Current Ranges- Fixed vs. Autoranging

Gamry potentiostats have either 9 or 11 current measurement ranges.  The value of the current range indicates the maximum current that the range can measure.  For example, these are the current measurement ranges of a Reference 600 potentiostat:

With a total of 11 current measurement ranges--with decadal spacing--and the ability to offset and gain on the Reference 600, it is possible to obtain a very high resolution over an extremely wide current range.  The current range can be dynamically switched during an experiment, and the potentiostat is said to be in autoranging mode.  Autoranging will adjust the range so that the measured current is always within the capabilities of the instrument.  Each range has a settling time associated with it, and the settling time is longest on the most sensitive ranges.  Generally, most corrosion experiments are run at slow scan rates (~0.187mV/s) and long sampling periods (~2s) so settling time does not affect experiment stability.  For this reason, all DC105 experiments default to autoranging mode.

Physical electrochemistry experiments are generally run with scan rates that are many times faster, and current range settling time can cause instability.  In these cases the instrument must be used in fixed ranging mode.  In all PHE200 and PV220 experiments the user has the ability to select which range mode is used via the I/E Range Mode setting.  The box directly below the I/E Range Mode selector is the Max Current parameter.  The user should enter the maximum current value that they expect to see for the experiment, and the instrument will lock into the next highest current range.  In cases where the user is not sure about the max current—a fresh CV scan perhaps—it is suggested to run the experiment first in autoranging mode, observe the maximum current, then run the experiment again in fixed range mode with an appropriate Max Current entered into the parameter box for best results.  In cases where autoranging is required but undesirable effects are noticed, we have the ability to make software modifications that can improve the result.  Contact Gamry for details.