Gas station without pumps

2012 June 20

More musings on circuits course: temperature lab

I’ve decided to do a lot of my musing about the course design on my blog, so that others could contribute to the course design (or at least participate vicariously).  I think that having an open log of our thinking on the course design will be useful to grad students or new faculty who are having to do their own course designs, to show how us old farts do it (whether they wish to copy our methods or avoid using them).

The relevant posts so far are

Changing teaching plans
More on electronics course design
Yet another project idea
Another way to think about course design

I’ve got my son working on Arduino software to act as a data logger, so that students can have an easy-to-use tool that requires no programming, but which is easily modified if they want to do their own programming.  He has Arduino code for alpha testing already, and I think he’ll have a user interface ready for beta testing by the end of the week, but he’ll only have tested it on a Mac—we’ll need to get him access to a Windows machine to do testing there, because there are some operating system dependencies in talking to USB devices like the Arduino from a Python program. He’s been consulting with me on desired specs for the data logger, but doing all the coding himself. It is good practice for him both in interrupt handling and in user-interface design. He’s also having to think about portability for the first time, as the data logger has to run on Windows and Mac OS X (I think that Linux users should have no trouble with the version that works on Mac OS X, but we probably won’t be testing it). He’ll have to write user documentation and installation instructions also. Some of the packages he likes to use (like PyGUI) are enough of a pain to install that he’ll probably provide a stripped-down interface without a GUI for those who want to do minimal installation (Arduino software, Python, and PySerial are only essentials).

The first lab I’ve been thinking about is for temperature measurements. For the small temperature ranges normally needed in biosensor applications, the sensitivity, low thermal mass, and low cost of thermistors make them probably the best approach, and I think that they are what are used in the cheap digital oral thermometers sold in drug stores.  I wonder what techniques the manufacturers use to get medically acceptable calibration for those devices.

I’ve been thinking about the thermistor lab—it seems like we could do that in stages: first just reading resistance with a meter, then adding a series resistor to make a voltage output, then adding a parallel resistor to try to linearize the exponential curve a bit, finally adding larger series resistors and an amplifier to avoid self-heating currents through the thermistor and to allow nearly the full range of the ADC to be used.  This series of lab exercises could be spread out over the quarter, as students learn more about circuits.

For them to calibrate the thermistor, we could use hot and cold water baths, if the thermistor was in a waterproof package. From what I can see, that raises the price of a thermistor with leads from about 25¢ (for something like the NTCLE100E3333JB0 by Vishay BC Components) to $1.55 (for USP10982 from US Sensor) or $3 (for USP10973RA from US Sensor).  [Prices from DigiKey 10-unit price.]

I think that having the students do their own waterproofing is probably not a good idea.  Potting components gets to be a mess, and adding a large blob around the thermistor will slow its response time a lot.  I wonder whether using 5¢ disposable thermometer probe covers would work, or whether they tear too easily.  I probably need to look at some at the drug store, to see whether there is thicker one on the market that the cheap thermistors would fit into.

If waterproofing a cheap thermistor turns out to be too difficult, we need to think about whether to use the more expensive parts or work out some cheap, measurable temperature sources that are not wet.  We could make something like is used in PCR machines, with a couple of blocks of aluminum and a Peltier device, but I don’t think that the price is worth it—better to use the sort-of waterproof probes, a cup of water, and a thermometer.

I’ve noticed that for some applications, people are choosing voltage-output temperature sensors that rely on the thermal
coefficient of a transistor, rather than on a thermistor, like the MCP9700-E/TO from Microchip Technology (25 cents).  They have a fairly linear 10mv/degree C output, but their absolute accuracy is even worse than thermistors. These may be a better choice than thermistors in many applications, but would not provide the same teaching opportunities for a circuits class.

Using a thin-film platinum resistor temperature sensor (RTD) like the US Sensor PPG102C1RD would allow more accurate temperature measurement without calibration. With calibration, RTDs can be the most precise electronic temperature sensors, though I don’t know if the high precision is available in thin-film resistors, or only in the more expensive wire-wound ones. I suspect that repeatability from part to part is higher in the wire-wound RTDs, but that the thermal coefficients are the same, so that calibrating at one temperature should give about equal accuracy either way.

The naturally linear response of RTDs (100Ω at 0°C and 138.5Ω at 100°C in a very straight line) does not lend itself to as much circuit design as thermistors. On second thought, converting the 3850 ppm/°C resistance change into a voltage range readable by the Arduino ADC is not a bad circuit exercise, particularly if self-heating is to be avoided, though it is not as difficult as flattening the highly nonlinear response of a thermistor.  The biggest problem with RTDs is their price: at over $10 for a bare sensor they may be too expensive for class use.

Another possible temperature sensor is a thermocouple, which generates a voltage based on the temperature difference of two electrically connected junctions of dissimilar metals. One article in the engineering toolbox claims that thermocouples are cheap and RTDs expensive, and I think that is true if you are looking for high-temperature devices (like thermocouples for detecting pilot lights in furnaces), but not so true if you are looking for careful measurement in corrosive wet materials (like most biosensing applications). See Choosing and using a temperature sensor for more info comparing RTDs and thermocouples. Thermocouples have relatively low precision and sensitivity, and they measure only the difference in temperature between two points, and so are probably not very interesting for biosensing.

Action plan for testing out a temperature measurement lab:

  • Get some thermistors and some thermometer probe sheaths and see if I can make adequate temporary waterproofing for pennies per student.  I’ll probably have to solder on wires to lengthen the leads.
  • Try calibrating thermistors using a multimeter, cups of hot and cold water, and an accurate thermometer.
  • Try reading the thermistor using a voltage divider and the Arduino ADC.  Plot the temperature and Arduino reading over a wide temperature range (say, as a cup of boiling water cools).
  • Try linearizing the thermistor readings  using a parallel resistor and voltage divider.
  • Try designing an amplifier to read the thermistor with much lower current through it (and so less self-heating).

 

 

3 Comments »

  1. […] thermistors that I ordered (see More musings on circuits course: temperature lab and Buying parts for circuits course) arrived today, about 2.1 days after I ordered them.  So […]

    Pingback by Temperature lab, part2 « Gas station without pumps — 2012 June 23 @ 22:52 | Reply

  2. […] the thermistor lab seems to have worked fairly well (see More musings on circuits course: temperature lab, Buying parts for circuits course, Temperature lab, part2, and Temperature lab, part 3: […]

    Pingback by Op-amp lab « Gas station without pumps — 2012 June 25 @ 17:39 | Reply

  3. […] More musings on circuits course: temperature lab […]

    Pingback by Order and topics for labs « Gas station without pumps — 2012 August 16 @ 23:38 | Reply


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