Home › Forums › Mooshimeter Support › I am see a significate amount of noise in buffer mode.
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- Owen BrinkerhoffGuest
I am seeing a large amount of noise with no leads connected in buffer mode.
Some kind of filtering is needed. It is a little too sensitive. If you connect 2 leads to the V and C posts separate and coil them, select DC voltage on the meter channel 2 then go to the buffered graph and disable channel 1 you will see a repeating pattern. The fun but not useful part is that if you move your body or wave your hand it will disturb the pattern. If the update could be sped up it would make a cool theramin.
I have more to go on. I live in an older house that is near an open delta primary (unbalanced EMF) and the house has some ground issues I have not tracked down yet. As a result the meter is in a 4.5 to 5 milliguass fluctuating AC field. Hand or body motion will temporarily affect affect the field. I went for a walk and I was able to get the field down to 0.6 milligauss. The pattern became more random as I got away from overhead power. I now believe the previously reported pattern was the resultant field from the missing leg on the overhead power line. Even in a 0.6 milliguass AC field non contact motion effects the meter. Since the the EMF around the equipment I work on is often much higher than the 5 milliguass field, the meter would have a good amount of error using an external current clamp that outputs in the milliamp or millivolt range. The buffered graph would have induced noise that may not be part of the measured signal. The noise would cause many problems with accurate power measurement if a millivolt signal were scaled to reflect measured current.
Interesting, thanks Owen. If I understand correctly, I would hesitate to call this signal “noise” in the meter because the meter is measuring a signal that’s actually there – it just happens not to be the signal you’re looking for.
In a few situations I’ve reduced induced magnetic voltages by twisting my test leads together. It’s not perfect but in my case it reduced the induced voltage to an acceptable level. Would that work in your case?
Thank you James,
I don’t think it would be practical. Besides the mooshimeter will pick up the signal with no leads at all. I install well pumps for a living and I was hoping to be able to be able to remotely see power changes happening while I moderate pump output away from the control box. Single phase motors are all about unbalanced magnetic fields. Without a way to zero out unwanted signal, I don’t think I can rely on the information I get. I know it can be done. I have a current leakage meter that can resolve down to the 10 micro amp range. The meter can be zeroed in the field it is in and then the clamp can go over the conductors to take a reading.
Simple Definition of noise
: a loud or unpleasant sound
: a sound that someone or something makes
: unwanted electronic signals that harm the quality of something (such as a radio or television broadcast or a digital photograph)
I see, thanks Owen. The context really helps. Would you mind sending a link to the current leakage meter you’re referencing?
Also – I didn’t realize you were referring to measuring current. This reminds me of another issue I discovered. On the RevH PCB, a batch shipped with a bad filter that adds some white noise to the current channel in normal operation. Is the noise you’re referring to only on the current channel or have you seen the problem on the voltage channel as well? And is your meter RevH or RevI? The revision is written on the PCB, you should be able to read it through the bottom of the meter.
If you are using a RevH PCB and seeing a lot of noise in buffer mode on the current channel only, I believe you may be affected by this problem. Please email me at email@example.com and I can arrange a replacement by a later model.
Hi James, Here is the link to the current leakage meter.
I have the RevI PCB. I am getting the signal on both voltage and current channels in different amounts under different conditions. It was not until I used just one buffered dc voltage channel 2 with leads coiled that I realized that the source of the unwanted signal was AC EMF. It is an unmistakable signal with one dc channel. On any other channel it could be mistaken for spurious noise. The reason this is a problem is that the unwanted signal peaks deep into the measurement range of the current clamps I have. I have three ways to get a current signal from a clamp: 1 mv=1 amp, 10 mv=1 amp, or 1 ma=1 amp. The pumps I most commonly work with, work between 8 and 12 amps and the peak on the unwanted signal is about 4ma at worst.
NEW STRANGE BEHAVIOR
While I was trying to explain my last issues I now have worse and stranger issues with the latest beta release. If I choose AC voltage (120VAC input) on channel 2 8000 hz sample rate then go to the buffered graph I get a sine wave with a saw tooth wave inserted on the second quadrant only, after a few updates I get a sine wave followed by lost connection. I have found only two ways to reestablish the connection.
One is the reset button and that causes the boot-loader to start. The only way to move ahead from there is to load the firmware again. The other way is to pull the batteries and reinsert them. The battery method will connect normally with the meter. There seems to quite a bit of randomness to the problems I am seeing they don’t always repeat.
One more try on the link.
Sorry for the slow reply, I’ve been wrestling with the issue because it seems quite nuanced.
Thank you for finding the issue with 8kHz buffered graphs in the beta firmware, I’ll investigate this.
Regarding the meat of the problem, measuring AC voltages in such a noisy environment: I will repeat some information you probably already know for the benefit of others who might come across this post. The Mooshimeter defaults to 4kHz sample rate and 256 samples to do its AC RMS math. Internally, the low pass filters on the input start to cut off around 20kHz (that number is rough and off the top of my head, but sufficient for this discussion).
The meter who’s datasheet you sent me has filters built in to cut off everything above 60Hz or so (one of the features listed is “Filter to isolate 50/60Hz fundamental from harmonics). Could this be part of the discrepency in readings? The meter you’re used to using shows you RMS of the fundamental, but the Mooshimeter is showing you the RMS of all the harmonics up to 2kHz or so. I recognize this does not address the magnetic pickup issue, but I think there may be multiple factors at play here.
A way to test this might be to manually lower the sample rate of the Mooshimeter, eg. selecting “Voltage AC”, then setting the sample rate to 500Hz instead of 4kHz. This may cause slight aliasing, but the number you get out should do a much better job rejecting all the higher harmonics.
Regarding magnetic pickup – You mentioned zeroing your other meter’s AC reading to compensate for the ambient field (in your May 12 post). If AC zeroing was added to the Mooshimeter, would that solve the issue?
Thanks for the detailed descriptions, sorry if I’m misunderstanding the problem. Screenshots might be helpful if I’m still not getting it. Best
I will start with the ground leakage meter. Most of the time the harmonic filter makes a small difference in the current reading. What the meter does do that makes a big difference is it has what appears to be a zeroing algorithm. When the meter it is initialized it shows a value above zero that slowly steps down to zero in a few seconds. The zero button is intended for relative measurements. I have used the zero button in a really big field to stabilize the reading.
I think AC zeroing might solve my problem with the mooshimeter. Being able to reject ac field noise would allow a magnetic pick up that outputs in millivolts to give accurate readings. This is more important for power functions because it would affect power factor calculation. (The noise is not likely to be in phase with the current.)
I my particular case I use power (KW) to watch pump performance under different flow conditions. If I only used current I would see little or no difference in the current on a single phase pump because the power factor changes relative to the load.
I’ve learned a lot in this thread. Presently zeroing in AC mode doesn’t do anything in the app because I didn’t understand the use cases for this feature. I understand better now and will make sure the feature is included in the next beta release.
Thanks again, really appreciate the insights.