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Thread: Mega Ohm's question

  1. #1
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    Mega Ohm's question

    If you take a resistance reading with a dmm and it is set on auto, and the reading shows "M" for Mega (example, 18.42 M Ohm's), I'm assuming that would be equivalent to 18,420,00 Ohm's?

    I had a brain fart today while troubleshooting some tile floor heat, related to their rated resistance and have been second guessing myself all afternoon!

    The heating element is rated for 16 Ohm's. It's not putting out heat. No current draw at t-stat.
    Not sure if heating element was compromised during tile install. Was ohm'ing out the leads to element and definitely didn't get 16 Ohm's. Tried a couple of meters with similar results.

    After figuring this out, then need to interpret those results...

    Moderators, if you feel this belongs in different section feel free to move it...
    Thanks!

  2. #2
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    Yes, M for Mega means that you multiply the reading by one million, 1,000,000.
    While K for Kilo means that you multiply the reading by one thousand, 1,000.

    A reading of 18Megohms is for all practical purposes an open circuit. (Although a higher voltage than is used by your DMM might break down a tiny insulating gap somewhere.)

    Note also that the presence of any residual voltage on the disconnected wires, even as little as phantom voltage in some cases, can interfere with the reading process on some DMMs.

  3. #3
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    A reading of 18 MΩ is likely to be the resistance of your fingers.

  4. #4
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    Yeah, I was just playing with the calculator, nameplate-> 16 Ohm's at 120v is 7.5 amps and 900 watts. What I saw-> 18.42M Ohm's is basically 0 amps and watts!

    I'm guessing the heating element got damaged...

  5. #5
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    Quote Originally Posted by sw_ross View Post
    Yeah, I was just playing with the calculator, nameplate-> 16 Ohm's at 120v is 7.5 amps and 900 watts. What I saw-> 18.42M Ohm's is basically 0 amps and watts!

    I'm guessing the heating element got damaged...

    Once they develop a hot spot for any reason eventually the heater wire will burn through at that point.
    Even nichrome wire increases in resistance when it gets really hot.

  6. #6
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    It would be wise to test the cable heating element and cold extension leads, before the floor installers show up. That way you would know who owns the problem.

    MTW

  7. #7
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    Quote Originally Posted by drcampbell View Post
    A reading of 18 MΩ is likely to be the resistance of your fingers.
    Yes, if OP was contacting probes while testing he could have been reading through himself, always make sure you are never touching more then one probe when testing results is in meg-ohm range.

  8. #8
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    I thought this was going to be the one about Mega Ohm and Milli Amp going out into an electric field and fluxing.

  9. #9
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    Quote Originally Posted by ggunn View Post
    I thought this was going to be the one about Mega Ohm and Milli Amp going out into an electric field and fluxing.
    They rode on a Mega Cycle.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  10. #10
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    Quote Originally Posted by LarryFine View Post
    They rode on a Mega Cycle.
    At one point Milli said, "Please stop, Mega; that Hertz".

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