Gas station without pumps

2014 April 24

Stainless steel electrode analysis

Filed under: Circuits course — gasstationwithoutpumps @ 00:20
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After yesterday’s lab I wrote

I do have to get the students to start lab more efficiently. Once everyone had their setup built, they took measurements fairly quickly, but they came to lab with no schematic of their test circuit and no table set up for recording their measurements. The first hour of lab was wasted by almost everyone doing the pre-lab work that they should have done over the weekend and asked about in class on Monday.

I started out today’s lecture talking about that—how everyone was very efficient in the last hour of lab, but how it had taken them forever to set up, because they hadn’t come to lab with their schematics already drawn, ready to build, nor their data tables set up, ready to fill in.  I’ll see if they are more efficient in Thursday’s lab.

I also wrote

Tomorrow I’ll spend some time helping them write gnuplot scripts to model their impedance data.  I’m assuming it will look a lot like the data I collected last year, which means that the conventional model of polarizing electrode will not fit all that well.

I hope that we also have time for some complex impedance and voltage divider problems, so that they have a little more practice before Friday’s quiz (which I still haven’t written).

We ended up spending almost the entire hour looking at gnuplot scripting and model fitting.  In fact, the data did not fit the polarizing electrode model ((R_{surface} || C_{surface}) + R_{bulk}) at all well, though the students got pretty clean data from 10Hz to 300kHz. We spent some time talking about whether the model was useful, even though it was as much as 20% off, and looked at a simple power-law model that was even further off.

I showed them how to fit data for log-y plots by using fitting the log of the function to the log of the data. I don’t know how many of them got the point of that, though, as my explanation was not as clear as it might have been, and I saw some blank looks.  I’ll probably want to revisit that next week, with a clearer explanation, when we do a similar measurement lab for the loudspeaker.

The problem with the conventional polarizing model is that it predicts a fairly simple Bode plot, with a constant resistance at high frequencies (Rbulk) and at very low frequencies (Rbulk + Rsurfacec. The data actually has no flat spot at frequencies as low as we could measure,
and impedance is still dropping, though slowly, at the highest frequencies we could measure.

I’m wondering whether it is worth setting up the PteroDAQ measuring system to look at much lower frequencies, to see whether impedance flattens out at a much lower frequency than the RMS voltmeters are good for.

Another possibility is try to set up a circuit to measure phase shifts, so that we can get a better idea of the complex impedance, rather than trying to infer the complex impedance from the change in its magnitude with frequency. Both of those are going to have to wait, though, as I have to write the quiz for Friday, provide feedback on some grad student papers, grade this week’s lab, grade the quizzes, and write an all-new lab handout for transimpedance amplifiers.

Actually, I’m thinking that we should do the simple one-stage or 2-stage audio amplifier first, then the transimpedance amplifier for the optical pulse monitor.  So probably this weekend will be on rewriting the audio amplifier lab.  I’m a bit torn, though, as it would be good to do the audio amplifier and power amp back-to-back, particularly if we could do the 2-stage op-amp version in one day and spread the class-D amplifier over three days.  Oh, well, I don’t need to decide until this weekend—I’ll be too busy with the quiz before them to re-order the schedule.

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