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Sensitive analog meters

Discussion in 'General Electronics Chat' started by dr pepper, Nov 20, 2017.

  1. dr pepper

    dr pepper Well-Known Member Most Helpful Member

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    After speaking to an old timer, even older than me I was told that if you wipe the front of a low current like 100ua moving coil meter a static charge can build up and the meter will not return to zero if it still has an impedance connected.
    Wiping said meter with a damp cloth removes the charge and the meter returns to zero.
    Kinda sounda like an old wives tale, anyone seen this happen?
     
  2. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I had a very low current multi meter. (large meter) The meter movement will follow my finger when I touch the meter face. In high humidity areas the meter worked well. In low humidity areas the meter would some times get stuck. I think the meter arm would stick to the meter face. I had to put my hand on the meter face or bang on the meter to get it to go again.

    Then my weather is no like "North west UK". I am at one mile above sea level and we get less than 6 inches of rain/year.
     
  3. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Most likely to happen with cheap, plastic meters. Breathing (moisture in exhale) on them will discharge the static.
     
  4. dave

    Dave New Member

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  5. crutschow

    crutschow Well-Known Member Most Helpful Member

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    In this case the old wife was telling the truth. :D
     
  6. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    From experiance; the old wife can tell if I am telling the truth.
     
  7. OBW0549

    OBW0549 Member

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    I don't believe it makes any difference whether the meter is connected to anything or not; either way, it's quite easy to unintentionally put an electrostatic charge on the meter face cover strong enough to attract the tip of the moving pointer and make it "stick."

    People should pay more heed to old wives, IMO (provided they're someone else's, of course).

    For a longer lasting fix than you can get with plain water, I've found that spraying the meter face with a light coating of anti-static spray and letting it dry works wonders. I don't know what's available over on your side of the Atlantic, but the one I've been using is Pledge Electronics Cleaning and Dusting Spray. Works great and lasts quite a while.
     
  8. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    I guess I could drag one of my old Simpson 260 meters up and brush it with a wool sweater. Then simply exhale on the meter plastic face and see how my warm moist (humid) breath. The Simpson 260 has 50 uA meter movement and 20K Ohm / Volt sensitivity. No it is not an old wives tale. More than once I removed a static charge from the face plate of an analog meter. This was before clothes dryer anti-static sheets were popular. There were also anti-static pump spray solutions used on these meters.

    Ron
     
  9. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    Here you go, I just dug out one of the old Simpson meters I have. Wearing a part wool part I don't know what long sleeve shirt I brushed my shirt sleeve over the meter face a few times. The before is about a 20% upscale as a result of static charge. I gently exhaled on that meter and in about 15 seconds had a nice zero. Another interesting fact is how these meter movements were actually balanced with tiny weights and counter weights on the needle. The meter, when properly mechanically zeroed should maintain a zero regardless of how the meter is positioned.

    Meter Static.png

    Meter No Static.png

    Ron
     
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  10. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    The 260 had so many different models with the same 260. It's not an old wives tale. The meter may not likey move if it's shunted or the "keeper" i in place. The "keeper" is a piece of spring loaded wire shipped with meter movements.

    High voltage capacitors build up a charge just sitting. They need storage keepers too.
     
  11. dr pepper

    dr pepper Well-Known Member Most Helpful Member

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    Aha so its not a charge that affects the meter coil, its a sheer electrostatic thing that directly attracts the needle towards the glass/plastic, that I can understand.
    In the Uk the quivalent to the simpson 260 is the Avo 8, its been with us a long time.
    Nice demostration Ron.
    Ok not an old wives tale, I'm not going to make reference to mine, she sometimes reads my posts.
    6 inches of rain a year would be nice, we had 4 inches last night, and they say another 4 tonight, with gales.
    I'm contemplating ressurecting a old project, a Lcr bridge, and mod it so's its also a esr meter, I have a couple of digi meters but the old analogue ones 'feel' better to use, th emovement is an antiquated 100ua one.
    I remember walking round a boot sale and there was a old meter up on a pile of stuff, every time there was a bit of wind the pointer danced, not sure if it was the wind blowing the needle or some static effect, must try it with this one.
     
  12. tomizett

    tomizett Active Member

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    I re-purposed a old plastic cased 500uA meter movement for a homebrew ESR meter. I made a custom scale and, like a fool, laminated it to make it look nice, sticking it over the original painted aluminium scale. It rendered the meter practically useless - you only had to look at if for the needle to swing around wildly or else lock up solid. Removing the plastic cover/window would restore it to normal working.

    I've learned my lesson.
     

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