PHY579:CChamber2

From MariachiWiki

1 Cooling the cloud chamber with Peltier Elements

Cooling the cloud chamber with Peltier Elements

Class 1

We tested the elements for the cloud chamber and established few parameters. First the Cold Plate, then the Peltier, then cooling few aluminum plates.

The cold plate is a plate used to cool hard disk. It has been puchased from Sharka Corporation and it is the Alphacool HDD3 CU/Plexi 3.5" Water Block. For the circulator we used a pump made by Beckett, Medium Fountain Pump. The specs for the fountain can be downloaded here.

The measured flow through the plate with the setup shown above is 137 liters/hour, and the plate temperature of 0 ^oC. Note that we had a full cooler with ice and water. With the peltier element mounted on top of the water cooled plate the temperature achieved was -33 ^oC, with approximate 1/2 power, i.e. ~7V and ~6 A.

We noticed that a good thermal coupling is very important. The rubber material was not very good and we opted to use silicone based thermal grease instead in our next iteration.

The challenges:

Find a power supply with enough current, perhaps a computer power supply.
Find out if we can cool a thinner plate for the size of the chamber we have.
Experiment lowering the ice/water mix by adding salt.

[edit]

Class 2

In this session we tried to decrease the temperature of the top of the aluminum plate by making a number of impovements. The first order of business was to make a thinner Al plate because we suspected last time that the plate was too thick to achieve the desired temperature of -30 to -35^oC. Also, the rubber thermo coupling material that Helio brought to class wasn't effective in filling in the air gaps, so this time we are going to try a Si based thermal compound. We also found last time that the temperature decreased more steadily when we applied an external pressure on the aparatus, which is mainly due to decreasing the air gaps as you increase the pressure. Last time we used a textbook, but this time we got a little more sophisticated with some clamps, courtesy of Jeremy. Furthermore, we decreased the airgaps between the Cu, Peltier, and Al plate, by screwing the Al plate and the Cu plate tightly together.

Temperature vs time for thermocouples positioned at the center and edge of the Al plate

Before screwing the Cu and Al plates together we had to sandwich the Peltier in between them, and we used the Si based thermo coupling to ensure we had good thermal contacts. Tom took care of fastening our aparatus together with the screws and thermal compound. Rich was responsible for bringing all the equipment to class and Jackie went to 7-11 to get some salt for us. Justin set up the power supplies while Steve connected the thermocouples to the Cu plate, center of the Al plate (just above the Peltier), and the outter edge of the Al plate. Jeremy was on water duty again, and he recorded the temperature readings as a function of time. In theory we would expect the ice-water/salt solution would lower the freezing point of the liquid due to more ions, which would lead us to believe that a CaCl solution added would decrease the freezing point even further. However, Ted carried out this experiment and found that the NaCl mix actually works better than the CaCl mix. I believe Ted was able to achieve -8^oC with the water/salt mix. In any event we added 1 pound of salt to our cooler of water and ice and let the baby pump! Adam was in charge of setting up our circuitry to the Peltier and taking measurements of the Voltage and the Current using a DMM.

With the aparatus completely insulated we were able to get the top of the Al just above the Peltier to be -23^oC. The outter most edge of the Al plate was consistently 10^oC warmer than the center. We seemed to hit a plateau at -23^oC, but the outter edge did reach -14^oC, which suggests that had we waited a sufficient amount of time the entire Al plate might have reached an equilibrium temperature of around -23^oC.

Once we realized we had reached an end to the cooling, we removed the insulation on top of the Al plate and replaced it with the tank. We would expect the temperature to drop significantly, but to our suprise it fell to about -20^oC in the middle of the plate, and -10^oC at the edge, which was fairly close to our cutoff. Jeremey has the data that was taken during the run and Rich has the pictures. Since the edge of the plate tends to be 10^oC warmer than the center of the plate, where the Peltier element is just below, we will need to get the middle of our plate to -40 to -45^oC to ensure the entire plate is at -30 to -35^oC. However, with the fishtank on top of the Al plate instead of the insulation we found the middle of the Al to be -20^oC, which means we still have -20^oC to go...we are only half way there! Although we still have the other half of the race to go, we need to take into consideration that our new power supply was non-existent. According the Rich the Peltier can handle 170W of power and 135 of those Watts will be used towards cooling. At first we were only using 42W, the same as last time, and after putting two power supplies in parallel we could only achieve about 90W, so we are still working well below the power capacity of the Peltier.

Before making further suggestions for next week let me first summarize what improvements we made.

Thinner plate
Thermal Compound
Salt Added to solution

Although there are three major improvements over last week I suspect the change that made the most impact is the thinner plate. It seems the plate that we have now is very durable and could be made thinner. Some suggestions for next week might be:

Make the plate thinner, possibily have the bottom of it ribbed for added strength.
Test to see which solution will drop the freezing point of the liquid the most, NaCl or CaCl? Also, what is the best mixture so that the temperature of the liquid is at its lowest?
WE NEED A BIGGER POWER SUPPLY
Better insulation, the pieces of pink insulation we have now are too small and don't fit the right dimensions for our aparatus, it would be nice to have brand new insulation that we could cut to our specifications.
Another Peltier device? The temperature just above the Peltier was ten degrees cooler than the temperature far away from the Peltier. Perhaps if we had two devices we would be able to spread out the Peltier's over the whole area of the Al plate.

Remember, we still have another -20C to go, so we do have a lot of improvements to be made. --SHick 12:43, 5 October 2006 (EDT)

Tom applies the thermal grease to assure strong thermal contact between the Peltier element and the cooling circuit.	Just checking. The pump is so quiet that every now and then, we need to make sure its still going. The flow was measured to be 156 ltr/hr.

[edit]

Class 4

Well, we did it! We got the chamber working!

Temperature vs time for thermocouples positioned at the center and edge of the Al plate

We were successful in cooling down the bottom plate of the cloud chamber to -34^oC, on top of the elements and -27^oC in the middle of the chamber.

To achieve that we had two 225Watts Peltier elements connected to a single power supply and cooled water (at 0^oC). There is a lot of room for improvement. We would like to have more power and then more water flow.

Tracks were visible mostly in the areas above the Peltier elements.