Poorman's Space Program
There are several simple tests that you should perform on an experiment before launching it into near space. One of those tests is the thermal test in which an experiment is chilled with dry ice to temperatures similar to what the experiment will experience during its mission. There are two reasons for performing a test like this. The first is to ensure the experiment will function at extremely cold temperatures. There'll be no data returned if the experiment freezes up during its mission. The second reason is calibration. Some devices and sensors are temperature sensitive, and they'll not perform to the same specifications in near space as they do on your bench top.
Figure 1. This article describes assembly of the thermal test chamber shown here.
The thermal test chamber (TTC) isn't just for near space. Any experiment designed for winter exposure (especially if it's an automatic remote experiment), is an ideal candidate for a run through the thermal test chamber. This article describes the design of a thermal test chamber that I've used for my near space program.
Materials
Some people make a TTC by tossing a chunk of dry ice into a Styrofoam ice cooler. While this works, it's not the best design for a TTC. For starters, the sides of a Styrofoam cooler usually aren't straight and they only come in a limited number of sizes. Therefore, it can be difficult to fit some test items. In addition, Styrofoam coolers don't have thick walls or air fans. So they don't get as cold as we'd like. The design I recommend is a large volume, thick walled, Styrofoam box with interior fans. However, the size and shape of the TTC are customizable. There's one final benefit of this design over a Styrofoam cooler. It's easier to make this TTC look high tech than a beer cooler.
List of Materials
Styrofoam sheet 5 centimeters (2 inches) thick
Styrofoam sheet 1-1/4 centimeters (1/2 half inch) thick
Exacto Knife (with a long blade)
Metal straight edge (a meter or yard stick)
T-square
Pencil
Hot glue
Package of Space Blanket
Aluminum tape
Modeling plywood 3 mm (1/8 inch) thick
Miniature 5 volt CPU fans (2)
#24 AWG stranded wire
Heat shrink tubing 3 mm (1/8 inch) in diameter
Packaging tape
#4-40 hardware (4 sets)
Five-volt household power supply transformer module*
Male DC Power Plug *
Wire mesh, desk top letter basket (2)
Wire mesh, desk top In-basket
* A commercial power supply transformer module usually terminates with a connector that is a female coax style, DC power plug. These connectors come in a variety of sizes and are categorized by their outside and inside diameters (e.g., 5.5 mm OD X 2.5 mm ID). The DC power plug connector you purchase for the fans must fit the DC power plug of the power supply. You can purchase an inexpensive power supply from surplus electronics companies like All Electronics and Electronics Goldmine. The DC power plug connectors are usually available from these companies or at your local Radio Shack. If you go to Radio Shack, you can test fit the DC power plug on your transformer module.
Cutting the TTC
My TTC is a cube measuring 50 cm on a side and large enough to test two BalloonSats simultaneously. However, you're free to assemble your TTC to different dimensions. Just remember that a larger TTC requires more dry ice and possibly larger fans.
Begin by laying out the top, bottom, and sides of the TTC on the 50 mm thick Styrofoam. Don't do this as you did with paper cubes in elementary school; you need to take into account the thickness of the Styrofoam. Since the lid of the TTC is its top face, I prefer the top and bottom of the TTC to be the outside dimensions of the TTC. The front and back sides are the width of the TTC and 10 cm shorter than the height of the TTC. This leaves the sides 10 cm shorter than the dimensions of the TTC in both height and depth.
There's one more piece of Styrofoam to lay out. This piece attaches to the inside of the lid of the TTC and forms a seal against air infiltration. Draw this on a piece of 1.25 cm thick Styrofoam with the dimensions that match the interior of the TTC.
Use a pencil, metal meter stick, and a T-square to keep the TTC sides straight and the corners at 90 degree angles. Use the yardstick to guide the Exacto knife as you cut the Styrofoam. Use care to avoid cutting yourself. Styrofoam cuts cleanly with a sharp Exacto knife blade, so if the foam begins chipping, try using a newer blade. Cut through Styrofoam with several passes rather than trying to cut through in one slice. The cut edge of the Styrofoam should be smooth when you finish cutting it.
You may notice that Styrofoam cuts better in one direction than another. I suspect that extruding Styrofoam creates a grain in the sheet and, if you cut against the grain, the cut isn't as smooth. As you cut out the pieces of the TTC, label them with the sides that they'll be. After cutting out the Styrofoam, test fit the pieces to make sure the TTC will go together without gaps
If you have access to a wood shop, you can try using a table saw to cut the Styrofoam. I've used a table saw with good results. However, I'm not an expert, so there may be some "gotchas" to look out for.
Assembling the Exterior of the TTC
Now fire up the hot glue gun; you've got some gluing to do. Styrofoam is a good insulator so melted hot glue doesn't cool down quickly on it. Therefore, you'll have time to align parts. However, also be aware that hot glue can melt Styrofoam. You may have to unplug the hot glue gun occasionally to give it a chance to cool a bit. The alternative is to use a glue gun with a temperature adjustment or to try a low temperature glue and glue gun.
Place the bottom of the TTC and the front face in front of you. Apply a bead of hot glue to the front of the TTC bottom and glue the front into place. Quickly check that all the edges line up. Now quickly glue one of the sides to the TTC bottom and front face. You'll use two beads of hot glue to do this. Repeat this with the remaining TTC side and then the back piece. Don't glue the top on! By being quick, you'll make the sides square before the glue has a chance to cool down.
To strengthen the TTC, wrap a length of packaging tape around its top and bottom edges. The tape puts the front, back, and sides under compression so they won't split apart.
Now attach the inner lip of 12.5 mm thick Styrofoam to the inside of the lid. First, test fit the lid's inner lip. It's easier to fit one edge of the inner lip at a time. That way, part of the inner lip remains outside of the TTC during the test fit. Be careful to avoid damaging the inner lip while trying to force it in or out of the TTC. Trim the inner lip if necessary to get it to fit. Apply hot glue to one face of the ½” thick Styrofoam and center it over the lid. Quickly fit the lid over the TTC and make sure the inner seal is properly centered. Before the glue cools, you'll have time to adjust its position on the lid. After the glue sets up, test fit the lid once more. If for some reason, the inner seal slipped, you can do some additional trimming.
Wrapping the TTC
Now you have an ugly looking Styrofoam box with a lid. You can add insulation and give the TTC a nice scientific look by wrapping its interior and exterior with an aluminized Space Blanket. To cover the interior, cut five pieces of Space Blanket into squares slightly smaller than the interior dimensions of each side and the bottom. Use short strips of double-sided tape to hold the Space Blanket in place. To prevent the edges of Space Blanket from lifting up, hot glue thin strips of ½” Styrofoam onto the inside edges of the TTC box. The glued strips will cover the exposed edges of the Space Blanket.
Next, wrap the exterior of the TTC in Space Blanket (consider using several wraps). After wrapping the exterior, tape the outside corners and edges of the TTC with aluminum tape to add durability to the Space Blanket exterior.
Extending the Fan Leads
First, before cutting wires, connect each fan to a battery to verify it works. Since the wires (leads) on the CPU fans are too short, you'll need to solder additional wires to them. If you know how to solder, you know that you first need to strip the ends of the wires that you'll solder together. Since you're using stranded wire, twist the bare leads tightly after you strip them. This keeps them from fraying and making a mess when you solder. The bare leads are next twisted together, soldered, and then covered with heat shrink tubing.
Figure 2. How the wires are soldered and insulated with heat shrinkable tubing.
Assembling the Interior of the TTC
Here's where you build the brackets for the PC fans and mount them inside the TTC. I'm recommending two fans in this article; however, I wouldn't be surprised if additional fans will chill the experiment more quickly.
Cut the thin modeling plywood into two squares that are 2.5 cm (or one inch) larger than the outside dimensions of the PC fans. The PC fans will have holes in each corner where they're bolted to the CPU heat sink. So lay a PC fan centered on a plywood square and mark two diagonally opposite mounting holes. Drill two holes through the plywood with a drill bit large enough for # 4-32 bolts (2.5 mm or 3/32 inches). Then bolt the fans to the plywood with # 4 hardware. One fan must be mounted right side up and the other upside down in order to circulate the air properly. Next, cut two triangularly shaped supports for each fan from 2.5 cm (½ inch) thick Styrofoam. Hot glue the supports to two opposite edges of the wooden fan base. See the diagram in Fig. 3 if any of this is unclear.
Figure 3. How the fans are installed.
Select two opposite sides inside the TTC to mount the fans. The locations should be high, but not so high that the lid can't shut properly and leave no clearance above the fans to circulate air. Mark the location of the fans with a Sharpie pen. Then trim the two narrow strips of Space Blanket underneath each fan's Styrofoam supports with a sharp Exacto knife. Next, hot glue the fan supports to the exposed Styrofoam of the TTC sides. If you cut the Space Blanket strips a bit narrower, the glued fan supports will sandwich the raw edges of the cut Space Blanket.
Electrical Work
Next, drill a small hole through the wall of the TTC near the top. This hole is where the fan wires exit the TTC. To prevent the Space Blanket from ripping while you drill the hole, cover the location with a small square of aluminum tape before you begin drilling. Pass the fan wires through the exit hole then push the fan wires inside the TTC against the interior of the TTC and immobilize them with tape strips. Once the fan wires are in place, seal their exit hole with a little hot glue, both inside and outside the TTC.
To make the voltage on each fan equal to the power supply voltage, wire the fans in parallel as illustrated in Fig. 4.
Figure 4. How the two fans are wired in parallel.
Notice that the negative leads of both fans are soldered together and the positive leads are soldered together. Be sure to use heat shrinkable tubing over your soldered wires. (If the positive lead of one fan was soldered to the negative lead of the second, the voltage on each fan would be halved. This arrangement is called a series connection.)
The jacket of the power plug unscrews to expose the two terminals of the DC power plug. Normally, as shown in Fig. 5, the center connector is positive and the outer jacket is negative. This is done so that if the connector drops on a grounded surface, no short circuit will be created.
Figure 5. How the power plug is wired.
Pass the two wires from the fans through the jacket of the DC power plug before doing anything else (see Fig. 5). Too many times, I've seen students make solder connections before passing their wires through heat shrink tubing or a jacket. When they forget to do this, they end up cutting out the splice and starting over again.
Strip the ends of the wires and solder them to the tabs of the DC power plug as shown in Fig. 5. I get lazy at this point and just put a drop of hot glue over the soldered tabs of the DC power plug to make sure they're well insulated. Slide the jacket of the DC power plug over the wires and screw it closed. Now when you plug the wall transformer into a household power outlet, the fans should start spinning. There's probably an optimal fan size for TTC. Larger fans will move more air and move it faster; however, they also generate more heat. Finding the optimum size requires experimentation.
Dry Ice Chamber
I use two desktop metal mesh letter baskets to support the experiment over the dry ice. A larger in-basket holds the dry ice chunks above the bottom of the TTC. Their open mesh design allows air to circulate freely around the dry ice while keeping the experiment and the space blanket inside the TTC from making direct contact with the dry ice. See Fig. 6 for details.
Figure 6. The dry ice racks are made from a pair of metal mesh letter baskets.
The completed TTC interior, loaded with a BalloonSat, is shown in Fig. 7.
Figure 7. A Balloon Sat loaded inside the TTC and ready for low temperature testing.
Using the TTC
First, find a victim for your test. However, please do not use animals, not even insects. The TTC gets cold enough to kill most living beings. Purchase about a pound of dry ice and break it into two or more pieces. Caution: To avoid frostbite, be sure to use tongs or to wear thick gloves when handling dry ice. Place the in-basket into the bottom of the TTC and place two chunks of dry ice on it. Next, place the letter baskets over the dry ice and then place the item being tested on top of the upside down letter baskets. Close the lid and wait while the TTC chills the instrument(s) being tested.
The temperature inside the TTC can drop to below -40 degrees C (-40 degrees F) within 30 minutes. The chart in Fig. 8, shows the temperature reached by two BalloonSats that were identical in size and method of construction. The only difference between them was the type of Styrofoam from which their airframes were constructed.
Figure 8. Temperature reached by two Balloon Sats tested in the TTC.
Background Information
This was originally published as an article in the May 2005 issue of Nuts and Volts magazine and will eventually become Appendix C of my book, The BalloonSat Principia.
Onwards and Upwards, Paul. 
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