Gas station without pumps

2013 January 24

Electrode lab went well

Filed under: Circuits course — gasstationwithoutpumps @ 23:28
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Today’s lab, the electrode lab,  went fairly well.  One group managed to finish in about 2:40, and most were done in 4:30 (still a bit too long for a 3-hour lab, though).

Some things that worked well:

  • Everyone kept their salt water in cups in the secondary containment—there were no spills.
  • Everyone set current limits on the power supplies for the electroplating—there were no new fuses blown in ammeters (though we had to replace 2 fuses at one station that had been blown by the EE students, who do not seem to get proper instruction in the care of the multimeters).
  • I showed everyone who cared how to use a micrometer and vernier calipers—I’ll have to remember to bring the micrometer in again next week so that students can measure the thickness of the packing tape for the capacitance touch sensor.
  • Everyone took lots of measurements (they should have 7 sets of about 25 points each, each point consisting of a frequency and two AC voltage measurements).
  • Between answering questions, I managed to do continuity tests on all the analog oscilloscope probes, finding 6 or 7 with broken ground wires and sending them back to the lab support staff.  There were enough good probes that I could equip all the analog oscilloscopes with two probes, and still have a few left to lock in the cabinet.
  • Several students noticed that the readings they got for the Ag/AgCl electrodes did not vary as much as with the stainless steel electrodes.  They were worried that they were doing something wrong (a reasonable fear with so much unfamiliar stuff), but this is precisely the observation I was hoping students would make—the reason that we use Ag/AgCl rather than stainless steel electrodes is that they are non-polarizable and do not change characteristics with frequency (at least not much—they aren’t an ideal non-polarizable electrode).
  • The electrode holders I made with the laser cutter worked well, and no one dropped and broke any of them, so I still have enough for next year.
  • We had enough of each of the stock NaCl solutions (2 L each of 1M, 0.1M, and 0.02M NaCl) that none of them ran out and there is enough left over so that students who did not complete the lab but need to work this weekend or on Monday should still have enough.
  • The multimeters could not provide steady readings at 3Hz, demonstrating clearly to the students the low-frequency limits of the meters. Students have a tendency to believe that the expensive lab equipment should be able to do everything by magic, so running into the limits of the meters is important experience for them. Unfortunately, exceeding the high-frequency limits of the meters does not cause obvious problems, just subtly wrong readings, so I did not have them push the high frequency limit.

Some things that did not work so well:

  • I had another moment of culture shock—not quite as surprising as finding that half the class didn’t know how to open a command line window in the first lab, but perhaps more disturbing.  All the students in the class have had 6 college chemistry courses (3 general chem, 2 o. chem, and 1 biochem), which is far more chemistry than I’ve had, but most of them knew nothing about electroplating, and did not understand how they were to get silver chloride onto their silver wires.  It only took a couple of minutes for me to ask them what happened to the Na+ and Cl ions in solution when we put a voltage across the electrodes and to write the  Ag + Cl → AgCl + e reaction on the board, but I was really surprised that electroplating had never come up for them before.  I learned about it in grade school, I think, though with copper salts and copper plating.
  • Several of the students still wanted to connect ammeters in parallel with what they were measuring, rather than in series. Luckily setting the limits on the power supplies had been drummed into them enough that they did not blow any fuses, but it was a close call.
  • The lab is a little too long.  I think that next year I’ll split it into 2 2-hour labs.  The first lab would just characterize the stainless steel electrodes, then we would analyze that data in class, then the next lab would make and characterize the Ag/AgCl electrodes.
  • Some of the students ended up with only one of the wires well-plated with AgCl.  I think I need to improve the instructions, as they need to put a double-thick layer on one electrode, then run the current for half as long in the other direction, plating the other electrode while stripping half the plating off the first electrode. No one was timing the plating, which limited their ability to apply this strategy.
  • Students are still not very good about putting the circuits they are using into their lab notebooks.  Given that many of them can’t seem to hold simple circuits in their head well enough to wire them correctly, it is doubly important that they have good written records of their circuits that they can compare their wiring to.  They’ll certainly need to improve their sketching of schematics by the time the op-amp, power-amp, and instrumentation amps come up, as those circuits have many more connections than the ones they’ve been doing so far, and really benefit from careful wiring from a clean schematic.
  • The students needed the surface area to figure out the electroplating current (about 1mA/cm2), but several students were unable to compute the surface area of their silver wire given its length and diameter, computing the volume instead.  They did not seem to be aware of a problem even when the number they came up with was off by a factor of 100 or more (either from computing the wrong number or computing the surface area in square millimeters). Sanity checking of results seems to be a foreign concept, not part of their standard repertoire, which is worrisome in senior engineering students. I’ll do what I can to drum it into them this quarter, but it usually takes years of doing sanity checks before it becomes unconscious and automatic, so I fear I may be four years too late.

This lab is the last “wet” lab of the quarter (not counting the EKG lab which has the electrolyte on the stick-on-electrodes), so after Monday I can put away the secondary containment tubs and relax about the risks of someone spilling salt water into the electronics. There will still be hazards, but they are the familiar ones of an electronics lab (burns from soldering irons, blowing fuses, blowing up electrolytic capacitors, … ), which will not bring down the wrath of the lab support people nor destroy expensive equipment.

Next week’s lab is the hysteresis lab, which is one of the few “light-weight” labs. There is not extensive measurement (just a few frequency measurements) and the design is only picking two parameters (a resistance and a capacitance). They will have to breadboard a circuit then solder it up on a PC board, and I expect the soldering will take them some time, as it will be the first time for most of them. I’ll have to remember to bring in Al foil, packing tape, scissors, spare copies of the hysteresis oscillator board, some spare Schmitt trigger chips, and perhaps a couple of board holders (my Panavise and my old alligator-clip board holder). I expect that everyone will be able to finish this lab on time, even though I will insist that everyone solder their own board (not just one per pair of partners) and demonstrate it working.

Tomorrow in class I plan to look at RC voltage divider circuits, plotting their amplitude response versus frequency using gnuplot. I’ll introduce the notion of Bode plots and RC time constants, to make it easier for students to visualize the plots without needing to run gnuplot every time. We’ll also talk some about stainless steel (what makes it a good material for implants but a poor one for electrodes) and what electrodes are popular for bioelectronics. Oh, and I need to explain RMS voltage to them, since that is what they measured with the multimeters, but we’ve only talked about amplitude, not peak-to-peak voltage or RMS voltage.

1 Comment »

  1. […] Yesterday I wrote my plans for today: […]

    Pingback by Eighth day of circuit class « Gas station without pumps — 2013 January 25 @ 22:35 | Reply


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