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Stray readings on my XDM 1041 Multi Meter

FreeBird

I just want to confirm my suspicions.

I recently read an article by Forrest Mims regarding using LED's as light detectors.  I decided to see how my cheap chinese LED's would react so I plugged 2 wires into 2 adjacent rows on one of my bread boards.  Connected my DMM to them.  I then selected 1 of each color LED from my kit which contains 5 different colors (Red, Blue, Yellow, White and Green).  I noticed however, that I was getting stray voltage readings with nothing connected in the bread board.  I decided to record a series of 500 readings to see what the min, max and average of them would be.  This is what I got:

Min -0.0003
Max 0.0001
Average

-0.000025

I suspect, but would like someone smarter than me to confirm, that this is stray EMF in my lab.  I have a window AC unit running, my computer, a florescent light, etc.  Am I correct in my suspicion?  How might I confirm this, other than asking that is?

Thanks 

Charlie

Parents

  •  Yes and no.  For minor fluctuations and tiny readings, I call this "meter magic" when teaching electricity and electronics.  As  has stated, it exists in every multimeter.  Not that I'm correct, but I attribute it to stray microcurrents in the sensing circuit.  Think of the precision needed at the input of an op amp not to have some error.  It is a differential comparison that is trying to obtain microvolt level accuracy.  To figure out your approximate noise floor, take your meter into an open field and see what kind of readings you get.  Each meter is going to be slightly different.
    I teach that meter magic readings indicate an open circuit - since we're not in the EMF business.

    On the "yes" front, hold one meter lead near a wall wart and the other away from it.  Vary the distance of the first meter lead from the wall wart.  Watch the reading change.  On my Fluke 87, I can hold one of the probes in the vicinity of a fluorescent light (or really cheap LED bulb) and get a consistent frequency reading even though the voltage still fluctuates.  You can try this with anything that you think might be generating EMF.  Just remember that the field strength falls off significantly with distance since the noise radiates in all directions (fills the sphere).

    Based on your averaged readings, I'd say that your input offset is finely tuned and you have a quality bench meter there.

Reply

  •  Yes and no.  For minor fluctuations and tiny readings, I call this "meter magic" when teaching electricity and electronics.  As  has stated, it exists in every multimeter.  Not that I'm correct, but I attribute it to stray microcurrents in the sensing circuit.  Think of the precision needed at the input of an op amp not to have some error.  It is a differential comparison that is trying to obtain microvolt level accuracy.  To figure out your approximate noise floor, take your meter into an open field and see what kind of readings you get.  Each meter is going to be slightly different.
    I teach that meter magic readings indicate an open circuit - since we're not in the EMF business.

    On the "yes" front, hold one meter lead near a wall wart and the other away from it.  Vary the distance of the first meter lead from the wall wart.  Watch the reading change.  On my Fluke 87, I can hold one of the probes in the vicinity of a fluorescent light (or really cheap LED bulb) and get a consistent frequency reading even though the voltage still fluctuates.  You can try this with anything that you think might be generating EMF.  Just remember that the field strength falls off significantly with distance since the noise radiates in all directions (fills the sphere).

    Based on your averaged readings, I'd say that your input offset is finely tuned and you have a quality bench meter there.

Children