Second Topic:Difference Between Average Response Principle and True RMS Principle
FLUKE’s digital multimeters and clamp meters differ between average response and true RMS. For example, the well-known F15B+/F17B+ series are average response multimeters, while the F115C/F117C series are true RMS multimeters. So what are the differences between them? How should users make their choice?
What is RMS Value?
Alternating currenti flowing through a pure resistive circuitR generates heat over one cycleT that is equal to the heat generated by a direct currentI flowing through the same resistance over the same timeT. The value ofI is referred to as the RMS value ofi.
Average Response Measurement Principle:
In the diagram below, the peak value is 1.414 times the RMS value, and the RMS value is 1.11 times the average value, which is also the form factor of a sine wave. Therefore, for sine waves, the average rectification principle can be used to measure the RMS value. This can be expressed with the formula:
After measuring the average value, multiplying by1.11 yields the RMS value. This technique is also known as “average reading, calibrated to RMS,” but the issue is that this measurement method is only applicable to pure sine waves.
True RMS Measurement Principle:
For the waveform in the diagram below, the form factor = RMS value / average value = 1.82. If the average response method is used for measurement, it will still multiply the average value by 1.11, resulting in a significant error between the calculated RMS value and the true RMS value. Therefore, the true RMS method must be used for measurement, expressed with the formula:
This measurement principle ensures that the RMS value can be directly measured for all types of waveforms.
Conclusion:
For DC signals or standard sine waves, both true RMS and average response instruments can measure accurately. However, for distorted waveforms or typical non-sine waves such as square waves, triangular waves, and sawtooth waves, only true RMS instruments can measure accurately. The table below shows the differences in readings between true RMS meters and average response meters for several typical waveforms.
In residential and commercial settings, electrical signals are generally standard sine waves. However, in factories or workshops with nonlinear loads or devices, the impact of unbalanced loads, as well as interference from variable frequency drives or inverter devices, can cause distortion in the power system signals. In such cases, true RMS instruments are required to ensure accurate measurement readings.
