Peltier cooler-based cloud chamber

Electronics

Mar 1

In this video, I show you how to build and operate a cloud chamber. This is a very fun instrument because it can show the trajectory of ionizing particles. So you can detect the particles of the natural background radiation or the products of a radioactive (and decaying) material.

The principles are super simple:

We need to build a transparent, fully sealed chamber and put a damp (from isopropyl-alcohol (“IPA”)) piece of cloth in it which will provide the vapour needed → hence the name “cloud-chamber”.
We need to cool the bottom of this chamber to create a layer of supersaturated alcohol vapour → This is achieved by two, stacked Peltier coolers. The hot side of the stack is cooled by a CPU cooler.
We need a concentrated light shining at the chamber from sideways at a tangential angle. The light will make the droplets shine which makes the trajectories easily detectable. A bike headlight is perfect for the purpose.
We need a radioactive specimen. Some fire alarms contain americium, but use them with huge caution because they are hazardous! The background radiation already provides enough particles.

Building procedure and extra resources

There are not too many components in this project, but on the other hand, the fun factor is pretty high.

First, I constructed the chamber. I used 2 mm thick plastic sheets for the top and the side walls. The side walls are made of two 44 mm wide and two 48 mm wide sheets. Each sheet is 80 mm high. The grooves in the brace and the clamp are specifically designed for the 44 and 48 mm wide and 2 mm thick sheets. The height was chosen arbitrarily, but this height seems to be good for producing a nice layer of supersaturated vapour at the bottom of the chamber.

The brace for the top of the chamber is covered by a 48 mm x 48 mm plastic sheet. Once I assembled everything, I closed the small gaps with hot glue.

Now I have the chamber, so I can assemble the cooler stack. I took the CPU cooler and applied a thin layer of thermal grease to it. Then I placed the hot side of the TEC12715 Peltier cooler on the CPU cooler. I squeezed the Peltier cooler against the CPU cooler to get rid of the excess thermal grease.

Notice: Always check which side the cables are facing to avoid any disturbance (i.e. cables interfere with the fan). I mounted the coolers in a way that the cables go sideways.

Once the bottom Peltier cooler is applied, I put a thin layer of thermal grease on the top (cold side) of the TEC12715 and placed the hot side of the TEC12703 (top Peltier) on it. Squeezed them together, and then removed the excess grease from the side.

Now the stack is done. We need a black background for the cloud chamber. You can paint the Peltier cooler, but it might be difficult because the IPA might start to dissolve the paint. Or, you can put black paper or a thin piece of cloth on top of the Peltier cooler. What I did I cut out a roughly 45 mm x 45 mm piece from a black nitrile glove. I sprayed a little IPA on the surface of the Peltier, then placed the rectangular piece of the glove on the cooler. I used a piece of fabric and stroked the glove to make it sit flat and tight on the surface of the cooler.

In the next step, I put the clamping piece on top of the Peltier stack.

Notice: Make sure that the Peltiers perfectly overlap. They can glide on each other easily and if they are misaligned, the cooling will be worse.

Once the clamp is sitting on the stack, I bolted it down using the counter piece which is also 3d-printable plastic. The holes are good for M5 bolts, but if you gently ream them with a 6 mm drill bit, then the M6 bolts will sit tightly in the holes Tighten the bolts “by feeling”. Stop before the ears of the top clamp piece or the whole bottom piece starts to bend. Be gentle but confident.

The chamber is ready for use. Use a piece of fabric and wet it with IPA. Insert it at the top of the chamber. I used a piece of felt because it is rigid and I folded it into a “C” shape. If you have something radioactive, put it on the cold surface, then close the chamber. The walls of the chamber should tightly sit in the channels of the clamping piece.

Now comes the important part. First, turn on the fan to keep the CPU cooler cold. Then, turn on the bottom Peltier cooler. Only the bottom! You need to regulate the voltage to limit the current flowing through the Peltier cooler. Aim for 10 Volts. In my system, the bottom was running at 10.32 V and 8.98 A. You should see similar values, within ~5% deviation.

Wait 2-3 minutes until the bottom of the chamber becomes wet. This liquid on the surface is the condensing alcohol vapour. Once you see some noticeable amount accumulating at the bottom, turn on the top Peltier cooler too. This needs a separate power supply, because this cooler should run at a lower voltage than the TEC12715. This cooler should be started at 5 V first, then the voltage can be gradually increased to 7 V where the current was 1.88 A.

Once again, you only need to regulate the voltage, the current will follow it since the Peltier obeys Ohm’s law.

With this setup and parameters, the cold side of the top Peltier cooler will be somewhere between -30°C and -40°C. I did not measure it in the video, but I measured it with a different construction where the chamber was not sealed as well as in the video and the temperature of the surface of the Peltier cooler was -34°C.

Also, if you want to maximize the utilization of the Peltier coolers, you can look up their performance charts and evaluate the cooling power and other parameters. I roughly did this exercise in the video, but I explain it below.

What you need to do is you need to put the dT vs. Voltage and dT vs. Qc charts on top of each other, then follow these steps:

Measure the hot side temperature of the cooler. In our case, it can be the temperature of the CPU cooler, plus 10-20°C. The real hot side temperature (inside the Peltier cooler, at the junctions) is always hotter than the CPU cooler
Read the voltage and current from the power supply or measure it with a multimeter
On the dT vs. Voltage chart, look up the voltage you got from the power supply, and draw a horizontal line from the value
Find the current that you read from the power supply or multimeter and find the point where the current meets the voltage.
Draw a vertical line from this point down to the X-axis. The point where the X-axis is met by the line gives you the dT. So, if the hot side was 40°C and the dT was also 40°C, it is safe to assume that the cold side is somewhere around 0°C
Now go to the dT vs. Qc chart and draw a vertical line from the above-obtained dT value (40°C)
Find the point where it intercepts the current you got from the power supply or multimeter and draw a horizontal line towards the Y-axis
The point where the line meets the Y-axis is the Qc. This is the amount of heat that is (or can be) pumped from the cold side to the hot side while maintaining the dT = 40°C

The above exercise gave around 50 W for the bottom Peltier. This can be used as a guideline for choosing the top Peltier cooler, as well as its parameters. Choose a Peltier cooler that can produce the largest dT while not dissipating more than 50 W. Remember that the dissipated heat is the Joule-heat (P = I * U), plus the heat pumped from the cold side to the hot side.