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Reading an Analog Meter

  1. Jan 13, 2014 #1
    1. The problem statement, all variables and given/known data

    1.
    For the pointer position in the diagram, what is the input signal if the range is set at 0.3v?


    (i was just wondering if my path to the solution is correct)
    (arrow pointing at 1.75)
    picture attached

    3. The attempt at a solution

    Range / scales maximum = #

    scale reading / #

    so .3/3 = .1
    1.75/.1 = .175

    is that right?
    thank for all the help
     

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    Last edited: Jan 13, 2014
  2. jcsd
  3. Jan 13, 2014 #2

    ehild

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    Gold Member

    The result is right, but / means division. 1.75/0.1=17.5. You have to multiply the reading with #.

    ehild
     
  4. Jan 13, 2014 #3

    NascentOxygen

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    Staff: Mentor

    I recommend that you adopt a reading technique which will avoid the need to do that second step of mental arithmetic, because we all know that mental arithmetic is where mistakes are going to be made.

    The switch points to 0.3, indicating 0.3 FSD. So that tells you that you will be taking readings off the scale marked 0-3. So in your mind you should now see those scale markings as 0 ... 0.1... 0.2 ... 0.3 and take each reading directly from this "virtual" scale. You will become quite adept at this, with practice.
     
  5. Jan 13, 2014 #4
    ok now if the input signal is -0.25 what range should you select to give maximum deflection without going off scale? where would i draw the pointer at?
     
  6. Jan 13, 2014 #5

    NascentOxygen

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    Staff: Mentor

    What are your thoughts on this?
     
  7. Jan 13, 2014 #6
    I think that 100v range option should be used.
    100v / 10 = 10v

    ? x 10.0 v = -.25v
    ? =-.25v / 10.v
    -.025 is the position on the top scale where i should put my arrow?
     
  8. Jan 13, 2014 #7

    NascentOxygen

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    I think you mean you would choose the 10 v range.

    I'm assuming by -0.25 you are talking of -0.25 volts.

    attachment.php?attachmentid=65564&d=1389595619.jpg

    Where on the -10 .... 10 scale would -0.25 fall? Not very far from the 0 mark in the centre, really. Closer to the 0 than to the 2, anyway.
     
  9. Jan 13, 2014 #8
    Yes however dont you divide by 10?

    so you would get -.025v?
     
  10. Jan 13, 2014 #9

    NascentOxygen

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    Staff: Mentor

    If a voltmeter, as in this example, has scales marked 10 and 3 (obviously FSD), then you can expect there will be a choice of user-selectable switch positions marked (again referring to FSD) such as 0.01V, 0.03V, 0.1V, 0.3V, 1V, 3V, 10V, 30V. As you say, for best reading acuracy the optimum choice is usually that which gives maximum deflection of the pointer.

    So which of the switch positions I listed would you say causes maximum pointer deflection for a signal level of 0.25 V?
     
    Last edited: Jan 13, 2014
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