Advanced Measurement Technology 273A, User Manual

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Model 273A Potentiostat/Galvanostat User’s Manual
Fig. 6. I Monitor Waveforms as a Function of R Compensation
U
(Square Wave Applied).
is optimum for fast measurements. As the compensation is increased to the 95% to 100% range,
increasing ringing appears as the system becomes more unstable. This level of compensation is
not suitable for fast measurements. Any data taken during the ring-down will be of doubtful
validity. Also, even a slight increase in compensation from this point (or a real decrease in the
actual R ) could cause the potentiostat to oscillate. Compensation in the 95% to 100% range is
U
best suited to slow experiments (corrosion). Since Current Interrupt Compensation is ideal for
slow measurements, there will normally be relatively few instances where Positive Feedback
Compensation in the 95% to 100% range would be used.
When you are satisfied with the compensation level, turn off the cell and do the Scan Setup
appropriate to the intended measurement. Unless changed, the set compensation level will
remain in memory, ready to be applied whenever POSITIVE FEEDBACK is activated.
Current Interrupt IR Compensation
In this technique the current is periodically interrupted and a correction function developed that is
based on the time versus potential change that occurs at the moment of interrupt. Specifically,
the cell is disconnected (in less than 1 µs) for
190 µs at periodic intervals. (As fast as the
interrupts are, they nevertheless do affect the experiment, and a small artifact may be noticed in
the output for each interrupt.) At disconnect, a drop in the Electrometer potential equal to iR ,
U
occurs.
Thirty-two potential points are then taken at 5 µs intervals. Two of these points are used to define
a projection that is extrapolated back to the instant just after current interruption. By comparing
the potential at that instant with that before the interruption, a correction function is developed
that corrects for the uncompensated IR drop in the cell.
Several functions (see Section 5.2.4) are involved in setting the Current Interrupt parameters.
FUNCTION 13 sets the interval between current interrupts. The range is .004 s to 30 s, with a
default setting of 1 s. (In doing Curve Acquisition with the 273A controlled from a host computer,
the interrupt timing is controlled by the computer.) FUNCTION 17 sets the percentage of
correction, adjustable from 0% to 200%. FUNCTION 18 selects the first of the two extrapolation
points. The range is 2 to 1997 µs. The default selection is 10 µs on the 1 A and 100 mA Current
Ranges, and 75 µs on all other Current Ranges. FUNCTION 19 selects the time increment used
to determine the second point. The selection range for the increment is also 2 to 1997 µs. The
default selection is 10 µs on the 1 A and 100 mA Current Ranges and 75 µs on all other Current
Ranges.