Calibration of PMMC Voltmeter Using a Potentiometer

Procedure for the Calibration of Permanent Magnet Moving Coil (PMMC) Voltmeter:-

Calibration of Voltmeter Circuit
  1. Give connections as per the circuit diagram in above figure.
  2. Vary the voltage across the voltmeter so that it reads a rounded position, say 5 volts.
  3. Measure the voltage across the voltmeter by potentiometer and tally the voltmeter indication and potentiometer reading.
  4. Draw the calibration curve which is drawn between % error voltage Vs measured potentiometer readings. The expected curve is as follows:
    Calibration of Voltmeter using Potentiometer Expected Curve
  5. Repeat the above steps for different voltages and tabulate the results. The expected tabular readings are given below:

    S.No Voltmeter Reading Pot. Reading m.f(v) Measured Pot. Reading = [(m.f)x 30/1.5] % error
    1. 5 0.249 4.98 0.4
    2. 10 0.487 9.74 2.6
    3. 15 0.736 14.72 1.86
    4. 20 0.987 19.74 1.3
    5. 25 1.2195 24.39 2.44
    6. 30 1.461 29.22 2.6

Precautions

  • Connections should always be made with due care of polarity.
  • For measuring higher voltages better use a volt ratio box.
  • Standardize the potentiometer at regular intervals during prolonged tests to compensate it from any slight drift in the battery voltage.

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