I'm not sure the exact cause of the "hairyness" the CV's. I do know that it only seems to occur when using certain electrochemical setups. We don't see it with purely resistive loads such as dummy cells or even simple dummy cells with combined resistive and capacitive loads. You can do CV sweeps etc. and no hairiness . So it isn't really a matter of whether or not the potential is fixed - maybe it is some characteristic of the load in the cases where we see the hairiness. It would definitely be interesting to explore the issue. I've been considering designing some more general dummy cell PCBs for testing purposes and I would love some input on designs. It would be great to have a very general dummy cell design which could mimic all sorts of loading situations. In particular, could we find a dummy cell with the correct load characteristics to reproduce the issue? or is this really something to do with the electrochemical cell?
The capacitor in the feedback loop of capacitor of the control amplifier is a compensation capacitor used to improve stability. It is basically providing a high-frequency bypass. The load which the potentiostat is going to be hooked up to is essentially unknown (lots of different variations) so we wanted the control loop to stable under a broad range of conditions. Adding this compensation capacitor was found to help extend in stability over a broader range of conditions without causing a detrimental effect. I don't believe this capacitor is the cause of the hairiness you are seeing.
The compensation capacitors on the transimpedance amplifiers are also for stability. The values have been selected fairly conservatively. So we are probably getting a bit more lowpass filtering then absolutely necessary. For the most part this is more of an issue for the lower current ranges where the resistor in the feedback loop is larger. We've made some nA versions of the Rodeostat where we had to go to larger capacitor values in order to ensure stability. For example, for a +/- 60nA range with a 10MOhm resistor we needed a 100nF compensation capacitor for stability. Again I don't think these capacitors are the issue.
I would recommend running the cyclic voltammetry experiment from 0.1 V to 1.0 V, at 50 mV/s for a few cycles (3-4 should be plenty). That would be a direct comparison to the experiment I described.
Let me know how that experiment works and we can go from there.
The colorimeter footprint is 15 cm x 9 cm. Height of the assembled colorimeter is 5.8 cm tall (or 6.2 cm with the rubber feet).
Hope that helps. Let us know if you have any other questions.
So, yes, you do need to do a calibration each time you start a new measurement.
The calibration measures the light intensity that reaches the sensor after passing through the cuvette and your blank sample -- which should not be absorbing any light due to the substance you are measuring e.g. nitrate.
The calibration is used as a reference when calculating the transmittance of light through your actual sample.
For the API nitrate test, a 'blank' sample would be 5 mL of distilled water (zero nitrate) developed with the test kit i.e. 10 drops of bottle 1, 10 drops of bottle 2. The color should be yellow like the image shown below. You would pour some of this into a cuvette and calibrate the colorimeter.