dry vs wet electrodes
etc
Italy
Hey folks!
In the last months I read some papers and some post on OpenBci Forum about the comparision between dry vs wet electrodes. I am much interested, because for my needs I think that the time to build an experiment with the wet electrodes , but also with saline electrodes, is too long: for the time before the experiment and for the time after the experiment.
I think so:
1) the weak point of the dry electrodes, the electrode impedance, is not a big problem if the amplifier has a high input impedance as the Opnebci Cyton (1 GigaOhm); the loss of signal for a dry electrode with also an electrode impedance of 1000K Ohm will be not important;
(I read on https://openbci.com/forum/index.php?p=/discussion/2517/expected-impedance-range-with-ultracortex-cyton-resolved
“The ADS1299 chip used in the Cyton, has an input impedance of 1 GigaOhm. FIVE times higher than the 200Mohm in the paper. This might imply that (if quantities scale linearly), a skin impedance of 200K * 5 = 1 Mohm, still results in accurate (< 0.1% error) acquisition. And extrapolating further, if a 0.2% error is acceptable, then 2Mohm skin impedance is still valid. For 0.4% error, 4Mohm skin impedance would work. I would venture to say that 4 or 5 megohm impedances are not anything to worry about, with this dry contact system.”);
2) I read that the cables from dry electrodes with a high electrodes impedance to the amplifier can be prey of the enviroment interference (50/60 Hz); the active electrodes work to reduce this problem. But if after we can use the software filters (Nocht, band pass), what is the problem?
If these points are correct, why are the wet electrodes evalutated so better than the dry electrodes?
I think that there are some other considerations that I didn't understand.
Thank you and sorry for my English!
Ettore
Comments
Passive wet electrodes will give a stronger amplitude EEG signal, than passive dry. If you use active dry, that supposedly will improve the amplitude. But your mileage may vary. Post processing cannot compensate for lower amplitude signal. But some applications are not so sensitive to EEG stream amplitude. But some applications ARE sensitive, such as P300.
Thank you very much! You were very clear!!! Non it is only a problem for the electrode impedance, but it is a problem connected the amplitude signal. Thank you!
ps a question: I would like to test the Openbci Combo dry electrodes (5 mm.) with the Openbci EEG Electrode Cap.
Do you think that is it possible to assembly these dry electrodes with your Cap?
Thank you again! You are always a person available and very gentle!
Ettore
Hi Ettore,
The Electrode Cap in the Shop already has electrodes installed (permanently) at the scalp positions. So you cannot swap in other electrode types. However there are other alternative fastening systems for the dry electrodes, such as:
https://fri-fl-shop.com/collections/electrodes-eeg-electrodes
https://fri-fl-shop.com/collections/electrodes-eeg-electrodes?page=2
The velcro (self-adhesive) straps shown there are actually this double sided tape, with holes punched.
https://www.amazon.com/VELCRO-Brand-ONE-WRAP-Double-Sided-Multi-Purpose/dp/B000078CUB/
You can also find electrode caps online, that have holes instead of electrodes mounted.
https://www.google.com/search?q=eeg+electrode+caps&tbm=isch
Just one example:
https://brain-trainer.com/shop/products/caps-sensors-supplies/
https://brain-trainer.com/product/tc26-trainers-cap/
William
Thank you!
2 doubts! I know that these questions are a little stupid... !
1) I saw these items:
https://brain-trainer.com/product/tc26-trainers-cap/
https://fri-fl-shop.com/products/19-channel-eeg-headband
How can I be sure that the holes in these caps are good for the electrodes that I want to use?
2) If I try to use velcro, how can I build the whole "cap"? I think one strip for the holes Occipital, one for the holes Central, etc... .
But how can I block all the strips on my head?
And have I to find an item to do the holes of the correct dimension?
Thank you!
Ettore
The caps / headsets I mentioned all depend on appropriate fit and pressure being applied. The advantage of the velcro is that it is adjustable. For the caps, you have to pick one size.
FRI sells a pre-made item with all 10-20 positions, but again, for completely precise 10-20 scalp positioning, that is only achievable with a properly fitted cap, or manual measuring.
You can eyeball the hole sizes in caps or headsets that have holes. Compare to the images of the electrodes.
Thank you for your suggestions!
The TC26 cap / headset is made of neoprene foam (cloth faces with foam core). It is only slightly stretchy.
More typical electrode caps are made of lycra / spandex type elastic fabric. And these adapt better to differing head sizes around the size category chosen. An advantage for dry applications, is that the velcro systems allow customized tension. But at the expense of increased setup time.
Similar stuff to the TC26 here, for example the Tenocom.
https://www.aliexpress.com/w/wholesale-electrode-caps.html
Thank you for the lot of suggestions!
I think that the only one problem is to know, before to buy one of these solutions, if the holes size (not the position) is good for my electrodes.
You were very gentle!
Ettore
The holes required for the dry electrodes are very small. The caps that have holes are made of elastic material. Hence if the cap hole is smaller than the actual electrode post size, the cap hole will stretch to fit.
Ok!
Then I have to control that the cap hole is smaller than the electrode size.
Thank you.
Ettore