Green impedance but the signals are railed
attha
Bangkok
in Electrodes
Apology for poor picture quality.
I have a cython+daisy with an electrode cap, and the version of OpenBCI gui is 5.1.0. I checked the impedances, and they were green. From my understanding, I would expect eeg signals that should not be railed opposing what I saw (see figure below). Could somebody please suggest me what could be possible reasons? Thank you in advance.
Comments
Hi Attha,
An electrode cap with injected gel, should be capable of substantially lower impedance than the 150K to 400K ohm figures you are seeing. Has this cap ever worked correctly for you, and this poor connection is a new situation?
https://docs.openbci.com/AddOns/Headwear/ElectrodeCap/
What gel are you using? Are you following the cap instructions? Since many of your impedances are close to each other, it may be possible that your reference electrode (shared between all channels), has poor contact and is driving all the channel impedances to a higher range. If you are using an ear clip for reference, ensure that the ear lobe is clean and also gelled.
Here is a thread (more devoted to Ultracortex) discussing solutions to railed conditions.
https://openbci.com/forum/index.php?p=/discussion/1481/how-to-become-de-railed
Regards, William
Hi William,
Thanks for your quick response. Yes, it has been working before. I will go to the lab on Monday to check the equipment. I went through the links, and found the test of the cython+daisy board by bridging the three pins with suspect channels. How could we check the cap? Can we test using the bucket test?
Best, Attha
As mentioned before, I would suspect your reference electrode connection, as this electrode is shared with all channels. You can try exchanging the reference electrode with another electrode. Be sure the reference electrode is making good skin contact and has gel.
You can check conductivity between electrode and connectors with a multimeter / ohmmeter.