Chapter 4 -- Installation--Calibration
4 - 15
Potentiostat calibration is only required infrequently. You should recalibrate your Reference 3000 under the
following circumstances:
• It has been about one year since your last calibration.
• Your potentiostat has been serviced.
• You notice breaks or discontinuities in the data curves recorded with your system.
• The system is being run in an environment that is very different from the previous operating
environment. For example, if the Reference 3000 was calibrated at 15 °C and you are now
operating it at 30 °C, you should recalibrate.
Low I Range DC Calibration
The standard Reference 3000 calibration is performed with the cell leads connected to a 2 kΩ resistor. During
the calibration procedure, DC current range offsets are recorded with the cell switch turned off. A DC current
measurement is made on each of the 11 current ranges in the Reference 3000. The measured current on each
range is the sum of current contributions from:
• The input current of the I/E Converter input amplifier,
• the input current of the Working sense input amplifier,
• the input current of the Reference input amplifier,
• the input current of a Counter Sense input amplifier,
• current leakage in the cell switch.
In most real-world experiments, the cell is turned on, and the I/E converter does not measure the last three
terms listed above. These currents still exist, but they are generally sourced by the low impedance counter
electrode lead. During Low I DC Calibration the first two terms are directly measured in order to improve pA
accuracy of the instrument.
The current contributions from each source on the above list are (at most) a few pA, so they are insignificant on
all but the most sensitive Reference 3000 current range – the 300 pA range.
This effect can cause differences of up to 8 pA between DC current measured with the cell turned off and
current measured with the cell turned on. In most cases the difference is smaller – one or two pA.
Only two Reference 3000 applications are sensitive enough that this current measurement offset causes
problems. One is Physical Electrochemistry (Cyclic Voltammetry for example) with small electrodes and the
second is EIS on high impedance samples such as barrier coatings. Incorrect DC current readings in EIS can
slow the experiment, since the automatic ranging algorithms in the EIS300 software can make poor range
choices given incorrect data. This can significantly lengthen the time needed to measure an EIS spectrum.
Most corrosion experiments or macro-electrode measurements involve currents much too large to be affected
by this difference.
The Gamry Framework now includes a special “Low I DC Current” calibration procedure that corrects the
Reference 3000 offsets to minimize this problem. The procedure uses a script that:
1) asks you to disconnect the reference, counter, and counter sense leads from the calibration cell,
2) measures the I/E input and Working sense amp input currents on the 300 pA range,
3) replaces the 300 pA current range offset measured in the full DC calibration with the improved value.
To Next Page
Loading...
