High impedances with Ultracortex dry electrodes & Ganglion

sztahodavidsztahodavid Budapest, Hungary
edited November 2017 in Headware
Dear OpenBCI!

We have purchased a Gangion board with Ultracortex headset. We assembled it according to the documentation. We managed to record the signals on desktop computer. However, we experience high impedance (~600kOhm) on the original 3rd and 4th electrode (position C3 and C4). The impedance of the front electrodes and the ear references are also high in the beginning, but decreases to ~30 kOhm after 5-6 minutes. However, the impedance of the other two electrodes do not decrease at all. It doesn't help if we change the position (we tried it also in O1 and O2). Also, we can't see the increase of the alpha energy band in relaxed state. Maybe it is due to the high impedances?

How can we lower the impedance of the dry electrodes?

Sincerely,
David

Comments

  • lzAlayanlzAlayan
    Hi David,

    Had you solved the problem? What kind of dry electrodes were you using please?

    I have the same problem. I cannot detect meaningful brain signals. I use the TDE­200 Disposable /Reusable Dry EEG Electrode, without any gel, along with Ganglion board and Ultracortex cap. I observed that:
    1) when I short the electrodes with the ground, the impedance could be as low as 0.5 K ohm.
    2) when I contact the electrode and ground on my hand (different fingers), the impedance is as high as 700 K ohm (and I didn't see a decrease after a while).
    3) when I wet my figures and repeat 2), the impedance drops to 5~10 K ohm.

    These electrodes were purchased around 1 year ago. Unfortunately I didn't measure the impedance at that time.

    I was wondering if anyone can help with the following questions:
    1) why is the measured skin impedance so high?
    2) could this due to "oxidate" effects for this type of electrodes?

    Thanks,
    Li

  • sztahodavidsztahodavid Budapest, Hungary
    Hi Li,

    We have not managed to solve the problem so far. We applied a workaround: we wetted the skin under the electrodes (achieved 20-40 kOhm impedance with nicely visible alpha waves). It works until the skin has been dried (5-10 minutes).
    We use the dry electrodes that came with the Ultracortex Mark IV headset.

    We have two of these caps and this high impedance is measured with both (using separate Ganglion boards). So, what I don't understand is, that on the demo video of the OpenBCI software and hardware, it is not mentioned that any adjustments (such as wetting) is needed. And in the video, alpha waves can be immidiately measured. However, we can only achieve appropiate signal, if we apply wetting.

    Anyone from OpenBCI could tell us, why do we have this effect?


    Thanks,
    David
  • wjcroftwjcroft Mount Shasta, CA
    David and Li, hi.

    Have you looked over the tips in this other thread?


    Regards,

    William
  • lzAlayanlzAlayan
    Hi David and William,

    Thank you for your response.

    I have contacted the technician of OpenBCI. I was told that this high impedance might be caused by the oxidation of the electrodes, since that our electrodes had been stored for around one year.
    I also tried the wetted electrodes and it did improved the impedance.
    However, I was told that wetting or putting gel might also hurt the coats of electrodes. So we should avoid to do that.

    I ended up ordering new electrodes. Hopefully they can solve out.

    Thanks,
    Li
  • sztahodavidsztahodavid Budapest, Hungary
    Hi Li,

    We are having this effect since the first use of the cap and we immidiately started using it after it arrived. Does this mean, that the electrodes should have been oxidated the time they arrived?
    Then I will ask for a replacement of the electrodes too. Please tell us if the new electrodes work correctly when they arrive!

    Thank You,
    David 
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