impedance problem

An eeg measurement experiment is in progress using cyton and daisy in open bci.
In impedance measurement, the lower the better, but I have tried various things such as twisting the line, changing the line, changing the computer, checking the saline solution, and changing the device, but the impedance does not decrease.
Basically, according to the impedance set in the open bci gui, it shows more than 1000, that is, yellow.
I would appreciate it if you could tell me what to do in the above case and how other users who are using the gui are progressing.

Comments

  • wjcroftwjcroft Mount Shasta, CA
    edited August 2023

    You did not say what electrodes or electrolyte you are using. Is this an Ultracortex?

    You mention 'saline', but electrodes using gel, paste or saline, generally get much lower impedance that what you show. This set of impedance looks more like dry electrodes.

  • A gel free electrode cap is being used. I am using saline to measure the eeg, but I see the impedance seen when using a dry electode.
    Even if I change other variables, this problem continues. Is it a problem with the gui? Or is there any other solution?

  • wjcroftwjcroft Mount Shasta, CA

    @openbxai said:
    A gel free electrode cap is being used. I am using saline to measure the eeg, but I see the impedance seen when using a dry electode.

    Not sure I understand. You are measuring both the impedance and your data collection with the same saline cap and same session setup?

    Where is your reference electrode and ground electrode? If you are using a dry ear clip for that, it is obviously not going to work. Your impedances will be determined by the highest resistance element. And your reference is reflected in the measurements for all channels.

    You should be using saline throughout. And most probably a reference location on the scalp. Common reference locations for caps include midline locations such as AFz or POz or CPz or FCz etc.

  • wjcroftwjcroft Mount Shasta, CA
    edited August 2023

    This doc shows reference at CPz. Ground at AFz.

    https://docs.openbci.com/AddOns/Headwear/GelfreeElectrodeCap/

    If you see similar impedances on all channels, suspect something wrong with your reference. You could also try swapping GND and REF to see if you have a bad sponge, holder, etc.

  • https://docs.openbci.com/AddOns/Headwear/GelfreeElectrodeCap/

    As shown in the above document, I specified the location of ref and gnd.
    I measured it according to the document, but can I change the location of red and gnd at will?
    Even though saline solution was applied to all channels and proceeded before it dried, such a problem occurs. I haven't had this problem before, but since the same problem occurs even after changing to a new device and equipment, could there be another cause?

  • wjcroftwjcroft Mount Shasta, CA

    Has this cap EVER worked correctly? Is it a recent purchase? REF and GND can be interchanged as I suggested previously.

    With saline silver chloride electrodes used in this cap, your impedances should be much much lower than what you report. The fact that this is not the case, implies something is malfunctioning. You are seeing impedances say between a half megohm and 1.6 megohm. Normal electrolyte based electrodes should be well below 100K, likely below 20K.

  • wjcroftwjcroft Mount Shasta, CA

    This table in the doc confirms your scalp impedances should be below 20 kilo-ohms.

  • Thanks for the advice.
    I'll pay attention to the reference and ground and talk back if I run into any problems.

Sign In or Register to comment.