Selecting Potentiostat Current Range & Auto Ranging
Potentiostat Current Range
Using the correct current range is important for getting the maximum benefit from your potentiostat’s capabilities. Ametek potentiostats include auto ranging as an option, which provides high accuracy and resolution, with reduced noise. However, it can cause data spikes, so in some cases manual selection is more suitable.
Blue Scientific is the official distributor for potentiostats, galvanostats and electrochemical testing instruments in the UK and Ireland, from Princeton Applied Research and Solartron Analytical. For more information and quotes, please get in touch:
What is Current Range?
Potentiostats control the potential (voltage) of electrochemical cells, as well as measuring the current response. Current flowing through a resistor is directly proportional to the voltage difference across that resistor, as defined by Ohm’s law (E = IR). The potentiostat forces the current to flow across a resistor (Rm) and measures the current by the resulting voltage drop across Rm.
The current range defines the maximum amount of current that can be measured and the accuracy of the potentiostat as a function of its available ranges.
Instruments from Princeton Applied Research have a variety of current range specifications, and all of them feature auto ranging.
With auto ranging, the ideal range is adjusted dynamically throughout the experiment. The instrument automatically selects the lowest range for which the measured current will not exceed the range magnitude.
This delivers high accuracy and resolution throughout the experiment, with reduced noise. The disadvantage of auto ranging is the possibility of data spikes. These typically occur during fast potential pulse/step or voltammetry experiments, or when using a highly capacitive electrode.
When to use a Fixed Current Range
A fixed current range is recommended for:
- Fast pulse/step experiments
- Scans greater than 20 mV/s
This is a general guideline and not an exclusive rule. The maximum scan and pulse/step rates for using auto ranging depend on the experimental conditions, including the cell’s capacitance. Higher capacitance cells tend to be more likely to produce data spikes.
The best practice is to use auto range whenever warranted. If spikes or overloads affect your data analysis, or if you need “presentation quality” results, repeat the experiment with a fixed range. This can be determined by the initial auto ranging experiment, or sample data at a slower rate.
For some high capacitance samples (eg thick oxide on the surface of an alloy) spikes can still occur even when using a slow rate. In this situation, it is recommended to simply remove the spikes from the data set. This can be performed easily in VersaStudio in the Data menu (Delete Selected Only or Delete All Overloaded).
Auto ranging should always be avoided for:
- Very fast scans >1 V/s – such as the blue scan in the example below
- Pulsing or double‐step techniques eg square wave voltammetry or charge measurement actions; Data gaps will occur due to time losses associated with the range change itself.
Batteries and Fuel Cells
Special care should be taken with experiments on low input impedance cells such as batteries, fuel cells or super capacitors. Low impedance cells are susceptible to fast changes in current, which are orders of magnitude in size, with only small changes in potential. When set to auto range, the potentiostat is measuring this current and trying to use the appropriate range. Large, fast current changes cause fluctuations between current ranges. This results in overloads and unnecessary wear on mechanical and electronic components in the circuit. For this reason, it is best to avoid auto ranging if possible.
Fuel cell experiments should first be conducted using the highest current range (eg 2A). Lower ranges should only be used if appropriate for that particular experiment.
In VersaStudio, the Auto Current Range Setup action in the Advanced Actions tab can be used to set an initial, minimum and maximum current range. This action should be added as a step prior to the experiment action. Further instructions are available in the the VersaStudio manual.
Further background on how potentiostat/galvanostats measure current, with examples of how current range affects data, is available in an an application note from Princeton Applied Research.
Blue Scientific is the official distributor of the full range of Ametek potentiostats in the UK and Ireland, including the new PARSTAT 3000A-DX bipotentiostat. If you have any questions or would like a quote for any of their instruments or accessories,please get in touch: