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Thread: Instrument, measurement, results

  1. #11
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    180508-1245 EDT

    junkhound:

    Your waveform is not what I suggested, but it is just fine for illustrating the calculation.

    My RMS calculation agrees with yours for your waveform.

    I do not agree on the average calculation, although the answers are the same. I get
    9*0.9 - 1*0.1 = 8.1 - 0.1 = 8.0

    Had the two peak values been 8 and -2 with the same duty cycle, then I get an average of
    8*0.9 - 2*0.1 = 7.2 - 0.2 = 7.0 which is not 8 - 2

    Or for 0 and -10 I get an average of
    0*0.9 - 10*0.1 = 0 - 1 = -1 which is not -10

    .

  2. #12
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    for illustration purposes, might be good to take pics of your meter gear AND the signal as seen on a scope.

    and if its a signal gen, change the frequency on the different waveforms, and adjust % duty if its true square wave PWM. see what the meters do.

  3. #13
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    180508-1606 EDT

    FionaZuppa:

    junkhound provided a good plot of the signal he analyzed.

    If the voltage levels are held constant, the duty cycle is constant, the bandwidth is sufficient to not distort the waveform, and frequency is high enough that good averaging occurs, then frequency does not matter.

    If the peak-to-peak voltage is held constant, and the above criteria are met, then as average DC voltage is changed the RMS voltage will change. This is the problem with the Fluke and similar meters and calling them true RMS.

    An electrodynamometer type meter is a true RMS meter within its limitations, and can provide a transfer measurement from DC to AC.

    There are various persons on this forum that are insistent that an RMS meter should be used. But blindingly doing this may not provide the wanted information. You need to know your waveform, how your instrument works and its limitations, and what your measurement is going to do for you (provide useful information).

    At this point I don't even know who those proponents are. That is not important.

    Knowing how your meter works, and the meaning of its measurements are what is important.

    .

  4. #14
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    Quote Originally Posted by gar View Post
    180508-1606 EDT



    If the peak-to-peak voltage is held constant, and the above criteria are met, then as average DC voltage is changed the RMS voltage will change. This is the problem with the Fluke and similar meters and calling them true RMS.
    so are you talking about cases where +Vpk != -Vpk , yet pk-pk is constant?

  5. #15
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    FionaZuppa:

    Yes.

    In that paragraph, and the waveforms I presented in post #1 the peak-to-peak values were the same, and will remain the same when the waveform is coupled thru a capacitor that removes the DC component.

    When you have a meter or amplifier that removes the DC component, then the output waveform remains the same independent of the that original DC component. A Transformer does the same.

    .

  6. #16
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    After looking on line for the AMPROBE instruction manual I came to the conclusion that this particular instrument was measuring the peak current & then converting it into a RMS value. An averaging instrument would read the compact fluorescent lamp low, at least the Fluke T6 does at .2A.

    Here are a couple more pics of the pair, except this time I have turned on the PEAK HOLD function in the 285. Many upper tier Flukes have this same feature. They can capture transients greater than 250micro-seconds.

    Remember the decimal point must be shifted two digits to the left on the Flir285.
    Compact Fluorescent Bulb
    In first picture the where the AMPROBE is displaying 1.4A the 285 is showing a peak to peak reading of 5.26A (2.62+2.64) . If you divide that by 2.8 a normal sine wave RMS value would be 1.88A. The RMS meter & known value are much lower, around .6A. Looking at the data provided on the 285 one can tell the sides of the sine wave are much steeper than normal.

    Incandescent Bulb
    In the second picture the AMPROBE is spot on at .6A considering full scale is 200A. The 285 is displaying .656A with a peak to peak value of 1.929A (.966+.963). Again dividing by 2.8 one gets .69A. The sides of this sine wave is pretty much normal. We know the top of the waves are a little flattened out all across the country from a previous thread. That may account for the slight difference between the calculated & measured.
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  7. #17
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    GAR, I will try to set aside some time this coming weekend to run the experiment with some of my instruments. My biggest problem will most likely be setting up the function generator.
    Advise is a dangerous gift, even from the wise to the wise.

  8. #18
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    Here is a picture of the waveform that the AMPROBE was having a hard time with.
    Attached Images Attached Images  
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  9. #19
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    Quote Originally Posted by gar View Post
    180507-0912

    Problem:

    Using various instruments and calculations, a 10 ohm resistor as a load, and rectangular wave function generator what are the results of measurements across the load?

    1. Average DC volts.
    2. Actual RMS volts.
    3. Fluke 87 AC RMS volts.
    4. Fluke 27 AC volts.
    5. Simpson 260 on AC volts.
    6. Simpson 260 on Output.

    For the following waveforms where the frequency is high enough for satisfactory coupling and averaging of the signal for the time percentages.

    1. +10 V for 10% and 0 V for 90%.
    2. +9 V for 10%, and -1 V for 90%.
    3. 0 V for 10%, and -10 V for 90%.

    How do a Simpson 260 and a Fluke 27 measure AC and get their readings? Define the philosophy.

    .
    In 2 above why is the +9 associated with the 10% & -1 with the 90% ?
    The AC Peak to Peak Function is showing +1 & -9 on the Fluke 289 & Flir 285.

    The 289 is measuring 3.03VAC / 5.04V AC+DC / A Crest Factor of 3 / 60HZ / 89.96% Duty Cycle / 4.03VDC
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  10. #20
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    SG-1:

    Your post #19.

    In 2 above why is the +9 associated with the 10% & -1 with the 90% ?
    The AC Peak to Peak Function is showing +1 & -9 on the Fluke 289 & Flir 285.
    Because I chose to create a waveform rectangular in shape, with a low duty cycle (not symetrical in time), and with the same peak to peak value independent of DC offset.

    The reason for this choice was that many meters in their AC mode remove the DC component of what is being measured. This may or may not be important to the measurement being made. Fluke 27 and 87 for example do strip DC, while Simpson 260 does not unless you switch the + probe to Output.

    RMS calculation of a rectangular waveform is very easy. A +/-5 V sq-wave (10 V peak-to-peak) has an RMS value of 5. If offset to +10 and 0, then RMS is 7.07 V.

    The Fluke 27 and 87 will produce incorrect true RMS readings for all waveforms that do not have a zero DC component.

    Using my scope to set time and voltage at 1 mS + direction and 9 mS - direction, and 10 V peak to peak the Fluke 87 reads 2.97 V. This is slightly low by about 0.03 V for zero bias. The reading does not change with a change of DC bias.

    At +10 V and 0 V the reading should be 3.16 V, and Fluke read 2.97. At +9 and -1 it should be 3.00 and the Fluke 87 read 2,97, this is about correct, At 0 and -10 the reading should be 9.487, but it is still 2.97 .

    .

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