Topic | Discrepancy with Fluke meters on AC measurement

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    • #16822 Reply

      I’ve been doing some testing of the Mooshi alongside a Fluke 117 and Fluke 28-II. DC measurements have been great, but AC has been giving some trouble.

      My application (cathodic protection) requires measuring 500-1000Hz AC rms voltages, typically in the 10-40mV range. When measuring close to the current injection point (strong signal) the Mooshi and Flukes display the same voltage, and both Fluke meters are able to calculate the correct AC frequency.
      Moving further away (weaker signal) gives varying results:
      – Mooshi: 0V
      – Fluke 117: 20mV, unknown frequency
      – Fluke 28-II: 35mV, ~60Hz

      There is undoubtably a lot of noise present, including some line frequency. I’m wondering if the Mooshi is determining that only noise is present and so displays 0V?

      The interesting thing about this application is that the measured AC value doesn’t really matter, but it is important to detect the presence of any AC voltage. Is it possible to have the Mooshi behave more like the Flukes?

      When I get a chance I will be taking a scope out to get some actual waveforms of what the signal looks like.


    • #16825 Reply

      Have you checked what mooshimeter sees there in burst mode?

      I’m quite sure that 4 (or 8) kHz sampling can’t basically even see that 1kHz signal, or at least datapoints would be so random, that it is impossible to guess that there is sinewave or anything else..

      Pure sinewave at bit higher voltage might be possible to nearly 1kHz, but i have no idea what mooshimeter would show then.. Damn, i actually did play with amplifier and some android “signal generator”, but cant remember what happened :D .. Somehow that 1kHz seemed familiar and i think that at least in burst mode it was (human) readable.

      Did you use voltage terminal for your measurements, or “ohm”-terminal, that does “precision” voltages to +/- 1,2 volts?

    • #16826 Reply

      No, I had tried playing with burst mode before and didn’t seem to be working on my phone. Just tried again though and it works, I will try it out and see.

      The signal I got the above results for was at 512Hz, so I thought the mooshi should be able to see that ok. I’ll have access to a function generator soon so I’ll do some more testing with that.

      I was trying both the voltage terminal and the “ohm” terminal on precision voltage with the same results.

    • #17065 Reply

      Ok, I figured out what is going on by looking at the burst mode graph!

      My AC signals are around 20 mV rms, and often have a small DC bias such that there are no zero crossings. While playing with the function generator I realized that the mooshimeter shows 0V when there are no zero crossings in the signal.

      My understanding is that most DMMs measure the AC coupled rms, discarding any DC. Would it make sense to change the mooshi to do that as well? Or even offer a setting for combined DC+AC rms or AC only rms (Fluke 289 has this)?

    • #18495 Reply

      AC only rms, AC+DC rms and DC only. Add in viewing AC only rms and DC only at the same time as a bonus. In many troubleshooting situations these are the measurements that really help.

      Based on the above comment the mooshi would never be able to display ripple or DC offset. Huge limitations. Even the most basic non-rms meter would give me some clue if a DC supply had ripple. I also expect a reasonable response to to a failing triad that is outputting half wave rectified DC.

      Another huge cleanup would be being able to graph just one channel at a time. Voltage or Current or Power…

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