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class 18.08.2020.pptx

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  1. 1. Measurements done in a laboratory or at some other place always involve errors.  No measurement is free from error. If the precision of the equipment is adequate, no matter what its accuracy is, a discrepancy will always be observed between two measured results.
  2. 2.  No measurement can be made with perfect accuracy but it is important to find out what accuracy actually is and how different errors have entered into the measurement.  A study of errors is a first step in finding ways to reduce them. Errors may arise from different sources and are usually classified as under : ·  1. Gross Errors.  2. Systematic Errors.  3. Random Errors,
  3. 3.  This class of errors mainly covers human mistakes in reading instruments and recording and calculating measurement results.  The responsibility of the mistake normally lies with the experimenter.  The experimenter, may grossly misread the scale.  For example he may, due to an oversight, read the temperature as 31.5°C while the actual reading may be 21.5°C.
  4. 4.  Gross errors may be of any amount and therefore their mathematical analysis is impossible.  However, they can be avoided by adopting two means. They are : 1. Great care should be taken in reading and recording the data. 2. Two, three or even more readings should be taken for the quantity under measurements. These readings should be taken preferably by different experimenters and the readings should be taken at a different reading point to avoid re-reading with the same error.
  5. 5. I. Instrumental Errors. 2. Environmental Errors. 3. Observational Errors.
  6. 6. These errors arise due to three main reasons : (i) Due to inherent shortcomings in the instrument (ii) Due to misuse of the instruments, (iii) Due to loading effects of instruments.
  7. 7.  These errors are inherent in instruments because of their mechanical structure.  They may be due to construction, calibration or operation of the instruments or measuring devices.  These errors may cause the instrument to read too low or too high.  For example, if the spring (used for producing controlling torque) of a permanent magnet instrument has become weak, the instrument will always read high.
  8. 8. The procedure of measurement must be carefully planned. Substitution methods or calibration against standards may be used for the purpose. Correction factors should be applied after determining the instrumental errors. The instrument may be re-calibrated carefully.
  9. 9.  There is an old saying that instruments are better than the peop1e who use them.  Too often, the errors caused in measurements are due to the fault of the operator than that of the instrument.  A good instrument used in an unintelligent way, may give erroneous results.  Examples which may be cited for this misuse of instrument may be failure to adjust the zero of instruments, poor initial adjustments, using leads of too high a resistance and so on.
  10. 10.  One of the most common errors committed by beginners, is the improper use of an instrument for measurement work.  For example, a well calibrated voltmeter may give a misleading voltage reading when connected across a high resistance circuit .  The same voltmeter when connected in a low resistance circuit, may give a more dependable reading.  These examples illustrate that the voltmeter has a loading effect on the circuit, altering the actual circuit conditions by the measurement process.
  11. 11. These errors are due to conditions external to the measuring device including conditions in the area surrounding the instrument. These may be effects of temperature pressure, humidity, dust, vibrations or of external magnetic or electrostatic fields.
  12. 12.  Arrangements are made to keep the conditions as nearly as constant as possible. For example, temperature can be kept constant by keeping the equipment in a temperature controlled enclosure.  Using equipment which is immune to these effects. For example, variations in resistance with temperature can be minimized by using resistance materials which have a very low resistance temperature co-efficient.
  13. 13.  Employing techniques which eliminate the effects of these disturbances. For example, the effect of humidity dust etc. can be entirely eliminated by hermetically sealing the equipment.  Applying computed corrections: Efforts are normally made to avoid the use of application of computed corrections, but where these corrections are needed and are necessary, they are incorporated for the computations of the results.
  14. 14. There are many sources of observational errors. As an example, the pointer of a voltmeter rests slightly above the surface of the scale. Thus an error on account of PARALLAX will be incurred unless the line of vision of the observer is exactly above the pointer.
  15. 15. The quantity being measured is affected by many happenings throughout the universe. We are aware of and account for some of the factors influencing the measurement about the rest are unaware. The happenings or disturbance about which we are unaware are lumped and called "Random" or "Residual”
  16. 16. Moving coil type instruments a) Permanent magnet type b) Dynamometer type
  17. 17. Principle : when a current carrying conductor is placed in magnetic field it is acted upon by a force which tends to move it to one side and out of the field. This movement of coil is used to measure current or voltage.
  18. 18.  This instrument consists of a permanent magnet and a rectangular coil of many turns wound on a light aluminium or copper former inside which is an iron core • The sides of the coil are free to move in the two air gaps between the poles and core • To the moving coil spindle is attached, a pointer is attached to the spindle to move over a calibrated scale.
  19. 19.  A magnetic field of sufficient density is produced by the permanent magnet.  The moving coil carries the current or a current proportional to the voltage to be measured.  Hence, an electromagnetic force is produced which tends to act on the moving coil and moves it away from the field.  This movement makes the spindle move and so the pointer gives a proportionate deflection
  20. 20.  Deflecting torque : It is directly proportional to the current or the voltage to be measured. So, the instrument can be used to measure direct current and dc voltage.  Control torque : Spring control.  Damping torque : Eddy current damping.Damping is electromagnetic by eddy currents induced in the metal frame over which the coil is wound. Since the frame moves in an intense magnetic field, the induced eddy currents are large and damping is very effective.
  21. 21. The permanent-magnet moving coil (PMMC) type instruments have the following advantage and disadvantages: ADVANTAGES 1. They have low power consumption 2. Their scales are uniform and can be designed to extend over and arc of 1700 degree or so 3. They possess high (torque/weight) ratio. 4. They can be modified what the help o f shunts and resistances to cover a wide range of currents and voltages. 5. They have no hysteresis loss. DISADVANTAGES 1. Due to delicate construction and the necessary accurate machining and assembly of various parts, such instruments are somewhat costlier as compared to moving iron instruments. 2. Some errors are set in due to the ageing of control springs and the permanent magnets.
  22. 22. Principle An electrodynamic instrument is a moving coil instrument in which the operating field is produced, not by a permanent but by another fixed coil. This instrument can be used either as an ammeter or voltmeter but is generally used as a wattmeter.
  23. 23.  Fixed coil (F) is made in two sections.  In the space between two, Moving coil (M) is placed.  Moving coil is attached to the spindle to which pointer is attached.  The pointer is allowed to move over a calibrated scale
  24. 24.  The fixed coil and the moving coil carry currents. Thus, two magnetic fields are produced.  Hence, an electromagnetic force tends to act on the moving coil and makes it move. This makes the pointer gives a proportionate deflection.
  25. 25. As voltmeter: The two coils are electrically in series. Deflecting torque is proportional to square of voltage to be measured. Hence used for measuring ac and dc voltages. As ammeter: The two coils are electrically in series. Deflecting torque is proportional to square of current to be measured. Hence used for measuring ac and dc currents. As wattmeter: Fixed coils carry the system current. Moving coil carries a current proportional to the system voltage. The deflecting torque is proportional to V ICos φ i.e. Power to be measured Control torque : Spring control. Damping torque : Air damping.

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