A Comparison of Differences Between Average-Response and True RMS Multimeters
FLUKE's digital multimeters and clamp meters can be classified into average response and true RMS. For example, in the data, the 110 series true RMS multimeter and 170 series true RMS multimeter are introduced, while only the 15B and 17B digital multimeters are introduced for 15B and 17B; So what are the differences between them? How should users make choices?
What is a valid value?
If the heat generated by the alternating current i passing through the pure resistor circuit R in one cycle T is equal to the heat generated by a direct current I passing through the same resistor at the same time T, then the value of I is called the effective value of i.
Principle of average response measurement:
For a sine wave, the peak value is 1.414 times the effective value, and the effective value is 1.11 times the average value, which is also the waveform factor of the sine wave. So for sine waves, the principle of average rectification can be used to measure the effective value. After measuring the average value, multiply it by 1.11 to obtain the effective value. This technique is also known as "average reading, calibrated according to the effective value". The problem is that this measurement method is only applicable to pure sine waves.
Principle of true effective value measurement:
For the waveform shown in the figure below, the waveform factor=effective value/average value=1.82. If the average response method is used for measurement, the average value will still be multiplied by 1.11, resulting in a significant error between the effective value and the true effective value. Therefore, the true effective value method must be used for measurement, which can be expressed as follows: This measurement principle determines that effective values can be directly measured for all characteristic waveforms.
Conclusion:
For pure sine waves, both true RMS and average response instruments can accurately measure them. However, for distorted waveforms or typical non sine waves such as square waves, triangular waves, and sawtooth waves, only true RMS instruments can accurately measure them.
