Simulated High Altitude Pressure/Temperature Chamber

In high altitude ballooning, payloads are sent into the Stratosphere where temperatures reach negative 60° C and pressure is greatly reduced. Since each flight is costly, my research team wanted to build a chamber in which we can run tests on our equipment before sending it up. The goal was to build a pressure chamber in which we can get to very cold temperatures. We chose to base our chamber design off of a design by Howard Brooks:

http://www.depauw.edu/files/resources/highaltitudechamberconstructionmanual.pdf

http://www.depauw.edu/files/resources/operatingthehighaltitudevacuumchambermanual.pdf

The first step in the process was to research the materials needed and to modify the design to fit our space and needs.

- Vacuum pump

- Large pieces of inch thick Styrofoam

- Aluminum Foil

- Large PVC pipe, 14"

- Various fittings to connect chamber to vacuum pump

- Lexan

- Garage door weather strip

- (3) Threaded rods

- (6) Nuts (size of threaded rods)

- Fans

Potentially Helpful Tools:

- Duct Tape

- Packaging Tape

- Styrofoam Cutter

- Epoxy

- Super glue

- Sandpaper

- Drill

- Jigsaw

We chose to use Styrofoam (because it is a great insulator) to build a box around the PVC pressure chamber, which will hold dry ice to freeze the chamber. An alternative to this is to use a freezer chest or any insulated chamber. We built our box 24” x 26” x 48” to fit into the space we wanted to store it in, but a different size could be made. Duct tape was then used to connect the Styrofoam pieces.

Since the pressure chamber will sit on the bottom of the Styrofoam box, we built a bottom insert from used Styrofoam pieces that can be taken out and replaced as needed to avoid damaging the bottom of the box. Also, this insert protects the bottom of the chamber from being ruined by the dry ice (Multi-purpose materials – YAY!). The ends of the box were both double layered for a secure closure and two strips of Styrofoam were added to the top of the box for the same reason, as shown in Figure 3.

In order to reduce infrared radiation to the box (to keep it from heating), we added aluminum foil to the outside of the entire box. We used epoxy to glue it on and packaging tape to tape down the edges. The top of the box is taped down on only one side so it can hinge open.

The PVC pipe we ordered was five feet long, but luckily we were able to have it cut to 34” by the carpenters at our university. We used sandpaper to sand down the ends of the PVC pipe. Next, we had to drill a hole for the hose connection. The hole was gradually drilled to a size of 7/16”, and then a ¼” pipe tap was used to create the threads in the hole. Next, we applied a small amount of epoxy to the threads of a ¼” NPT male to ¼” hose end and screwed the fitting into the drilled hole.

For the ends of the PVC pipe, we first traced out the cuts to make on the sheet of Lexan. Both lids fit on a single 2x2 foot sheet, as shown. In the diagram, the orange lines are the cuts we made using a jigsaw. Next, we drilled the holes for the threaded rods to go through to hold the lids on, shown by the green dots on the diagram. We clamped the two cut out Lexan pieces together to drill the holes so they lined up perfectly. As you can see in the figure, the octagons are not perfect – but as long as they fit over the PVC pipe, that’s all that matters. We gradually drilled the hole up to 7/16”.

The last step to building the PVC chamber was to put the seal on. We cut a piece of the garage door weather strip to be just longer than the circumference of the PVC (about 44”). We used gorilla super glue to first glue a portion of the seal to the outside of the PVC pipe. We used the crease already on the weather strip as the folding line for the seal. After the super glue around the outside dried, we bent the seal along the crease and glued it to the edge of the PVC pipe. This completed the pressure chamber portion.

The final product of the pressure chamber in all of it's glory!

Next, we created a rolling base to be able to move the entire chamber in and out of the space we will be storing it in without dragging it along the floor. We repurposed four old dynamics cars we had in the lab and added two pieces of wood for the chamber to lie on.

Next, we created a window on the end piece so we can look into the chamber while it’s running. This is important for us because we are putting electronics into the chamber and we want to have the ability to see if they are still on and running properly. We used two 10”x10” squares of Lexan left over from the lids as the window. After cutting a square into the Styrofoam end piece, we Gorilla glued the two squares on either side. This end of the chamber, like the other, is also double layered, so we cut a square into the larger Styrofoam piece and glued it to the inner end piece.

Since there are many ways the same goal could be achieved, the pieces we used may not be necessary for another similar project. We used NPT (National Pipe Thread) pipe fittings. Wikipedia has basic information about NPT and what all of the different numbers mean, which is helpful when trying to determine which pieces to purchase (https://en.wikipedia.org/wiki/National_pipe_thread). The diagrams show the design (labeled and not) that we chose to connect the vacuum pump to the pressure chamber.

The next step was to connect all of the fittings and hoses. We bought the hose in a ten foot section and cut it into two hoses with appropriate lengths for our chamber and space. We poked a small hole through the side of the Styrofoam box for the hose to go through and attached the hose to the hose end on the PVC pipe.

The other end of the hose connected to a hose end to ¼” NPT male fitting. This piece screwed into the end of a ¼” female pipe tee. Teflon pipe tape was wrapped around all pipe threads before making connections. The other end of the pipe tee has a ball valve connected to it to let out the air from the pressure chamber.

The third side of the pipe tee has a ¼” NPT male fitting to hose end, then a hose runs from here to the vacuum pump. Before the hose reaches the vacuum pump, we used a ¼” female pipe tee to add a pressure gauge. We also needed a ¼” NPT male to ½” NPT male adapter to connect to the vacuum pump.

We used chicken wire and gutter lining to create baskets for the dry ice to sit in, which are placed on the bottom of the box. We also created two baskets to put inside of the pressure chamber to keep the temperature cooler inside.

We put fans in the chamber to help move the cold air around from the dry ice since cold air tends to stay low. We connected each of three fans to their own AC adapter. The fans are labeled as DC12V (dc circuit, 12 voltz to power). These fans need AC adapters which convert 120 voltz (outlet) to 12 voltz (fan) with approximately the right amount of current. The AC adapters we found were INPUT: 120V, OUTPUT: 12V/1.0A.

The fans won’t work if they are connected in series together because the resistance of the fans is too high. First, we cut the barrel jacks off of the ends of the AC adapters. Then the wires were stripped, soldered together, and the connections covered with shrink wrap. *Note: If you use this method, some fans will only run in one direction, so be sure they run before soldering.

To connect the fans to the inside of the box, we zip tied them to an extra piece of Styrofoam and taped the Styrofoam to the inside of the box.

Operating the chamber can be as easy as turning on the vacuum pump; however, there were a few things we’ve noticed along the way:

1. The Lexan pieces will cave in while the chamber is under partial pressure. We weren’t sure we liked this at first, so we tried to eliminate it from happening by combining two Lexan pieces. Once we did this, we were unable to pull partial pressure. We think that the caving in is necessary for the chamber to hold a seal.

2. The hex nuts cannot be screwed onto the ends too tight. If they are, the Lexan will start to bend outward and the parts along the seal, which are not held down by hex nuts, will leave gaps. We tightened the hex nuts by hand and let the seal do the rest.

3. On that note, the hex nuts aren’t even required to seal the chamber. All it needs is to be held shut for long enough to start pulling air out. However, they can be helpful.

4. Because of the insulation properties of the PVC pipe, the pressure chamber takes a long time to cool down (about two hours to cool down to -20 C). One solution is to keep the pressure chamber open while the entire chamber cools down, then add the equipment and pull air out. Another solution is to put some dry ice inside of the pressure chamber along with the dry ice outside of it.