This project was aimed at creating a testing apparatus that allows you to vary the temperature between (Below Freezing) to ~100F.
- Temperature Sensors
- PID Control
The reason for this project is to create a box which can cycle between 30F (or lower) and 100F so that we can use it to test the stability of various components over a wide range of environmental temperatures.
The main component I used in this project was a TEC, which is a Thermo-Electric Cooler. This is a device which uses the peltier effect to create a heat differential across two surfaces. Different TEC’s have different ratings on the thermal difference they can achieve, in this device we went with a standard 100 Watt TEC. The most important factor in the temperature in which you can achieve on one face(highest/lowest) is based on how well you can cool/heat the other face. In our device, it will be very easy to heat the inside of the box, the real challenge is getting the boxes overall temperature below freezing.
I decided to go with a standard water-cooled block method, using a plate of copper and aluminum. The reason I have two different metals is to reduce the number of operations I have to do in preparing the copper plate. This is because copper is a very gummy material to machine, and can be difficult to create any well done parts. Because of this, our copper plate is mainly cut to size and had a few holes drilled.
In the design of the aluminum plate, we are aiming to create the most heat-transfer possible between our metals (Cu/Al) and our liquid (water). To achieve this, we machined a path for the water to travel with increases the total surface area it will interact with and for an extended period of time, Mainly a Zig-Zag/S shape. To pipe in the water, we drilled and pipe-tapped two holes for our brass barbed connectors. The reason we are using brass barbed connectors is not important, any barbed connectors will work… I just had some lying around that were the right size. A good rule of thumb is to teflon tape the pipe threads, this will help reduce the chance of leakage.
For our mating surfaces, we fly cut them to improve the mating quality. This is not necessarily important, but having smooth clean surfaces will improve the quality any mating or sealants you might use. In our case, we used a caulk sealant which we applied in sparing amounts before bolting our two pieces together tightly.
After the sealant had dried we conducted some tests with a cup of water. Though it does get down to freezing, it takes time and ultimately does not have much heat capacity on its own. To improve this you can add some pieces of copper/metal to act as thermal sinks. This will average the environmental temperature by acting as a thermal mass in which you can store heat/lack-there-of.
In the end the project worked, but it is not as good as an industrial grade device which can cost upwards of $2K+. The only major down-side is any real tests will need to be conducted over days instead of hours because of the limitations of our single 100W TEC.