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EM unit 1 part 5.pdf

  1. Errors in Indicating Instruments
  2. • Error is Deviation • The deviation of Measured Quantity from the True Value or Actual Value is called Error. • True Value: It is not possible to determine the true of quantity by experimental means. True value may be defined as the average value of an infinite number of measured values when average deviation due to various contributing factor will approach to zero. • Measured Value: It may be defined as the approximated value of true value. It can be found out by taking means of several measured readings during an experiment, by applying suitable approximations on physical conditions.
  3. Types of Errors Basically Three types of errors are studied:- 1. Gross Errors 2. Systematic Errors 3. Random Errors
  4. Gross Errors Gross Errors mainly covers the human mistakes in reading instruments and recording and calculating measurement results. Example:- Due to oversight, The read of Temperature as 31.5 ̊ while the actual reading may be 21.5 ̊.
  5. Gross Errors may be of any amount and then their mathematical analysis is impossible. Then these are avoided by adopting two means:- 1. Great care is must in reading and recording the data. 2. Two , Three or even more reading should be taken for the quantity under measurement.
  6. Systematic Errors Systematic Errors classified into three categories :- 1. Instrumental Errors 2. Environmental Errors 3. Observational Errors
  7. Instrumental Errors These errors arises due to three main reasons. 1. Due to inherent shortcoming in the instrument. Example:- If the spring used in permanent magnet instrument has become weak then instrument will always read high. Errors may caused because of friction , hysteresis , or even gear backlash. 2. Due to misuse of the instruments. 3. Due to Loading effects of instruments.
  8. Environmental Errors These errors are due to conditions external to the measuring Device including conditions of the surrounding the instrument. These may be effects of Temperature, Pressure, Humidity, Dust, Vibrations or of external magnetic or electrostatic fields.
  9. Observational Errors There are many sources of observational errors:- -- Parallax, i.e. Apparent displacement when the line of vision is not normal to the scale. -- Inaccurate estimate of average reading. -- Wrong scale reading and wrong recording the data. -- Incorrect conversion of units between consecutive reading.
  10. Random Errors The quantity being measured is affected by many happenings in the universe. We are aware for some of the factors influencing the measurement, but about the rest we are unaware. The errors caused by happening or disturbances about which we are unaware are Random Errors. Its also known as residual Errors.
  11. Errors in PMMC Weakening of Permanent Magnet: ➢ To prevent the weakening of Permanent Magnet due to ageing and temperature effect, magnets are aged by heat and vibration treatment during manufacturing process. ➢ These processes make the magnet to retain its magnetism over a longer period of time.
  12. Errors in PMMC Weakening of spring: ➢ The weakening of Spring can be reduced by careful use of material and pre-aging during manufacturing process. Change in Resistance of Moving Coil: ➢ When the temperature changes, the resistance of the coil vary and the spring also produces error in deflection. ➢ This error can be minimized by using a spring whose temperature co-efficient is very low.
  13. Errors in MI Temperature error : • Due to temperature variation, the resistance of the coil varies. • This affects the deflection of the instrument. • The coil should be made of manganin, so that the resistance is almost constant.
  14. Hysteresis error: • Due to hysteresis affect the reading of the instrument will not be correct. • When the current is decreasing, the flux produced will not decrease suddenly. • Due to this the meter reads a higher value of current. • Similarly when the current increases the meter reads a lower value of current. • This produces error in deflection. • This error can be eliminated using small iron parts with narrow hysteresis loop so that the demagnetization takes place very quickly.
  15. Eddy current error: ➢The eddy currents induced in the moving iron affect the deflection. ➢This error can be reduced by increasing the resistance of the iron. Stray field error: ➢ Since the operating field is weak, the effect of stray field is more.Due to this, error is produced in deflection. ➢This can be eliminated by shielding the parts of the instrument.
  16. • Frequency Error
  17. ➢Deflection of moving iron voltmeter depends upon the current through the coil. ➢Therefore, deflection for a given voltage will be less at higher frequency than at low frequency. ➢ A capacitor is connected in parallel with multiplier resistance. ➢The net reactance, ( X L − XC ) is very small, when compared to the series resistance. ➢Thus the circuit impedance is made independent of frequency. This is because of the circuit is almost resistive.