Difference between Average-response Multimeters and True RMS Multimeters
Fluke digital multimeters and clamp meters are available in average-response and true RMS types. For example, the documents introduce the 110 Series True RMS Multimeters and 170 Series True RMS Multimeters, while the 15B and 17B are simply referred to as digital multimeters. What are their differences? How should users make a selection?
What is RMS Value?
If the heat generated by alternating current i flowing through a pure resistance circuit R within one cycle T equals the heat generated by direct current I flowing through the same resistor for the same period T, the value of I is defined as the RMS value of i.
Measuring Principle of Average-response Meters
For a sine wave, the peak value is 1.414 times the RMS value, and the RMS value is 1.11 times the average value. This factor is also the form factor of the sine wave. Therefore, the average rectification principle can be used to measure the RMS value for sine waves. The measured average value is multiplied by 1.11 to obtain the RMS value. This technology is also known as "average reading calibrated to RMS". The drawback is that this measurement method is only valid for pure sine waves.
Measuring Principle of True RMS Meters
For the waveform shown in the figure below, the form factor = RMS value / average value = 1.82. If the average-response method is adopted, the average value will still be multiplied by 1.11. The calculated RMS value will have a large deviation from the true RMS value. Hence, the true RMS method must be used. This measurement principle enables direct RMS measurement for waveforms of all characteristics.
Conclusion
For pure sine waves, both true RMS and average-response instruments can perform accurate measurements. However, for distorted waveforms or typical non-sinusoidal waves such as square waves, triangular waves and sawtooth waves, only true RMS instruments can deliver accurate readings.
