Calibration of PMMC Voltmeter Using a Potentiometer

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

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:
   
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.