Difference of measurements in oscilliscope and DMM reading

Discussion in 'Test & Measurement Forum' started by ashokraj, Aug 31, 2018.

  1. ashokraj

    Thread Starter Member

    Feb 1, 2018
    59
    1
    I am using a single axis industrial accelerometer which is very sensitive. While I am noting down the measurements from accelerometer the values i have got in oscilliscope are way different than multimeter I dont understand how come? In the oscilliscope if i am geting a value of 2.5VAC, in the multimeter it is showing only 0.9VAC. Oscilliscope is tecktronix which can take 1Gsamples/ second.

    Does it happen because of sampling??? undersampling by multimeter? Any inputs ??? I would like to know why does it happen ?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    5,630
    Multimeters often do not have a very high frequency response.
    What is the highest frequency of the signal and the response of the multimeter you are using?
     
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  3. MrChips

    Moderator

    Oct 2, 2009
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    What range is the DMM set to?
    DMM are designed to measure slowly moving DC voltages when set to VDC range,
    When set to AC range the DMM is ideally suited for measuring AC line voltages.

    The oscilloscope will give a more faithful measurement when used within its limitations.
     
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  4. ashokraj

    Thread Starter Member

    Feb 1, 2018
    59
    1
    it is set to VAC
     
  5. crutschow

    Expert

    Mar 14, 2008
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    Okay.
    But what is the frequency response range of the meter?
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    That could be the problem.

    In future,use the oscilloscope for situations such as this one.
     
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  7. OBW0549

    Distinguished Member

    Mar 2, 2015
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    Your multimeter is measuring the RMS value of the AC signal. On an oscilloscope we can observe the peak (or peak-to-peak) voltage of a waveform; the RMS value has to be calculated from that.

    If you are seeing 2.5 volts peak-to-peak on the oscilloscope, divide by 2.818 to get the equivalent RMS value for comparison against what the multimeter is telling you. In this case, 2.5 volts p-p is equivalent to 0.887 volts RMS.

    So your scope and your meter appear to be reporting the same value.
     
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  8. Reloadron

    Distinguished Member

    Jan 15, 2015
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    That would be my guess also. The scope is displaying a peak to peak signal value and your meter the RMS value which is what I would expect to see, right around 0.9 VAC or as OBW0549 points out 0.887 volts RMS.

    Ron
     
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  9. MrChips

    Moderator

    Oct 2, 2009
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    Be careful here. That conversion by dividing by 2.818 is not universal. It works with sine wave signal but not with all waveforms.
     
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  10. -live wire-

    Active Member

    Dec 22, 2017
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    Yeah, very true. Maybe the TS could post some scope pictures to see what the waveform looks like. But it seems very likely that only the scope or some computer program will be able to find the true RMS of it.
     
  11. KeepItSimpleStupid

    Distinguished Member

    Mar 4, 2014
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    Yep, lots of issues from crest factor, wave shape, frequency response, p-p, RMS, average.

    I had a DMM at work that had peak hold.
     
  12. KeepItSimpleStupid

    Distinguished Member

    Mar 4, 2014
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    nah. One of the handheld meters I had was this one https://www.artisantg.com/info/PDF__466C756B655F38303234425F4D616E75616C.pdf#page=33

    It could also be used to do DC measurements just looking at the probes.

    The peak hold would look at either positive or negative peaks. Essentially a positive or negative peak detector. So, if i wanted the p-p voltage of a sine wave. Set peak hold and do positive peak, the reverse the probes and do the negative one and then the math. Forget how the polarities were indicated.

    Another was a Tektronix 916, I believe.

    The GOOD PART of having these meters was that no one would "borrow" mine because it was too "complicated". Same thing with an RPN calculator. They ask to borrow it and it gets returned "immediately".
     
  13. OBW0549

    Distinguished Member

    Mar 2, 2015
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    Good point. I gotta watch my assumptions...
     
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  14. ebp

    Well-Known Member

    Feb 8, 2018
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    Waveforms from accelerometers depend entirely on the system under test and can be a long long way from sinusoidal. Very few situations would allow anything even remotely close to useful to be measured with a meter - with the exception of one used to tell the OS that you've just turned your phone on its side or upside down, or one used to tell you that your car is upside down.
     
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