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electrode lead. During Low I DC Calibration, the first two terms are directly measured in order to improve
picoampere 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 600+ current ranges: the 60 pA and 600 pA ranges.
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 600+ applications are sensitive enough that this current-measurement offset causes
problems:
• Physical Electrochemistry (Cyclic Voltammetry for example) with small electrodes
• EIS on high-impedance samples such as barrier coatings.
Incorrect DC-current readings in EIS can slow the experiment, because the automatic ranging algorithms in
the EIS software can make poor choices regarding ranges, giving 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 includes a special “Low I DC Current” calibration procedure that corrects the
Reference 600+ 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 amplifier input currents on the 60 pA and 600 pA
ranges,
3) Replaces the 60 pA and 600 pA current range offsets measured in the full DC calibration with
these improved values.
The error sources listed above are both time- and temperature-dependent, so we recommend frequent
“Low I DC Calibration”—if you need accurate measurement of absolute currents at pA levels. The
procedure runs fairly quickly, so daily or weekly calibration ought not be too inconvenient.
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