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Meter accuracy

Kelly Marie Angel
I recently purchased 2 little voltmeters they look like the sort that would go in a control or instrument panel they are connected with just 2 wires which provide the operating supply ( they light up green and red) however the green  one states it will work between 20and 500 volts and the red one between 60 and 480 volts. When they are both on the green one indicates normally around 241 volts the red one shows 235 volts why the discrepancy I know it's not much but makes you wonder if one of them is lying. Secondly I've noticed that the green one tracks voltage changes faster than the red one and that a few times the green one jumps down to 238 then up to 241 multiple times while the red one stays the same and I think can see a slight flicker in my filament lamps when this is happening incidentally both meters are connected to the same plug  a 2 pin 5 amp one
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    Ah, so many assumptions that things will be spot on because the digits are present :)

    if it had been a meter that only quoted in 5v steps, you would have been happy (in your expectations)


    So what was the quoted accuracy ? - that illusive value.

    only a few of those panel meters quote a meters resolution 
    (or worse, they say accuracy(sic) when then mean resolution)


    Now if you want to buy £50 Murata devices, they do quote (in their data sheet)

    but even they quote 2v typical, and 4v max error
    https://www.digikey.co.uk/product-detail/en/murata-power-solutions-inc/DMS-20ACV-3-R-C/811-3311-ND/5798918


    But looking at your numbers - difference of 6v, = +/-3v => 1.26% (in)accuracy, which sounds OK till you look at what that means in practice.

    so lets say true value of 238v +/- 3v = 235 on one meter, and 241 on another : both are in spec.

    of course it is even worse if, by chance, one meter is 6v off, and the other spot on :).
    Volts

     

     

     

     

        +/-

    6

    3

    1

    0.5

    238

    2.52%

    1.26%

    0.42%

    0.21%

    Min

    232

    235

    237

    237.5

    Max

    244

    241

    239

    238.5



    So your meters would appear to be within 1.3% (which probably sounded OK)

    from the above table, if you want

    +/- 1v Readings you need to have 0.4% (in)accuracy

    +/- 0.5v Readings, you need to have 0.2% or better (in)accuracy


    Time to get calibrate these ones against a True Known Measurement device

    Oh, and know if these are averaging over what period of time ?

    . .to know what offset to apply for a True-er value.


Reply
  • 0
    Ah, so many assumptions that things will be spot on because the digits are present :)

    if it had been a meter that only quoted in 5v steps, you would have been happy (in your expectations)


    So what was the quoted accuracy ? - that illusive value.

    only a few of those panel meters quote a meters resolution 
    (or worse, they say accuracy(sic) when then mean resolution)


    Now if you want to buy £50 Murata devices, they do quote (in their data sheet)

    but even they quote 2v typical, and 4v max error
    https://www.digikey.co.uk/product-detail/en/murata-power-solutions-inc/DMS-20ACV-3-R-C/811-3311-ND/5798918


    But looking at your numbers - difference of 6v, = +/-3v => 1.26% (in)accuracy, which sounds OK till you look at what that means in practice.

    so lets say true value of 238v +/- 3v = 235 on one meter, and 241 on another : both are in spec.

    of course it is even worse if, by chance, one meter is 6v off, and the other spot on :).
    Volts

     

     

     

     

        +/-

    6

    3

    1

    0.5

    238

    2.52%

    1.26%

    0.42%

    0.21%

    Min

    232

    235

    237

    237.5

    Max

    244

    241

    239

    238.5



    So your meters would appear to be within 1.3% (which probably sounded OK)

    from the above table, if you want

    +/- 1v Readings you need to have 0.4% (in)accuracy

    +/- 0.5v Readings, you need to have 0.2% or better (in)accuracy


    Time to get calibrate these ones against a True Known Measurement device

    Oh, and know if these are averaging over what period of time ?

    . .to know what offset to apply for a True-er value.


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