Q.1. What is measurement?

DRex Electronics

Answer: Measurement is essentially the act, or the result, of a quantitative comparison between a given quantity and a quantity of the same kind chosen as a standard or a unit.

Q.2. How are the performance characteristics of an instrumentation system judged?

Answer: The performance characteristics of an instrumentation system are judged by how faithfully the system measures the desired input and how thoroughly it rejects the undesirable inputs.

Q.3. What is tolerance? Is it a static characteristic of measuring instrument?

Answer: Tolerance is a term which is closely related to accuracy and defines the maximum error which is to be expected in some value. Strictly speaking it is not a static characteristic of measuring instrument.

Q.4. What is meant by loading effect?

Answer: The incapability of the system to faithfully measure, re3cord or control the input signal (measurand) in undistorted form is known as loading effect.

Q.5. Why standard resistors use single layer coils?

Answer: Standard resistors make use of single layer coils from the point of view of availability of maximum heat dissipation.

Q.6. Enumerate the properties that must be had by the materials used in the manufacture of a standard resistance.

Answer: The materials to be used in the manufacture of a standard resistance should have the properties of- High resistivity. Small temperature coefficient. Greater permanence. Low thermoelectric effects. Free from effects of oxidation, moisture, acids etc.

Q.7. Why is it necessary to use four-terminal resistance coils for low resistance standards?

Answer: In order to obtain uniform current distribution through the cross section of the resistance material, the current terminals should be at an appreciable distance from the tapping points of the potential leads. That is why the resistance coils for low resistance standards are fitted four terminals – 2 current terminals and 2 potential terminals.

Q.8. Why is air always used as the dielectric in primary capacitance standards?

Answer: Air is the only dielectric whose permittivity is definitely known and is free from absorption and dielectric loss.

Q.9. What are IEEE standards?

Answer: IEEE standards are slightly different types of standards published and maintained by the Institute of Electrical and Electronics Engineers with headquarters in New York City. These standards are not physical items that are available for comparison and checking of the secondary standards but are standard procedures, nomenclature, definitions etc.

Q.10. What is the difference between absolute error and relative error?

Answer: Absolute error is equal to the difference of the measured value and the true value of the measurand. But relative error is equal to the ratio of absolute error to the true value of the quantity under measurement.

Q.11. What is meant by resolution of an instrument?

Answer: The resolution of any instrument is the smallest change in the input signal which can be detected by the instrument. It may be expressed as an actual value or as a fraction or percentage of the full scale value.

Q.12. How do accuracy and precision differ from each other?

Answer: Accuracy is defined as the degree of exactness (closeness) of a measurement compared to the expected (desired) value, whereas precision is a measure of the consistency or repeatability of measurements, i.e. successive readings do not differ. Precision is the consistency of the instrument output for a given value of input.

Q.13. What are systematic errors?

Answer: Systematic errors remain constant or change according to a definite law on repeated measurement of the given quantity. These errors can be evaluated and their influence on the results of measurement can be eliminated by the introduction of proper corrections.

Q.14. What is the expression for probable error if the reading of measurement follows Gaussian distribution?

Answer: The probable error is given as r = 0.6745σ, where σ is the standard deviation.

Q.15. What for are null type instruments mainly used and why?

Answer: The null type instruments are more accurate and highly sensitive as compared with deflection type instruments, and therefore, are more suitable for calibration purposes.

Q.16. What are the various effects of electric current/voltage, upon which the principle of operation of electrical measuring instruments is based?

Answer: The effects utilized in the manufacturing of electrical instruments are magnetic, heating, chemical, electrostatic and electromagnetic effects.

Q.17. What are the essential qualities which the materials used for control springs must possess?

Answer: The essential qualities, for materials used for springs, are that they should be non-magnetic, proof against mechanical fatigue and in particular of low resistivity and low temperature coefficient specially when it is used as lead for current flowing to the moving elements.

Q.18. When is the instrument said to be dead beat?

Answer: When the degree of damping is such that the pointer rises quickly to its deflected position without oscillations, the damping is said to be “critical” and the instrument is said to be dead beat.

Q.19. How does eddy current damping torque vary with the variation in the velocity of the moving element?

Answer: The damping torque varies linearly with the variation in velocity of the moving element provided that the strength of the magnetic field is constant.

Q.20. How does eddy current damping torque vary with the radial position of the poles relative to the axis of the disc?

Answer: Damping torque decreases with the movement of the magnet towards the edge of the disc and becomes zero when the centers of the poles are at the edge of the disc.

Q.21. Why in some instruments, the end of the pointer is flattened and a strip of mirror glass is fitted below the graduated scale and behind the pointer?

Answer: In precision instruments, the end of the pointer is flattened and a strip of mirror glass is fitted immediately below the graduated scale and behind the pointer. The mirror assists the observer in positioning his eye so that it is directly above the pointer, when the pointer and its image coincide. Thus error in reading due to parallax avoided.

Q.22. What is the difference between an ammeter and a voltmeter?

Answer: Ammeter is a low resistance indicating instrument while the voltmeter is high resistance one.

Q.23. Why should an ammeter be of very low resistance?

Answer: Ammeter, which is connected in series with the circuit carrying the current under measurement, must be of very low resistance so that the voltage drop across the ammeter and power absorbed from the circuit are as low as possible.

Q.24. Why should a voltmeter be of very high resistance?

Answer: Voltmeter, which is connected in parallel with the circuit across which the voltage is to be measured, must be of very high resistance so that the current flowing through the voltmeter and the power absorbed from the circuit are minimum possible.

Q.25. How can an ammeter be changed in to a voltmeter?

Answer: An ammeter of low range can be converted into a voltmeter by connecting a high resistance in series with it provided the current through the series combination is within the range of the ammeter when connected across the voltage under measurement.

Q.26. What happens when an ammeter is connected across the circuit?

Answer: If an ammeter is connected in parallel to the circuit like a voltmeter, a very high current will flow through it which will produce such an excessive heat that the insulation of the wire carrying the current will be destroyed. The wire may itself melt away. Thus the instrument will get damaged.

Q.27. What happens when a voltmeter is connected in series with the circuit?

Answer: If a voltmeter is connected in series with the circuit, the circuit resistance will become too large and consequently a very small current will flow through it. The instrument will however, read almost the same emf acting in the circuit.

Q.28. Can the same moving iron instrument can be used for both dc and ac measurements?

Answer: Yes, the same moving iron instrument can be used for both dc and ac measurements because it has a square law response.

Q.29. Why is eddy current damping not possible in moving iron instruments?

Answer: Eddy current damping is not possible in moving iron instruments because of presence of permanent magnet required for each purpose would affect the deflection i.e. the reading of the instrument.

Q.30. How is controlling torque provided in PMMC instruments?

Answer: In PMMC instruments, controlling torque is provided by two phosphor bronze hair springs, either helical or spiral coiled in opposite directions normally equal in strengths.

Q.31. How is dynamometer type instrument used as an ammeter for measurements of small currents?

Answer: For using a dynamometer type instrument as an ammeter for measurements of small currents, its moving and fixed coils are connected in series.

Q.32. Why are the rectifier instruments indispensable for measurements in communication circuits?

Answer: The rectifier instruments are indispensable for measurements in communication circuits since because of their very high sensitivity, the power required for their operation is small enough to be within the capacities of the circuit.

Q.33. What do you understand by ammeter shunt?

Answer: An ammeter shunt is merely a low resistance that is placed across the coil circuit of the instrument in order to measure fairly large currents.

Q.34. Name the materials usually used for ammeter shunts.

Answer: Manganin for dc circuits and constantan for ac circuits.

Q.35. How do instrument transformers differ from power transformers?

Answer: The main difference between instrument and power transformers is that the instrument transformers are required to transform relatively small amounts of power because their only loads, called burdens, constitute the delicate moving elements of instruments.

Q.36. What do you understand by the term ‘burden’ of a CT?

Answer: The product of voltage and current on the secondary side, when the CT is supplying the instrument with its maximum rated value of current, is known as rated burden and is expressed in volt-amperes.

Q.37. What is meant by term compensation and why is it done?

Answer: The accuracy of a CT may be improved, at least with respect to transformation ratio, by modification of the ratio of turns. Correction can be made by a small reduction in secondary terms. The transformer so corrected may be marked as compensated.

Q.38. Why in a CT design the reluctance of the magnetic path and magnetic flux density are kept low?

Answer: The CT is designed with a view to minimize the current ratio and phase angle errors and for small errors it is necessary to keep magnetizing and energy components of exiting current small. Magnetizing component of the exciting current is made small by keeping reluctance of the magnetic path low while the energy or power loss component of exciting current is made small by employing relatively low values of flux density.

Q.39. What is the effect of change in power factor of secondary burden on the ratio and phase angle errors of a PT (Potential Transformers)?

Answer: A change from unity to a lagging power factor of secondary burden tends to increase ratio error but to decrease the phase angle.

Q.40. Why is not an ordinary electrodynamometer wattmeter suitable for measurement of power in low power factor circuits?

Answer: ordinary electrodynamometer wattmeter is not suitable for measurement of power in low power factor circuits owing to –

  • Small deflecting torque on the moving system even when the current and pressure coils are fully excited.
  • Introduction of large error due to inductance of pressure coil at low power factor.

Q.41. What is the need of providing copper shading bands on the pressure coil magnet of induction type wattmeter?

Answer: In order to cause the resultant flux in shunt magnet to lag in phase by exactly 90 degree behind the applied voltage, one or more copper rings, known as copper shading bands are provided on one limb of the shunt magnet.

Q.42. In what respect 3-ammeter method is superior to 3-voltmeter method of power measurement in a single phase ac circuit?

Answer: The drawbacks of 3-voltmeter method in comparison to 3-ammeter method are –

  • Even small errors in measurement of voltages may cause serious errors in the value of power determined.
  • Supply voltage higher than normal voltage is required.

Q.43. Why is the use of aluminium disc preferred over copper disc in energy meters?

Answer: An aluminium disc is usually preferred to a copper disc in order to have resistance per unit weight smaller.

Q.44. What is creeping in motor type energy meters?

Answer: In some energy meters, the disc continues rotating when the potential coils are excited but with no load current flow. This defect is known as creeping.

Q.45. What is the effect of bringing the shading band nearer to the disc on the speed of the disc?

Answer: The speed of the disc will decrease.

Q.46. Why is the speed of the rotating disc in energy meter kept as small as possible?

Answer: The full load speed of the rotating disc in energy meter is kept as small as possible so that the self braking action is reduced to minimum.

Q.47. How is moving-magnet type transducer superior to moving-coil velocity pick up?

Answer: Moving-magnet type devices are simple, robust and inexpensive and provide a sufficient high voltage output as they permit a quite lot of windings. Another advantage is that the connection to the leads does not suffer any motion so maintenance requirements are negligible.

Q.48. How can the range of vibrating reed frequency meter be doubled?

Answer: The range of vibrating reed frequency meter is doubled by polarizing its reed.

Q.49. On what principle does Weston frequency meter operate?

Answer: Weston frequency meter is a moving iron instrument which operates on the principle of variation of impedance in an inductive circuit with the variation in supply frequency.

Q.50. Why is moving iron power factor meter is less accurate than dynamometer type?

Answer: Moving iron power factor meter is less accurate than dynamometer type because of introduction of error owing to the losses in the iron parts of the instrument.