Procedure for the Calibration of Permanent Magnet Moving Coil (PMMC) Voltmeter:-
- Give connections as per the circuit diagram in above figure.
- Vary the voltage across the voltmeter so that it reads a rounded position, say 5 volts.
- Measure the voltage across the voltmeter by potentiometer and tally the voltmeter indication and potentiometer reading.
- Draw the calibration curve which is drawn between % error voltage Vs measured potentiometer readings. The expected curve is as follows:
- 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
- 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.