Define standard deviation.
Standard deviation or root mean square deviation is a statistical measure for analyzing grouped data by quantifying the dispersion of individual data points about their
mean value.
53. Define primary standards.
They are basic fundamental or derived unit standard maintained by a country’s national standards laboratory. They are absolute standards of high accuracy that can be used as ultimate reference standards.
54. Define sensitivity of an instrument.
Sensitivity of an instrument is a measure of the change in reading of an instrument for a given change in the measured quantity.
55. Define resolution of an instrument.
Resolution of an instrument is the smallest change in the measured quantity that will produce a detectable change in the instrument reading.
56.What is meant by systematic error?
Systematic errors are those errors that remain constant with repeated measurements, such as zero, scale, response time and loading errors.
57. Define limiting error.
In most instruments, the accuracy is guaranteed to be within a certain percentage of full scale reading. Manufacturers have to specify the deviations from the nominal value of a particular quantity. The limits of these deviations from the specified value are defined as limiting errors or guarantee errors.
Limiting error = accuracy x full-scale value.
58. Define probable error.
The probable error is defined as probable error r = ± 0.6745 σ
where σ is the standard deviation. Probable error is a measure of precision
UNIT II- BASIC ELECTRONIC MEASUREMENTS
1. List the different measuring methods in Q- meter.
There are three methods of connecting unknown components to the test terminals of a Q-meter: direct, series and parallel. The type of component and its size determine the method of connection.
2. What is the source of error in a Q-meter?
Distributed capacitance or self capacitance of the coil.
Residual inductance of the instrument.
conductance of voltmeter.
shunt resistor of Q-meter.
3. Give brief account on working principle of CRO.
The oscilloscope depends on the movement of an electron beam, which is then made visible by allowing the beam to impinge on a phosphor surface, which produces a visible spot. If the electron beam is deflected in either of the two orthogonal axes X and Y the luminous spot can be used to create two dimensional displays.
4. What is known as fluorescence?(May 2003/ECE)
A phosphor converts the electrical energy to light energy. When an electron beam strikes the phosphor crystal, their energy level is increased. This causes the phosphor crystal are excited and they emit light. This phenomenon is called as fluorescence.
5. What is sampling oscilloscope? (Nov’ 2003 ECE).
A Sampling Oscilloscope is used to examine very fast signals. Samples are taken at different portions of the waveform, over successive cycles, and then the total picture is stretched, amplified by relatively low bandwidth amplifiers, and displayed as continuous wave on the screen.
6. What deflection system is required for dual-beam oscilloscope? (Nov’ 2004 ECE).
It uses CRT with two separate electron guns generating two separate beams. Each beam has its own vertical deflection plates. But the two beams are deflected horizontally by the common set of horizontal plates.
7. What is the working principle of sampling oscilloscope? (Nov’ 2004 ECE).
A voltage comparator compares the ramp voltage with the voltage generated by the staircase generator, when the voltage are equal it produces a pulse to the sampling gate. Thus, each time a pulse is generated, the sampling of the input signal is done.
8. List the main parts of a Cathode ray tube. (May’ 2004 ECE).
a. Electron gun assembly
b. Deflection plate assembly
c. Fluorescent screen
d. Glass envelope
e. Base
9. What is the difference between digital and analog oscilloscopes? (Nov’ 2005 ECE)
An analog oscilloscope collects data after it has been triggered. A digital storage oscilloscope is always collecting data, and the trigger tells if when to stop. The advantage of the analog storage oscilloscope is that it has a higher bandwidth and writing speed than a digital oscilloscope, being capable of operating speed of about 15GHz.
10. Justify – ‘A CRO can be regarded as a X-Y recorder’(Nov’ 2006 ECE)
A CRO can be regarded as a X-Y recorder. The X-Y mode means, that the oscilloscope operates with channel A as the vertical signal and channel B as the horizontal signal. The accurate measurements can be done in this mode.
11. What is a vector voltmeter?(May 2004/May 2003/Nov’2006 ECE)
In vector voltmeter very high frequency RF signal is converted into an equivalent IF signal .Then the amplitude and phase is measured simultaneously. The vector voltmeter is used for the following measurements:
(i) Gain of amplifier and phase shift,
(ii) ‘S’ parameter of transistor,
(iii) Insertion loss.
(iv) Transfer characteristics of a filter.
(v) Two port network parameters.
(vi) Amplitude modulation index.
12. Give the characteristics of probes used in CRO. (May’ 2006 ECE)
Probes are used with the oscilloscope to connect the test circuit to the oscilloscope.
(i) The probe should not affect the circuit under test in ideal cases. To achieve this situation a probe should have high input impedance. High input impedance means high input resistance and low input capacitance at the probe tip.
(ii) Any signal going to the oscilloscope, will pass through the probe first, so bandwidth of the probe combines with the bandwidth of CRO. The probe bandwidth must be higher than the oscilloscope bandwidth.
(iii) The probe tip is the signal sensing circuitry. It must be active or passive. If it is passive, it consists of resistor and capacitors. If active it consists of FET.
13. What is the advantage of “pretrigger view” mode of operation in digital storage oscilloscope? (May’ 2006 ECE)
The pretriggering allows the input signal preceding the trigger points to be recorded. So can be set to record continuously, until the trigger signal is received, then recording is stopped. Pretrigger can be 25%, 50%, 75% for single shot, roll normal modes,
14. How is the electron beam focused to a fine spot on the face of CRT? (May 2007)
A focusing anode is mounted just ahead of the reaccelerating anode which gives the electron beam its last addition of energy before its journey to the phosphor screen. These electrons will diverge owing to variations in energy and would produce a broad ill-defined spot on the face of the CRT.
15. List the disadvantages of storage cathode ray tube. (May 2007)
The digital storage oscilloscope will continuously record a selected waveform, and when the trigger waveform appears signifying a failure, the storage will stop. The consecutive type of storage oscilloscope will provide this feature.
16. Mention some of the requirements of signal generator. (May 2007-R01)
The requirements for signal generator are as follows:
i) The frequency of the signal should be well known and stable.
ii) The amplitude should be controllable from very small to relatively large values.
iii) The signal should be free of distortion.