Home › Forums › Mooshimeter Support › Voltage not accurate
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Hello. As soon as I received the mooshimeter, I plugged it into a wall outlet. But instead of showing 120 VAC, it shows 131 VAC. So I played around different frequency settings, and the one that shows the most accurate reading is 500 hz. If I increase the sampling frequency, the VAC reading goes up (132 VAC), but if I lower the sampling frequency, the VAC reading goes down (38 VAC). The accuracy of the AC voltage reading has nothing to do with the “sample buffer depth”, but with the sampling frequency. Is this the expected behavior? Because I thought that my mooshimeter is not accurate.
The 120VAC voltage coming out of the wall is not very accurate – for example in our office it measures 108VAC. It’s not terribly unusual for a 120VAC wall socket to measure 130V, I wouldn’t worry about that.
Regarding sample buffer depth and frequency settings: The Mooshimeter works by gathering a bunch of samples in a buffer, then calculating the RMS value of the buffer. When you lower the sampling frequency, you end up smoothing out higher frequency signals which may be present on the power line, which decreases the calculated RMS. If you lower the sample frequency to the extreme, you end up smoothing out the power waveform itself (it’s a 60Hz waveform, if you sample at 125Hz it will not give you an accurate RMS value).
The default settings (4000Hz, 256 samples) represent good settings for capturing RMS components up to 1kHz. You should not have to mess with the sampling settings unless you have a very strange use case.
Does this answer your questions?
Thanks for the quick reply. I didn’t compare it with what I expected to find, but with the measurements done with another multimeter, which displays 120 AC. So I tried to use “buffer mode” to see the sine wave, and it does not work. It just hangs whilst displaying the spinning circle. I left it for several minutes to see if it responds, but it doesn’t, it just shows the spinning circle. I am using a Nexus 4 with Android version 4.4.4. Any ideas why the mooshimeter app hangs when trying to use “buffer mode”?
That’s interesting. What kind of multimeter is the other one? Some multimeters do not perform actual RMS calculation, but just give an estimate of AC voltage based on the maximum. The other multimeter may also not capture as high a frequency as the Mooshimeter. As a test, can you compare a DC reading?
Regarding buffer mode: Android 4.4 is supported but the Android BLE interface was only released in version 4.3, 4.4 still has a lot of know bugs. Can you try tapping the “Refresh” button in the lower left corner while in buffer mode? It should not take minutes to load, expect it to take 5-10 seconds to transfer the measurement buffer.
Please let me know how it goes, best
Thanks for your reply, James. The other multimeter is an economic one. So it is entirely possible that the cheap multimeter’s readings are below the actual mains voltage.
I tried your suggestion and it works! So immediately after holding my phone sideways, I press the refresh button, and I can see the sine wave. When the sample frequency is 4000 hz, its peak voltage is about 190 V. So the RMS calculations are correct. But can it be possible for the mains AC voltage to go as high as 137 V RMS?
I compared the mooshimeter’s VDC readings with the other multimeter and they are identical, which is great.
Hooray! I’m glad it’s working. The failure of the first buffer on transition to buffer mode was experienced by another use, so I’ll be sure it gets on the bug list.
Thanks for the feedback, and let me know if anything else comes up!