Acceptable resistance for multimeter probes

Discussion in 'General Electronics Chat' started by vpoko, Mar 14, 2012.

  1. vpoko

    Thread Starter Member

    Jan 5, 2012
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    I noticed that an older set of DMM probes is giving me about 7 ohms of resistance between them when shorted. These probes are from a $10 meter I got at Radio Shack years ago. Another pair of probes that I have give me 0.5 ohms. It's definitely the probes because I've tried both sets in both meters that I own. Is 7 ohms an acceptable resistance for meter probes (even if 0.5 ohms is better) or should I toss these?
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    There must be some corrosion on the probe ends, or lots of broken strands. Seven ohms would be about 4 meters of AWG 37 wire. If your probe wires are less than 2 meters each, the wire would have to be even smaller to get 7 ohms. I doubt anyone would make probes with such tiny wire.
    You can measure each probe separately. You might be able to rejuvenate them by reattaching all 4 probe ends.
     
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  3. KJ6EAD

    Senior Member

    Apr 30, 2011
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    I usually read less than 0.2Ω per probe. That includes not only copper and connector losses but also any probe tip oxidation.
     
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  4. Lundwall_Paul

    Member

    Oct 18, 2011
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    Toss them.
     
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  5. creakndale

    Active Member

    Mar 13, 2009
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    Check each one independently to see if one is measuring ~6.5 ohms and the other ~0.5 ohms. If so, cut the bad probe in the middle. Then check the banana plug section and the probe tip section. Figure out which part measures high resistance then cut it's end off leaving you with a good piece of stranded wire. This way you recycle the good parts for some future use and only get rid of what's actually bad.

    If they both measure ~3.5 ohms, cut them both in the middle and repeat the above process. Only throw out the bad. Keep the good parts to make up future custom probes, jumpers, etc.

    creakndale
     
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  6. #12

    Expert

    Nov 30, 2010
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    I'm with KJ6 on this one. I've worked with some incredible equipment and could never get below .15 ohm even after I cleaned the connectors with a q-tip and solvent and wiggled everything to rub new surfaces on the metal.

    If my leads measured 5 to 7 ohms I'd mark them by tying a knot in each one and toss them in the scrap box. That way I would never mistake them as reliable and never try to use them until I had put them through the creakndale process.
     
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  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I wouldn't start by cutting them in half. Probe wires generally fail at the point of maximum repeated flexing. That is at or right behind the body of the probe, or the meter end. I would cut off the probe end first, including about an inch of wire, then remeasure the wire resistance. If it's still bad, cut off the other end, and again remeasure. I would cut the wire in half as the last resort.
     
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  8. creakndale

    Active Member

    Mar 13, 2009
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    I agree that the high resistance will typically be located at near one end or the other where the wire lead enters the plastic so you have a 50/50 chance of cutting off the correct end. Knowing my luck (and Murphy's law), whichever end I cut first will be the good, low resistance end (rats!), thus leaving me with the bad end connected to a long piece of wire. The only recourse now is to cut off the bad end leaving just a piece of stranded wire.

    By cutting it in the middle, Murphy's Law is circumvented, leaving either:
    1) a nice molded banana plug with half the wire, or...
    2) a probe tip with half the wire.

    creakndale
     
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  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Yeah, I thought about that. I guess it's a matter of personal choice. :)
     
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