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Electric Test Equipment FAQ - Tech Tips

Electric Test Equipment FAQ - Tech Tips

Question: My Digital Multimeter & now my new Digital Clamp-on seem to indicate readings when NOT connected to a circuit, are these instruments defective?

Answer. Your Instruments are NOT defective. Almost all makes & models of Digital Multimeters (including newer Clamp-on products) have millivolt AC & DC Voltage ranges. These meters have a very high input impedance on their voltage ranges, typically 10 Megohms. This desirable feature permits these instruments to pick up "stray" electrical signals and display them as a non-zero reading. If the instruments test leads are shorted together (tips touching each other), the reading in the display should go to zero. This quick test confirms correct instrument zero on its voltage range. The "stray" pickup does NOT interfere with the instruments ability to measure real voltages, since real voltages are usually generated from sources that have much lower impedance and thus will not contain "stray" voltages.

Question: Why should I buy a True RMS (TRMS) reading Instrument?

Answer. A quick answer is TRMS instruments are accurate on any AC waveform. There are two basic AC measuring systems. One is "Average Sensing, RMS Indicating." This has been the most common system, since it is low in cost and easy to manufacture. An instrument with this type of measuring system measures the average of the sine wave, then multiplies it by 1.112, to allow the instrument to indicate RMS. This measuring system is called "Average Sensing, calibrated to the RMS of a sine wave," and is accurate ONLY ON SINE WAVES!

The second measuring method is called "RMS Sensing, RMS Indicating." An instrument with this type of measuring system computes the RMS value of the measured waveform (sine or otherwise). This is referred to as "True RMS" reading. This measuring system is accurate for ANY waveform, sine or otherwise. When an Average sensing instrument is used on a non-sinusoidal waveform, measurement accuracy could be as much as 50% off! Since you may not know what type of AC waveform you are measuring, a TRMS instrument should always be used.

-Mel Hendrickson, Amprobe Technical Support Specialist

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