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Thread: Standards For Voltmeters Ohms

  1. #1
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    Standards For Voltmeters Ohms

    Why Are The Industrial Standards For Voltmeters 20,000 Ohms Per Volt For Dc And 5000 Ohms Per Volt For Ac?


    Please Help Answer This Question.........

  2. #2
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    Quote Originally Posted by senior
    Why Are The Industrial Standards For Voltmeters 20,000 Ohms Per Volt For Dc And 5000 Ohms Per Volt For Ac?


    Please Help Answer This Question.........
    My semi-educated guess: AC measurements are invariably for power systems, where the source is of relatively low impedance, whereas many DC circuits are of low current, and a low-impedance meter can load down and alter the voltage.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  3. #3
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    I doubt there is any standard for today's digital meters. Ohms per volt was a common spec. when meters had an analog display that actually needed some input current to drive the needle.

    Today's digital meters usually have MOSFET inputs which are basically an open circuit. The input capacitance is often more important than the input impedence.

    For example, one Fluke meter specifies a straight 5 megaohm or 10 Megaohm input resistance depending on an AC or DC input.

    Steve

  4. #4
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    Because back in the day, that was the best they could make a cost effective meter. Many cheap analogue multimeters were never that good, 1000 ohms per volt on DC was common. You needed to know the ohms per volt because if the meter's resistance (or impedance on AC) was significant compared to the source impedence, then you neeeded to calculate out what the meter was doing to the voltage source.

    Now, of course, meter imedence is pretty muich infionite, compared to most things you would ever be measuring...

  5. #5
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    "industrial standards", if there are any, are to provide meters with as many ohms/volt as practicable vs cost, which also provides for better safety (and the class ratings for the testers) taking into account ratings for the insulation parameters of the equipment. Here's a link that explains the loading of the circuit by a poor meter.
    http://mrtmag.com/mag/radio_technically_speaking_test/

  6. #6
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    Back in Mr. Buckley's day (yeah, mine, too :smile, the best meters for electronic work was the VTVM, the vacuum-tube voltmeter.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  7. #7
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    Quote Originally Posted by LarryFine
    Back in Mr. Buckley's day (yeah, mine, too :smile, the best meters for electronic work was the VTVM, the vacuum-tube voltmeter.
    You may have some time on me then, my first "high impedence" meter used a FET rather than a tube...

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