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100G ohm resistor only measures 50G ohm

Discussion in 'General Electronics Discussion' started by (*steve*), Jan 25, 2017.

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  1. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,491
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    Jan 21, 2010
    I got my brand new 100G ohm resistor (it's sealed in an evacuated glass envelope) and it measures only 50G Ohm!!!

    What could be the problem?

    If I disconnect the coax with the alligator clips on the end, it measures (open circuit) 100G ohm.

    If I disconnect the coax, it measures something in excess of 10T ohm (the max reading is 10T ohm).

    Interestingly, the PL-259 to BNC adapter, if removed makes no noticeable difference to the reading. For a very cheap adapter (it cost me 5c) that's not too bad. From the look of it, it has the same teflon insulation as the built-in PL-259 socket. (Dick Smith Electronics sold me about a dozen of them for 5c each as they were closing down. Looks like they were reasonable quality!)

    In the 10^13 ohm range, it takes several minutes for the needle to move across the dial and then to pin against the end stop. I could probably time the difference with and without the PL-259 adapter (what a good idea).

    It takes 3 min 33 sec to reach full scale with the PL259 adapter, and 2 min 40 sec without it. Clearly a measurable difference, but I'm not going to hazard a guess at the possible resistance.

    Should I shell out for a 1T ohm resistor? What about a 10T ohm resistor?

    I'm happy that my 100G resistor is pretty close to 100G though
     
    OBW0549 likes this.
  2. OBW0549

    OBW0549

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    Jul 5, 2016
    I have a few 1 GΩ resistors on hand for oddball projects such as signal conditioning circuits for piezoelectric accelerometers, but I can't even imagine what one would use a 1 TΩ or 10 TΩ resistor for. What are you making?
     
  3. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Did you clean the glass envelope of your 100 GΩ resistor? I would use a mild dish-washing detergent in de-ionized or distilled water followed by an 90% isopropyl alcohol rinse and careful drying. Use nitrile gloves to handle it and a fresh Kimwipe or a hot-air gun set to a low temperature to dry it. It doesn't take much contamination to lower the terminal resistance from its stated value. How are you measuring the resistance? BTW, unless there is an evacuation nipple, I doubt the glass envelope is under vacuum. Probably was sealed under dry nitrogen atmosphere, or possibly argon.
     
    Last edited: Jan 25, 2017
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I'm making.... sure that the electrometer is working :)

    Testing the resistance ranges of the easiest, and high value resistors are not too expensive.

    At the moment I'm just having fun playing with, and in some cases repairing, the equipment I've bought.

    And because it's a hobby, I don't have to justify it to anyone other than my wife :)

    I haven't found a cheap source of 1T or 10T resistors. There's someone in Russia selling a set from 100M to 1T for about $25, but I've got all the values except 1T, so it's a bit expensive for just one resistor, and they're only 10% tolerance.

    For checking my multimeters I have 0.01% and 0.005% resistors in decades from 100Ω to 100kΩ (and the 0.005% 0.2ppm/C° resistor is the most expensive resistor I've ever bought). I now have extended that in ever loosening tolerance up to resistors that are for most practical purposes, insulators.

    Hop,

    The high value resistors arrived sealed and (except for one) have only been handled using the crocodile clips on my test lead.

    I'm measuring them using my Keithley 600B electrometer. This is the same one that I created the murcury cell replacement for.

    The 100G resistor measures 50G because it is measured in parallel with the insulation resistance of the coaxial test lead, which, coincidentally is pretty damn close to 100G. The leakage is not dirt on the resistor. I will admit to thinking about handling the 100G resistor to see what happens to the resistor, but so far I have resisted the temptation.
     
    Last edited: Jan 25, 2017
    hevans1944 likes this.
  5. OBW0549

    OBW0549

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    Jul 5, 2016
    Well, I'd say 10 TΩ oughtta do the job...

    No wife, here-- therefore, infinite hobby! :D
     
  6. hevans1944

    hevans1944 Hop - AC8NS

    4,608
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    Jun 21, 2012
    I would give in to the temptation, just to see whut hoppens. Well, maybe I would want to have an untouched spare, in case something awful happened. If there is factory-applied identification on the outer glass envelope, it is possible this could be worn off, dissolved, or otherwise erased after handling and cleaning. I (personally) would go ahead and "experiment" with handling it, but it's your toy, not mine. The largest-valued resistor (also glass encapsulated) that I ever got my mitts on was only 1000 MΩ IIRC. And also, IIRC, the ink did dissolve and wash away when I cleaned the glass envelope. Made a pretty good feedback resistor for a pyroelectric sensor though, using the transimpedance amplifier connection.
     
    OBW0549 likes this.
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