BioSemi.nl

I wish to test the BioSemi GSR sensor with a potentiometer. If I understand correctly, the way to do it is to use the pot as a rheostat (i.e. leave the center terminal disconnected) and attach the GSR electrodes to the outer terminals of the pot. What I don't understand is how to connect the CMS/DRL. Intuition tells me that the correct thing to do is to put a very high resistance between each GSR electrode and the CMS/DRL. Does anyone know the proper procedure? I want to see the GSR trace go down exponentially when I increase the resistance.

Connect the GSR and DRL electrodes to one of the GSR electrodes (no resistor needed). Connect your rheostat between the GSR electrodes. ActiView displays the conductance in nanoSiemens in the left sidebar (for example: 100 kOhm displays as 10,000 nanoSiemens)

Best regards, Coen (BioSemi)

Thank you for the swift response. Unfortunately, I don't quite understand. What is meant by 'Connect the GSR and DRL electrodes to one of the GSR electrodes (no resistor needed)'. Did you mean connect CMS/DRL to on of the GSR electrodes? I get the rheostat between the GSR electrodes bit.

Coen means that you should press the AgAgCl tips of the CMS, the DRL and 1 of the GSR electrodes together.
In practice, this will be difficult, because you may absolutely not use any metals when doing this. When metals will touch the AgAgCl tips, the tip will be contaminated with metal particles and the electrode will become noisy.
The 2nd GSR electrode should be connected with the resistor to the other 3 (CMS,DRL, 1st GSR electrode)

In practice, this could be done by using 2 plastic or glass bowls filled with water and a sniff of salt.
Bowl 1: CMS,DRL, 1st GSR electrode.
Bowl 2: 2nd GSR electrode.

Now place your variable resistor between bowl 1 and bowl 2.
Do note that the material that will be used for connecting the resistor to bowl 1 and bowl 2 should be the same material as all the other electrodes.
When a different material is used, offsets will be created. How much offset? that depends on which material you will use.
You probably do not want to cut off 2 electrodes of an old P32 electrode set only for being able to do this test 100% correctly.
Because BioSemi electrodes are active electrodes, this is also not very easily done.
The best alternative would be to use silver wire or standard passive AgAgCl electrodes.

Also note that when using 2 bowls of water, the resistance of the water will be added.

Cool. Thank you for the very detailed explanation. This now makes perfect sense. Typically I couple electrodes for things like this by putting them in globs of electrolytic gel. Obviously whatever electrolytic material is used to couple the electrode leads will not have infinite admittance and this will need to be adjusted for.