In March we received lots of great questions from the community! These were submitted to firstname.lastname@example.org or via our Contact Form. We are sharing four of these along with our responses this week. We will be compiling these and other frequently asked questions into a web page shortly.
Question 1 - Rodeostat Potentiostat Max Current Range
Yes the maximum current range for our standard Rodeostat is ±1mA . We also have a new higher current version with a maximum ±10mA range. However, both have limits. There are a couple of aspects which determine this limit.
First, the transimpedance amplifier converts the current at the working electrode to a voltage which is proportional to the current. Subsequent amplifier stages then scale and shift this voltage to a range which is suitable for an analog input (ADC) on the microcontroller. The analog ADC has a finite (voltage) range and we want to make the best use of that range. So for a given current range, say - 1mA to 1mA, the output voltage of the transimpedance amplifier is shifted and scaled to cover the full range of the ADC. However, because of this, currents outside of this range lead to voltages which are outside of the range of the ADC - either too low or too high. The analog inputs are protected so that when this happens the signal is just clipped at the minimum or maximum values. However, it does prevent you from making measurements outside of the given current range.
Second, there is a limit on how much current the control and transimpedance amplifiers can source/sink. The op amps used in these circuits determine the absolute maximum currents for the device. In general if the sole current source from your test cell is the Rodeostat itself (i.e. the Rodeostats counter electrode) then there isn't too much danger in over currenting the device. However, if there is some external source of current then I would try and limit the current to ±10mA.
Question 2: Potentiostat Sample Rate
The "sample rate" determines the rate at which measurements (samples) are sent from the Rodeostat to the host PC. The units are Hz or (samples per second). Note, this value only sets the rate at which data is collected - it does not control how quickly the output voltage is changing. When performing a linear sweep with the web app the "scan rate" in V/s controls the rate at which the output voltage is changing. When using the Python library (http://stuff.iorodeo.com/docs/potentiostat/) the scan rate is set by the (finalValue - startValue)/duration. Note, the tests are implemented in firmware on the Rodeostat so the same underlying test is being run regardless of whether you are using the web app or the python API. The various tests are documented more completely in the python library documentation. For example, the documentation for "linear sweep" test can be found here http://stuff.iorodeo.com/docs/potentiostat/tests.html#linear-sweep-voltammetry and the documentation for cyclic voltammetry can be found here http://stuff.iorodeo.com/docs/potentiostat/tests.html#cyclic-voltammetry. That said, for a given test the input parameters aren't always exactly the same for the web app and python library. However, usually the translation between the two is pretty straight forward - such as with the scan rate in the linear sweep test.
Question 3: Additional Rodeostat functions
The Rodeostat doesn't have this ability built in. It is a potentiostat and controls the potential between the working and reference electrodes. It doesn't have built-in galvanostat functionality. That said if the timing requirements aren't too demanding it might be possible to mimic this behavior in software from manual/direct mode. This would require writing a program in python which implements a little control loop to keep the current fixed by adjusting the output voltage in real-time. There are some limitations as this would have to be done over USB so the fastest you could probably run this loop is about 500Hz and you would have to implement this control loop yourself.
Question 4: Educational Colorimeter Kit Software
The source for the latest version of the Python software for the previous Educational Colorimeter Kit can be found here https://github.com/iorodeo/colorimeter (see the python/Colorimeter sub-directory).
After downloading the repository the software can be installed using pip from the python/Colorimeter directory e.g. something like "python -m pip install ." I've added a "install_requires" to the setup.py file so that pip will automatically download all of the required dependencies when it is installed. Note, You will need a working Python installation. It will work with Python 3.10.5.
Here is a link to the Educational Colorimeter Kit documentation: https://sites.google.com/iorodeo.com/colorimeter/home?ref=io-rodeo-resources