Sunday, April 12, 2020

Diffusion Cloud Chamber (Part 4)

The Liquid Cooling Plant (LCP) was constructed on the same 1/2" plywood material (10" x 17") as the Cloud Chamber's top surface. 
One of the design goals for this project was a modular system approach. Usually, desktop space is very limited during a Science Fair at my son's school and we wanted to preserve it for supporting materials. By having a physically separate Liquid Cooling Plant, this cooling module can be placed on the floor or somewhere else, out of the way from the Cloud Chamber's Main Unit. The "Static Head lift" of the water pump is listed as 5 meters - more than enough!
Another benefit of a stand-alone LCP is that this module can be used for other experiments requiring liquid cooling - all is needed is another pair of male Quick Connect/Disconnect adapters.

The Liquid Cooling Plant for Stage 4 cooling is using a large, densely finned Heat-Exchanger / Radiator with 18 channels (normally used for Laser or PC water cooling), 3 large fans, a centrifugal water pump, coolant tank, flow meter, two digital thermometers and Quick Connect/Disconnect ports.
All liquid interconnects are done with 3/8" ID 1/2" OD vinyl tubing. 
I had to replace the G1/4 to 5/16" barb fittings for G1/4" to 3/8" fittings on the water pump and flow meter to accommodate the larger diameter tubing. The tubing management is not fantastic - the 1/2" OD tubing and the small footprint of the base made it a bit difficult to route all tubing in a neat manner while avoiding small radius bends.

The fans are configured to suck the air thru the radiator and can be used at the same time to blow air at and cool down the high current Stage 3 (30A) Power supply when placed in front of the fans.
A flow-meter with a spinning propeller gives a visual feedback about the speed of the coolant flow. This housing is also used to accommodate the inlet temperature sensor.

The Centrifugal Water Pump is 12V / 19W unit with a specified flow of 800L/h and Head lift of 5 meters. The pump is gravity fed from a coolant tank just above it. It is configured to pump water into the cooling water blocks and the return line goes thru the flow meter, into the heat exchanger and the output of the radiator is fed back into the coolant tank.

Two digital thermometers display the output and returned water temperatures. The output water temperature is measured at the water tank just before the pump inlet. The sensor for the returned water temperature is located at the flow-meter housing.
The female Quick Connect/Disconnect ports were a bit tricky to mount. We had to fabricate special aluminum L-brackets to mount the assembly firmly in place. I used 3 x 1/4" Female-Female brass couplers soldered together side-by-side in order to mount the Inlet/Output ports on one side and the adapters MIP to 3/8" barb to the other. The whole QC assembly was mounted by threading a long brass screw thru the middle 1/4 adapter (acting as a spacer) and the two supporting L-brackets already mounted on the plywood.
Using Quick Connect ports makes it extremely easy and fast to put the whole setup together and to drain the coolant when done.

The heat-exchanger / radiator  (393 mm  x 120 mm x 32 mm) is attached with L-shape aluminum angle bracket to the plywood base. It has 18 internal finned channels side-to-side.
The entire cooling system, including tubing and water cooling blocks takes about 600 mL of coolant and it is fully sealed.

The water lines 3/8" ID 1/2" OD and male Quick Connect/Disconnect adapters on the Cloud Chamber side.

Due to the fast coolant flow and overall efficiency, the temperature differential at the heat-exchanger is not large but the system cools the water efficiently, maintaining a constant temperature of about 3-4°C above the ambient air temperature.
The Digital Thermometers are powered from the +5V line on the main wire harness. The 3 cooling fans and the water pump are powered with the +12V line.

It takes about 10 min for the temperature to drop down to the nominal -36°C due to the large thermal mass of the copper Cold Plate - thickness of the plate is 6.3mm. Again, due to the same large mass the temperature is very stable and rises slowly when power is turned off.  First particle tracks appear when the cold plate reaches about -28°C. Due to the plate thickness, I am probably loosing a couple of degrees min temperature but on the other hand, temperature is very evenly distributed across the surface.

Using the right concentration of Isopropyl Alcohol (Isopropanol) is absolutely critical for proper operation. The higher the concentration is, the better the Cloud Chamber will work. We use 99.9% Lab Grade pure Isopropyl Alcohol and results are excellent. In theory, everything above 91% should work but using >95% is highly recommended! Isopropyl Alcohol has the lowest Ionization Energy (IE) level - 10.10eV of the 3 popular types of alcohol and it is the best choice! Ethanol (IE ~10.48eV) or Methanol (!Poison!)  (IE 10.85eV) will also work.
A couple of Squeeze Alcohol Wash-Bottles are a very handy accessory - one bottle, filled with alcohol to saturate the felt reservoir of the Alcohol Evaporator Unit when priming the Cloud Chamber for startup and an empty bottle to periodically collect the alcohol condensate from the Cold Plate and recycle it.
A "must-have" accessory is a "Lead Pig" (Lead Lined Container) to store radioactive samples - Americium-241 pellet from a Smoke Detector, Thorium Gas Mantles, Thoriated Welding Rods, Uranium Glass, Tritium Vials, Ores, Minerals etc., as well as pair of tweezers to handle the samples. I have a few different sizes for storing radioactive samples. Plastic covered Lead Pigs are the best choice for small, "Cloud Chamber ready" samples = due to the lead's toxicity one should avoid handling bare lead metal.
For larger specimens, I prepared lead lined boxes.

This is complete Main Unit of the Diffusion Cloud Chamber!
Building the Cloud Chamber took about a week of build time but it was actually stretched over a month due to parts delays and issues with the eBay shipments.

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