Fluke - Facts about True-RMS measurement

Our thanks to Fluke for allowing us to reprint the following article.
By Chuck Newcombe

A while back I was asked by a young acquaintance, "How did you measure ac accurately before there were true-rms DMMs?" My answer was, "Very carefully," and it's true!


In 1961, when I first began work in a measurement standards laboratory, there were three kinds of precision rms meters available to me. The available types were all analog; the electrodynamic, the iron vane, and the thermocouple. They were all fragile, requiring careful handling to achieve accurate measurements and avoid damage. They were also slow to respond, and presented a significant load to the measured circuit.


How different it is today, with many digital multimeters featuring true-rms ac measurement capability. Today's meters are rugged, respond quickly, are more sensitive, and present the results with easy to read, digital clarity. But there are some subtle points you should be aware of when making rms measurements...


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