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Thread: Digital Multimeter for resistance

  1. #1
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    Digital Multimeter for resistance

    There are serveral ranges for ohms on the meter 200, 2K, 20K, 200K, 2M and 20M. When you get a read-out thats a decimal. What multiplier do you use to get the correct ohm reading?

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    The numbers you posted are full-scale readings for each range. For example, if you get a reading of 3.87 with the scale set to 20K, then the resistance is 3.87K ohms. If you had the same reading with the scale set to, say, 2M, then the resistance is 387K ohms.

    The reason they use a "2" is to use what is known as a half of a digit. In other words, a typical 8-segment display can show all 10 digits, whereas a 1 can be displayed with a vertical line (the 'half digit'). So the max readout is actually 199. (with a 2-1/2 digit readout)
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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    On the 200 scale Iam getting 44.9 Is this the ohm reading because the resistance did not go over 200?

    On the 2K scale .045 Do you multiply this by 1000 unless you go have a whole number?

    On the 20K scale .04 Do we multiply this by 1000 too?

    on the 2M scale .000 If there was a digit to the right of the decimal point, what would that be multiplied by?

    Also doing the math to find resistance of 25W, 120 volt bulb = 576 ohms
    Measuring the resistance of that bulb while off (cold) = 44.9 ohms @ 200 ohm scale
    Question: Does the resistance change that much?
    Last edited by EEC; 02-02-11 at 02:19 PM.

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    On the 200 scale Iam getting 44.9 Is this the ohm reading because the resistance did not go over 200?
    Yes.

    On the 2K scale .045 Do you multiply this by 1000 unless you go have a whole number?
    It's .045 K = 45 ohms, so, yes.

    On the 20K scale .04 Do we multiply this by 1000 too?
    It's .04 K = 4[0] ohms, so, yes.

    on the 2M scale .000 If there was a digit to the right of the decimal point, what would that be multiplied by?
    Full scale is 200,000,000 ohms but not all the digits are displayed, though there are probably laboratory meters that can do this.
    Such a meter displaying this many digits would read
    200,000,045 ohms
    so you would see a zero in the fourth place of your meter.

    If you really want a challenge, try to decode the accuracy specs on your meter and check if these readings indicate your meter is within tolerance, assuming the 44.9 ohms is exactly correct.

    You might also Google
    "significant figures"
    and
    "scientific notation"

    Also doing the math to find resistance of 25W, 120 volt bulb = 576 ohms
    Measuring the resistance of that bulb while off (cold) = 44.9 ohms
    Question: Does the resistance change that much?

    Wiki says the cold resistance is 10x to 15x less than the hot resistance.
    Since the resistance drops as the current drops an incand. bulb in series with an outlet [a "voltage source"] acts somewhat like a "current source."
    Last edited by G._S._Ohm; 02-02-11 at 02:28 PM.

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    Quote Originally Posted by EEC View Post
    On the 200 scale Iam getting 44.9 Is this the ohm reading because the resistance did not go over 200?
    Yes. 44.9 ohms.

    On the 2K scale .045 Do you multiply this by 1000 unless you go have a whole number?
    No. That means .045 thousands of ohms, a.k.a. .45 hundreds, or 4.5 tens, or 45 ohms (rounded).

    On the 20K scale .04 Do we multiply this by 1000 too?
    On that scale you should read .004 or something useless.

    on the 2M scale .000 If there was a digit to the right of the decimal point, what would that be multiplied by?
    Nothing. Switch scales. Stop thinking of multipliers.

    The closest scale that's higher than your measurement gives you the most accurate reading.

    Also doing the math to find resistance of 25W, 120 volt bulb = 576 ohms
    Measuring the resistance of that bulb while off (cold) = 44.9 ohms
    Question: Does the resistance change that much?
    Absolutely.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  6. #6
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    Correction


    Full scale is 200,000,000 ohms but not all the digits are displayed, though there are probably laboratory meters that can do this.
    Such a meter displaying this many digits would read
    200,000,045 ohms
    so you would see a zero in the fourth place of your meter.

    should be

    Full scale is 2,000,000 ohms but not all the digits are displayed, though there are probably laboratory meters that can do this.
    Such a meter displaying this many digits would read
    2,000,045 ohms
    so you would see a zero in the fourth place of your meter.

    Sorry.

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    Now I am really confused

    Quote Originally Posted by EEC View Post
    On the 200 scale Iam getting 44.9 Is this the ohm reading because the resistance did not go over 200?

    On the 2K scale .045 Do you multiply this by 1000 unless you go have a whole number?

    On the 20K scale .04 Do we multiply this by 1000 too?

    on the 2M scale .000 If there was a digit to the right of the decimal point, what would that be multiplied by?

    Also doing the math to find resistance of 25W, 120 volt bulb = 576 ohms
    Measuring the resistance of that bulb while off (cold) = 44.9 ohms @ 200 ohm scale
    Question: Does the resistance change that much?
    The ohmmeter reading are actual readings. Take my 20K reading of .04. Larry says "For example, if you get a reading of 3.87 with the scale set to 20K, then the resistance is 3.87K ohms.

    So that means my resistance would be .04 ohms, but my reading at the 200 ohm scale was 44.9 ohms.

  8. #8
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    This a simple math explanation that I came up with.

    V=IxR
    P=VxI

    Line voltage=120V

    25W=120VxI

    25W/120V=.208A

    120V/.208A=576 ohms


    Meter=Approx 9V

    9V/.208A=43.2 ohms

    9Vx.208A=1.872W



    25W/1.872W=13%

    120V/9V=13%

    576 ohms/43.2 ohms=13%
    "Electricity is really just organized lightning." George Carlin


    Derék

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    Quote Originally Posted by jumper View Post
    This a simple math explanation that I came up with.

    V=IxR
    P=VxI

    Line voltage=120V

    25W=120VxI

    25W/120V=.208A

    120V/.208A=576 ohms


    Meter=Approx 9V

    9V/.208A=43.2 ohms

    9Vx.208A=1.872W



    25W/1.872W=13%

    120V/9V=13%

    576 ohms/43.2 ohms=13%
    I'm not sure that your 9V calculations are applicable since most DMMs aren't using the full battery voltage for Ohms readings but are calibrated to show a standard value. If they weren't, my 6V meter would show something different than my 9V meter and neither would show the same as a 3V meter. It doesn't work that way. Aside from that, the reading of a cold filament will be very different than that of a hot one which is why you can't use the wattage of a bulb to figure out its resistance when not lit. The same temperature issue comes into play when measuring resistance of heating elements (ex. floor heating cable or water heater elements) as well - the specs will give you a specific resistance but should also state at what temperature the measurement was taken.
    Peter A.

  10. #10
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    Quote Originally Posted by EEC View Post
    The ohmmeter reading are actual readings. Take my 20K reading of .04. Larry says "For example, if you get a reading of 3.87 with the scale set to 20K, then the resistance is 3.87K ohms.

    So that means my resistance would be .04 ohms, but my reading at the 200 ohm scale was 44.9 ohms.
    No, it means .04 K ohms. Some meters put a small K or M at the far right as you change scales so it isn't so confusing. All you're losing as you change scales is precision, so try to keep leading zeros off the meter. 0.04 K ohms is the same as 40 ohms. Close to your first measurement of 44.9.
    Mark
    Kent, WA

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