Difference between the average response multimeter and the true RMS response multimeter
FLUKE's digital multimeter and clamp meter has the average response and true RMS points, such as in the information on the introduction of the 110 series of true RMS multimeter, 170 series of true RMS multimeter, and for the 15B and 17B only introduced 15B and 17B digital multimeter; so what kind of difference between them? How should users make a choice?
What is RMS?
The heat generated by an AC current i through a purely resistive circuit R in one cycle T is equal to the heat generated by a DC current I through the same resistor in the same time T. The value of I is said to be the RMS value of i.
Average Response Measurement Principle:
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, and this value is also the waveform factor of the sine wave. So for a sine wave, the average rectification principle can be used to measure the RMS value. The RMS value is obtained by multiplying the average value by 1.11. This technique is also known as "averaging readings and calibrating to the RMS value", but the problem is that this method of measurement is only applicable to pure sine waves.
True RMS measurement principle:
For the waveform in the figure below, the waveform factor = RMS/average = 1.82. If the average response method is used, the RMS value will still be multiplied by 1.11, and there will be a large error between the RMS value and the true RMS value, so the true RMS method must be used, which is expressed by the following formula: This principle of measurement determines that RMS can be measured directly on all waveforms.
Conclusion:
For pure sine waves, both true RMS and average response instruments can measure accurately, but for distorted waveforms, or typical non-sinusoidal waveforms such as square waves, triangular waves, and sawtooth waves, only true RMS instruments can measure accurately.
