More non-RMS vs. RMS meter movement errors.

Discussion in 'Feedback and Suggestions' started by The Electrician, Feb 20, 2010.

  1. The Electrician

    Thread Starter AAC Fanatic!

    Oct 9, 2007
    2,281
    326
  2. The Electrician

    Thread Starter AAC Fanatic!

    Oct 9, 2007
    2,281
    326
    In an earlier post, I pointed out some errors in the eBook:

    I only just now went back and re-checked this page (because someone on another forum referenced it, thus spreading disinformation), and noticed that all the RMS vs. non-RMS errors didn't get corrected.

    In this paragraph on the page:

    "Peak and peak-to-peak measurements are best performed with an oscilloscope, which can capture the crests of the waveform with a high degree of accuracy due to the fast action of the cathode-ray-tube in response to changes in voltage. For RMS measurements, analog meter movements (D'Arsonval, Weston, iron vane, electrodynamometer) will work so long as they have been calibrated in RMS figures. Because the mechanical inertia and dampening effects of an electromechanical meter movement makes the deflection of the needle naturally proportional to the average value of the AC, not the true RMS value, analog meters must be specifically calibrated (or mis-calibrated, depending on how you look at it) to indicate voltage or current in RMS units. The accuracy of this calibration depends on an assumed waveshape, usually a sine wave."

    The second part in red is not true for the iron vane and electrodynamometer movments.

    Furthermore, this page:

    http://www.allaboutcircuits.com/vol_2/chpt_12/1.html

    has a paragraph:

    "With some of the advantages and disadvantages of these meter movement technologies having been discussed already, there is another factor crucially important for the designer and user of AC metering instruments to be aware of. This is the issue of RMS measurement. As we already know, AC measurements are often cast in a scale of DC power equivalence, called RMS (Root-Mean-Square) for the sake of meaningful comparisons with DC and with other AC waveforms of varying shape. None of the meter movement technologies so far discussed inherently measure the RMS value of an AC quantity. Meter movements relying on the motion of a mechanical needle (“rectified” D'Arsonval, iron-vane, and electrostatic) all tend to mechanically average the instantaneous values into an overall average value for the waveform. This average value is not necessarily the same as RMS, although many times it is mistaken as such. Average and RMS values rate against each other as such for these three common waveform shapes:"

    with the same errors.
     
Loading...