# How to measure Skin Conductance?

Discussion in 'General Electronics Chat' started by PSIR, May 10, 2009.

1. ### PSIR Thread Starter Member

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Hello every one:

What is Skin Conductance? How to measure it?
Why connected to the fingers when measuring?

For example the sensor below:
http://www.bio-medical.com/product_info.cfm?inventory__imodel=SA9309M

Best Regards

2. ### t06afre AAC Fanatic!

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Hi
First of all I would recommend using 3 electrode system and a low frequency AC voltage say 100-200 Hz. The exitation voltage should be around 100mV amplitude. Could you tell us more about how you plan to use your measurement system

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4. ### PSIR Thread Starter Member

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Hi every one,
I found an article about explaining "Skin Conductance" below.
But I don't know the definition clearly and how to construct it to measure.....
especially what is the purpose of 1 megOhm feedback resistor?
Dose this circuit use OPAmp?

//---------------------------------------------------------------------
Conductance (G), as expressed in units
known as Siemens, is the inverse of
Resistance (R):

G = 1 / R

The device presents a voltage of 200mV
across the resistance being measured, so
that the Current (I) flowing across the load
from one electrode to the other is equal to
applied voltage divided by the resistance:

I = 200mv / R = 200mV × G

In the device, the Current (I) flows through a 1megOhm feedback resistor to produce
the Output Voltage (Vout), so that:

Vout = 1 / Rfeedback = I × 1 megOhm = 200mV × G × 1megOhm

Solving for Conductance (G):

G = Vout / 0.200V × 1megOhm

Since 1 megOhm is the reciprocal of 1 μSiemens, the Conductance (G), in μSiemens, is equal to 5 times the Output Voltage:

G (μSiemens) = 5 . Vout

Therefore, for the device, 1 Volt of output equals 5 μSiemens. This relationship along withthe calibration function of the recording device can be used to convert the voltages recorded from the subject to μSiemens of conductance.
//---------------------------------------------------------------------

5. ### t06afre AAC Fanatic!

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I think you forgot to post a link or a picture
But no harm done

6. ### PSIR Thread Starter Member

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Excuse me.
How many Ohms while 1 μSiemens?
In other word, 1 μSiemens is equal to the number of resistance?

Thank you very much.

7. ### StayatHomeElectronics Well-Known Member

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Siemens are the inverse of Ohms. Siemens = 1 / Ohms. Ohms = 1 / Siemens.

8. ### t06afre AAC Fanatic!

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Last edited: Jun 30, 2009
9. ### ELECTRONERD Senior Member

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Conductance is the reciprocal of resistance - C = 1/R

So what you can do, although it isn't very accurate, is hold the probes of a multimeter in each hand and set it to Ohms, you will get a resistance and you can apply this formula to find the conductance. The problem is that if you pinch the metal on the probes harder, you'll get lower resistance. So it isn't very accurate. Just for a general Idea though.

10. ### t06afre AAC Fanatic!

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Let me add to your idea. Put some coins on the skin. This will give you a better idea about the skin conductance. But the skin is NOT a pure resistor. The very simplified model is a a capacitor in parallel with resistor. So the most correct thing is to apply a constant AC voltage and measure the current

11. ### ELECTRONERD Senior Member

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Indeed, the skin is not a pure resistor, you may eat something one day and have a varying resistance and devour something another day and have a completely different resistance.

12. ### yusim Member

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the resistance of the skin varies according to the amount of sweat in the sweat glands

the smallest changes in the nervous system changes the amount of sweat in the sweat glands - therefore its used in the polygraph test

here's a simple circuit i built a while back to test GSR as part of a bigger biofeedback circuit - it works beautifully

13. ### t06afre AAC Fanatic!

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But it is still DC based and measure the skin conductance of two electrodes in series. With this system you a unable to isolate the conductance under a single electrode

14. ### DC_Kid Well-Known Member

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does "skin" here mean the skinning effect of hi freq currents moving to the edge of the conductor? RF, etc

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16. ### ELECTRONERD Senior Member

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Well, we all have some RF absorbed by our bodies.