dry electrodes, no signal (400uV, 4Mohms impedance)

JaybJayb
edited February 2016 in Electrodes
I've been getting high uV values (~400) with my electrodes and the FFT waveforms basically look as uniform as noise. My impedance level is in the range of 3-4M Ohms.

I'm not sure what I'm doing wrong, or if perhaps my board is broken. I'm using Florida Research Instruments' dry electrodes. I have an SRB2 electrode and a BIAS electrode on my earlobes and I've tried putting the measurement electrode all over my head. The impedance will not go down. I've also tried measuring impedance with both the OpenBCI_GUI in Processing as well as my own Nodejs code with their openbci_js_sdk. Both give the same results. Unfortunately I don't have access to wet electrodes to see if those work better.

Any ideas what might be causing this problem?

Comments

  • wjcroftwjcroft Mount Shasta, CA
    Jay,

    What happens if you clip together all three leads: SRB2, Bias, and IN1N? (Use alligator clips or jumper wires, e.g. with a breadboard or twisted ends.) That should produce close to 0 uV.

    Are you sure you are using the IN*N pins and not the IN*P pins? IN*N row is closest to the board.

    Is this a brand new board or was it ever used before? 

    William
  • JaybJayb
    Putting together all three leads produces <1 uV so that's good. I'm definitely using the IN*N pins; the bottom ones.

    The board is about a year old, I remember working with it when I first got it and it seemed to work fine, but I didn't know what impedance measuring was at the time so I never looked at that. I'm now starting to use the board again after a few months of idleness.

    The really strange thing is that on very random occasion, I see the impedance level hit about 40k Ohms, but only for a minute or so and the scenario in which this happens is very unreproducible as far as I can tell.
  • wjcroftwjcroft Mount Shasta, CA
    I assume you're using the FRI ear clips, which apply some spring pressure to flat Ag-AgCl disks. Combs or cones would likely not work taped to the ears.

    Your readings sound like not enough contact is being made with the skin. Are you using some type of band or elastic for holding the head electrode in place?

    If the OpenBCI worked fine when you first got it, what is different now?
  • biomurphbiomurph Brooklyn, NY
    @Jayb,

    Can you confirm that you are getting good impedance and signal quality using a different electrode than the FRI dry ones?

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