Calibration of PMMC Voltmeter Using a Potentiometer
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 |
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.