Trouble-Shooting Your Gamry Potentiostat with the Universal Dummy Cell 3

The Universal Dummy Cell 3 (UDC 3) is a printed circuit board used for calibration and troubleshooting of a Gamry Potentiostat and is supplied with every Gamry Potentiostat.  It has 3 sides, equipped with terminals, used for test functions.  The fourth side is available for custom applications.  The terminals are marked with labels corresponding to the leads on a Gamry Cell Cable.  

Click here for a photo of the Universal Dummy Cell 3.

Calibration
The calibration circuit is a 2000 ohm precision resistor (with some protection components).  Its value measured with a potentiostat is typically between 1.994 kohm and 2.006 kohm.   

Early revision UDCs had a 100 ohm resistor in the calibration circuit.  PCI4 family potentiostats require a UDC 2 with its 2000 ohm resistor.  If a dummy cell isn’t marked as a UDC 2, assume it’s an older dummy cell.  100 ohm Universal Dummy Cells (shipped before May 2003) will not calibrate a PCI4 family Potentiostat to its full accuracy.

You can also use the calibration circuit to test for proper operation of your Potentiostat.  Use any technique.  The current at any potential should be given by Ohm’s law, I = E/R.  At 1 Volt, for example, the current should be 500 microAmps (1 V/2000 ohms = 5 x 10^-4 Amps).

More specifically, we recommend you use one of these techniques if you have concerns about your potentiostat’s health:

   EIS300    Potentiostatic EIS
   DC105    Polarization Resistance
   PHE200    Cyclic Voltammetry  

EIS 
The EIS Dummy Cell is a Simplified Randle’s Cell (as described in Gamry’s Help system).   If you have a license for Gamry’s EIS300 Electrochemical Impedance Spectroscopy software, you can record this Cell’s EIS spectrum to check your system’s EIS operation. 

A circuit diagram of the EIS cell and a typical spectrum of the cell in Bode format is seen below.
 

A visual examination of the spectrum is usually sufficient to determine if your EIS system is working.  Most system malfunctions totally prevent recording of the spectrum or cause gross errors that dramatically change the shape of the curve.

If you want a numerical confirmation of the system’s performance, fit the dummy cell’s spectrum to the Randles model in the Echem Analyst.   The calculated values should be between:

R_p = 2.95 kohm and 3.07 kohm

R_u = 196 ohms and 204 ohms

C_f = 0.90 µF and 1.10 µF

DC Corrosion
The UDC 3’s DC Corrosion circuit simulates current versus voltage behavior of an ideal corrosion cell.  A voltage scan on this cell, with current range auto-ranging enabled, is particularly useful when testing Potentiostat operation because almost all current ranges will be used during the scan.

The circuit diagram of the DC Corrosion cell is shown above.  The logarithm of the current through a Schottky Diode (SR106) is proportional to the applied voltage, modeling a kinetically-controlled electrochemical process.  Unfortunately, a Schottky Diode-based dummy cell does not model corrosion perfectly.  A corroding electrode has a non-zero I_corr (corrosion current), while I_corr is essentially zero for this test cell.

You must have a license for Gamry’s DC105 software to fully utilize this cell.  We recommend using the Tafel Plot. Your scan rate should be slow (0.50 mV/sec) and should use a Sample Period of 0.5 second per point.

This cell gives acceptable log I versus E linearity for potentials ± 175 mV of Ecorr (zero volts on this cell). Current interrupt iR compensation during the scan expands the linear region to ± 300 mV, but adds noise to the data.

A Tafel fit of a typical scan (both with and without iR compensation) can be seen below.  (Curved Plot = no iR Compensation. Straight Plot = with iR Compensation.  Dark Blue Line = the fit from the Echem Analyst.)  Both the anodic and cathodic Tafel constants were calculated to be between 60 and 70 mV per decade.  The calculated corrosion rate is quite small (about 10^-3 mm/year) because of the very small I_corr (about 10^-10 Amps/cm²).

 
 

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Last revised on Wednesday, December 20, 2006