The lab for characterizing the thermistors went well—more smoothy than the last two years. It may be because the lab is now in the second week, rather than the first week, or it may be because I required prelab homework be turned in on Monday, so that students came to lab already prepared, or it may just be that this year’s class is more prepared than previous ones. I suspect that the main difference is in the prelab requirement, but that may be because that is about the only thing I can really control.
I went into the lab an hour early, to set up the coffee urn with hot water, get ice from the ice machine in the lab upstairs, and check each of the thermometers in ice water. The cheap student glass thermometers all ready between -1°C and 0°C, but several of the digital thermometers were way off (reading as much as 4°C). I separated out the digital thermometers that read over 0.5°C and marked them with blue tape as miscalibrated. (I can believe temperatures slightly below 0°C, since the ice water was tap water, not distilled water, but temperatures over 0°C were almost certainly wrong.)
I started by showing the students how to interpret the part number for the NTCLE428E3103F520L thermistor we were using, since half the class had not figured out that it was a 10kΩ thermistor on the homework. The data sheet is rather tricky, but in a common way: it is the data sheet for a family of parts, with a key for decoding the part number.
The students managed to collect 20–90 data points of temperature and resistance, with varying levels of noise. I had them plot the data after they had collected some points, and suggested to students that they fill in more points where they had gaps in the curve, or where they had “bumpy” spots that did not fall along a smooth curve (suggesting measurement errors), and pushing them to get lower and higher temperature measurements. Students with good lab skills managed to get a lot of data points from 2°C up to 69°C. The water in the coffee urn was a bit hotter than that, but by the time they got the water back to their benches and made measurements it had cooled off. It might be interesting some year to have a hot plate at every bench to let them measure up to 100°C, but I don’t think that the expense of the hot plates would be justified for this one lab—and I’d worry about possible spills.
I was worried enough about spills on this lab, reminding students frequently to carry the secondary containment tubs in both hands, and to put it flat on a level surface whenever they weren’t carrying it. As it turns out, about the only real spill was mine, trying to pour ice water from one thermos to another, and getting a chunk of ice splashing out. The spill was small (a few cubic centimeters of ice) and not near the lab equipment, and I could mop it up with a couple of paper towels. I was probably made a bit clumsier than usual by having only gotten 4 hours sleep last night (between grading prelab homework and adding material and exercises to the chapters of the book that the students need to read for Friday and next Monday). Still I felt bad about being the clumsy one in the room.
There were two glass thermometers broken—one was taken in its case by a student to be properly discarded in a broken-glass disposal bin, but the other was just quietly returned to the bin of thermometers. I have a lot more respect for the student who reported the broken thermometer than the one who tried to hide it. The thermometers themselves are cheap (about $3), so the breakage doesn’t bother me, but leaving problems for someone else to deal with does.
I had a few of the students send me data, which I will use in class today to show students how to use gnuplot to fit models to data. I also plan to go over the homework problem that only one student got—the optimization to maximize the sensitivity of the voltage divider at a particular operating temperature.