The following procedure is a simple test to determine whether your VOM or DVM produces true RMS voltmeter readings.

Connect a diode, such as a 1N4004, in series with the voltmeter under test and provide a switch to bypass the diode. If the meter's input impedance is greater than 100 k Ohms, install a bypass resistor of 33 k 1/2 W across the meter. Using this setup, with the switch closed, measure the ac power-line voltage. Open the switch and take a second reading. Now you are measuring the half-wave rectified voltage of the AC power line.

If the meter is a true RMS meter, the second reading will be 70.7% of the first reading. On the other hand, if the second reading is only 50%, this indicates that the meter is responding to the average instantaneous voltage and has been calibrated by the manufacturer to output a value equal to the RMS equivalent for a sine wave input and will produce erroneous RMS readings for non-sinusoidal input.

Condensed from an article Herman P. Raab in Electronic Design, April 1, 1996.