Cloud Chamber Cooling Analysis Heather B. Brown December 4, 2006

Motivation From experience, we know that the bottom of the chamber must be cooled to a rather low temperature, generally as cold or colder than dry ice (-70 deg C). Dry ice is easy to acquire but entails maintenance every few hours and does not provide a flat surface. Since chambers have been made successfully and consistently with dry ice, the next step is to devise a perpetual cooling system to provide constant entertainment. The continuous cooling would ideally be provided indirectly through an electrical outlet.

Thermoelectric Module (TEM) How they work Thermoelectric modules are solid state devices (no moving parts) that convert electrical energy into a temperature gradient. They are inefficient and little power is produced. They are typically 1.5 inches square (40mm x 40mm) or smaller and approximately 0.25 inches (4mm) thick. 1. 2. 3. 4.

Initial heat sinks used to cool the ceramic hot side The hot side was the motivation for heat sinks. Theoretically, with a Delta T of 70 deg Celcius, cooling the hot side would further cool the cold side. The heat sinks are the same length and width as the TEMs. Ice water was circulated through the heat sinks as Figure 1 shows to cool the ceramic hot side of TEMs. Result: These heat sinks did not have enough cooling power needed for the TEMs (cool side reached maximum -11 deg C). Figure 1. Schematic setup with TEMs Figure 2. Top and bottom views of heat sink.

Heat Sink / Fan Design A CPU cooling fan was purchased from Fry’s to cool TEMs. This heat sink / fan combo consumes 2.4W of power and has optimum operation at 12V. Its dimensions are 83 X 73 X 61 mm. Result: This attempt at cooling the ceramic hot side was the worst. The lowest temperature reached with the TEM was +15 deg C. Figure 3. Upside down view of the “Copper X478.” The TEM hot side sits on the copper side.

Solid Copper Heat Sink Design Purchased from the same company as the TEMs so we were hoping for better results. Was hooked up directly to our water pump with rubber tubing. The dimensions of this all copper liquid heat exchanger are 89 X 64 X 12.7 mm. This was the first all copper heat sink we used. Result: It did not transfer the cold from ice water as well as we needed. The minimum temperature reached with the TEM cold side was +5 deg C. This was the end of our TEM usage. Figure 4. (Top) All copper constructed liquid heat exchanger made by the same company that sold us the TEMs. (Right) Yay! No more TEMs!

Liquid Nitrogen Cooling Purchased from MSC distributors, this all aluminum, 2-fin system provides better heat exchanging than copper. The ‘recommended’ liquid to be used with this plate is ethyline glycol (typical antifreeze for a car or CPU). Dimensions are 279.4 X 198.12 X 19.05 mm. We glued polyethyline tubing into the fittings and tested ways to create a flow of liquid nitrogen through the cold plate. This was by far the most expensive item purchased for the cooling team. Figure 5. All aluminum cold plate used with liquid nitrogen testing.

Liquid Nitro Experiment Competency We achieved the lowest temperatures experienced yet (avg. -18 deg C) in various placements of the cold plate (i.e. the plate exhibits the same behavior in many different positions). Figure 7. Horizontal transfer of liquid With a direct flow of liquid nitrogen into the plate, the temperature went below -70 deg C (thermometer’s measuring limit). Figure 6. Vertical transfer of liquid

Liquid Nitro Experiment Incopetency Proper fittings and funnel would reduce or eliminate the leakage found at the unification of tubing and cold plate and provide a safer method of transferring the liquid nitrogen. We now understand that the cold plate must be cooled to roughly -60 deg C before trying to circulate the liquid nitrogen due to the plate being too warm and rejecting the liquid. The pump used was made for 3 V but needs at least 20 V to work constantly with the liquid. Figure 8. Unsafe method of transferring liquid

Conclusions TEMs are completely incapable of providing a low enough temperature. Liquid Nitrogen was definitely the best method used so far because of the temperature results attained. A more independent circulating system would need to be devised to continue using the liquid nitrogen. A manufactured chiller would be the best idea.

Future Plans www.thermo.com has many types of chillers, circulators and baths. One example of a circulator/bath combo is the Neslab ULT-80 and it operates from -80C to +10C. Ultimately, a similar apparatus would be the most effective for achieving our desired temperatures constantly. Figure 9. Neslab ULT-80; Work area (L X W X D) in cm is 13.7 X 17.8 X 24.1; weighs 336 lbs.; 4 gallon bath; cooling capacity 250W at -70C; costs $13,533 + Tax