Lab 4: Heat Transfer

In lab 4, you'll build a thermally insulated box according to some variety of possible configurations, drive a power load inside, and measure temperatures to understand the magnitudes of various heat flow mechanisms.

First, figure out which configuration you want to pursue, and sign up at the sign-up sheet on the front table of MHA 3574 near the supplies.

Decide how you want to proceed. Does it make sense for you to have a phase A (as a control measure) before implementing the full scheme? Can you apply the foil later? Can you install a fan but not turn it on at first? Best to figure out the plan before beginning construction.

Construction and Setup

  1. The nominal box has a 4×4 inch inside, with 1-inch thick insulation. A couple of 7×7 boxes are also prepared. You will need to tape your box together, inside and out, being careful to leave no gaps. Tape the inside seams as you go, to allow easier access.
  2. For boxes with reflective surfaces, use shiny tape. Be careful not to extend tape through gaps to the outside, or you will have a cold finger.
  3. Solder leads onto the power resistor, deciding whether you want to attach the fan in parallel (in which case solder also to resistor), or keep separate for independent power control.
  4. Secure the fan in place (preferably on the top lid) with tape. Make sure the tape has a large contact base with the foam, otherwise it may drop off during the experiment.
  5. Check that the RTDs you have are well calibrated to each other by pushing each onto a large block of metal (preferably at the same time). Don't use your finger for the pressing, or anything thermally conductive. Note any offsets, and keep track of which is which.
  6. Make sure your Python program for recording time stamps and RTD channels is working correctly. Once you start the thermal campaign, any mistakes might take a long time to thermally settle out back to the starting point. Be completely happy that this is ready to go: you get reasonable temperatures, and the right cadence.
  7. Secure the internal RTD(s) with tape. Ideally, the internal RTD will be at the fan exhaust for the most "representative" measurement. Label the wire/connection end of the of the RTD according to its function.
  8. Run all the wires out (perhaps at a single feed-through point) and secure the lid to the box. Attach the ends of the RTDs to the current sources, either on the breadboard or the packaged 4-channel unit.
  9. Construct a stand using the boards and dowels in the supply area: the box should have all sides open to the air.
  10. Establish the power supply to deliver the appropriate voltage(s) before actually hooking up to the internals. You don't want to start heating up the system until you're ready for the data campaign. The fixed 5 V supply may be all you need (make sure you can measure current from this supply if you decide to use it). Make sure the current limit knob is turned up (about halfway) so that it won't wimp out when you turn on the system. Pay attention to the fact that power, P = V2/R in establishing your operating point.
  11. When you think you're ready, take an initial suite of RTD readings (everything should be steady, so timing is not critical for this measurement). Turn on the power supply briefly (2 sec) to check that A) the voltage does not drop, and B) the light stays in constant voltage (CV) and does not switch to constant current (CC). If the latter happens, either you have a short or the current knob is not turned up high enough. Repeat these short burst experiments until you're steady. Once steady, start the Phython program and turn on the power supply for goot to begin the data campaign.
  12. Note the voltage(s) and current(s) as presented by the power supply.
  13. Do not at any time exceed 1300 Ω (1.3 V) on the RTD: this means you are approaching 80°C. Stay below this temperature!

During the Data Campaign

Beyond Equilibrium

Once you reach equilibrium, measure the skin temperatures (maybe top, a side or two). Press the RTD to the skin with a thermally non-conductive item.

Now you can go to a second configuration if appropriate. Turn on an external fan; coat the outside with aluminum foil; compromise the insulation by letting the top loose, poking an air hole, inserting a conductive "cold finger" (perhaps some coathanger). Use the opportunity to learn about the ins and outs of thermal control and heat flow. Be creative: you have a little thermal laboratory—what can you learn from it?

Making data available

Submit your raw data (times, RTD resistances) to me via e-mail. I will make these data available through this link to all. The most universal and appropriate format is comma-separated-value, or CSV, which almost every spreadsheet program can produce and read (also plain-text-readable). Include information on any RTD offsets, clearly identify the columns in your data, provide all supplementary information such as: your names, configuration, voltages/currents, occasional RTD measurements (e.g., skin temperature), etc. Imagine someone wants to use your data to construct your lab report: what info do you have that they need.

This way, groups can make meaningful intercomparisons. Probably the most relevant info will come from understanding the equilibrium temperatures established in each configuration. But making the whole series available may be of some utility.

Lab 3/4 Write-up

In the write-up:

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