Activity: Assembling an Air Circulation System

Materials

A schematic diagram of the space habitat

Figure 2: A schematic diagram of the space habitat. The left habitat has no provision to condition the air within it. The habitat on the right has a charcoal filtration system.

  1. 2 clear plastic water jugs or similar containers with a large opening
  2. 3/8 inch plastic (vinyl) tubing
  3. clear plastic soda or water bottle (bottom cut off)
  4. small 9-12 volt DC computer fan
  5. BBQ charcoal or activated charcoal
  6. 9-12 volt DC power supply
  7. clean sterile sand

Miscellaneous materials

Construction of the Air Circulation System

Construction of the space habitat begins with the assembly of the air circulation system. Begin by cutting a circular hole into the plastic lid of a peanut butter jar (or similar jar). Cut the hole so that it is approximately the same size as a small 12 volt computer fan. Computer fans are easily scrounged from old discarded computers or used electronic stores.

Attach the fan over the hole using epoxy (don't get epoxy on the fan blades) or using a couple of small bolts.

Attaching computer fan

Figure 3: Attaching the computer fan over the hole in the jar's lid.

The next step is to attach the fan's exhaust tower to the lid of the jar using a clear plastic soda bottle from which the bottom has been cut off. When cutting off the bottom of the bottle leave two plastic tabs with which to anchor the soda bottle to the lid. See Figure 4 below.

Plastic exhaust tower

Figure 4:ıı Detail of the plastic exhaust tower attached to the plastic jar lid (left) and a close-up of the 9 volt power supply (right) adapted to power the computer fan.

The Power Supply

The 12 volt computer fan will run quite well and quite safely by adapting a small 9 volt power supply which has been scrounged from an abandoned household appliance such as an old cell phone or a discarded rechargeable appliance.

Simply cut the low connector off the low voltage wire and connect them to the computer fan using a couple of Marr connectors as illustrated in Figure 4 above.

Assembly

  1. Using the 1/2 inch drill, make a hole in the top of the water jug (a), a hole in the lid of the soda bottle (c) and a hole in the side of the plastic jar (e). Hint: place a sturdy piece of wood behind the plastic that you are drilling.
  2. Insert the 3/8 inch grommets into the holes.
  3. Fill the filter jar with BBQ charcoal and then screw on the lid of the jar. This is the adsorbing medium.
  4. Cut the vinyl tubing to the appropriate lengths and assemble them as shown. Note that the opening in the water spout of the plastic jug is exactly 3/8 inch and the tubing will fit snugly into it.
  5. Add the clean sand to the floor of you habitat and slightly moisten it. Plant a few seeds in the moistened soil.
  6. Put a small open dish of water in the habitat and then seal the container.
  7. Plug in the air circulation fan.
  8. Set up a second habitat identical to the first except omit the air filtration system. Plant seeds in the soil and seal the habitat.

Habitat

Figure 5: Habitat without filtration system on the left. Habitat with filtration system on the right.

Theory

Because the charcoal has been exposed to the air in the classroom the charcoal will have adsorbed enough oxygen, nitrogen, carbon dioxide and water vapour to be in equilibrium with the atmosphere. This means that the rate at which the charcoal adsorbs gases is exactly balanced by the rate at which the charcoal releases the gases. The adsorbed air in the charcoal are said to be in a state of dynamic equilibrium with the air in the classroom.

Absorbing charcoal

Figure 6: The huge adsorbing surface area of the charcoal means that in as far as the air molecules are concerned, the charcoal has the ability to accommodate enormous volumes of air like a huge room.

When you seal up the system which contains the charcoal filter, you also seal in all the gases that the charcoal had adsorbed when it was exposed to the air in the classroom. The amount of adsorbed gas is huge and it's all inside your sealed habitat.

Key idea: The concentrations of various gases adsorbed by the charcoal and the concentrations of the gases which make up the air in the habitat are exactly balanced.

For example, if the concentration of oxygen inside the habitat drops, the charcoal will release oxygen to restore the balance. Similarly if the concentration of carbon dioxide raises inside the habitat, the charcoal will adsorb the carbon dioxide to restore the balance. Whenever the concentration of gases in the habitat changes, the charcoal will release or adsorb the gas to maintain the balance.

Because of the large capacity of the charcoal to "hold" gases by adsorption, it's almost as if the habitat were not sealed at all, but left open the classroom air.

Absorbing charcoal

Figure 7: The charcoal both adsorbs and releases the gases into the air. If any of the gases becomes more concentrated in the air then that gas becomes adsorbed by the charcoal. If the gas becomes less concentrated, then the charcoal will release that gas into the air.

Absorbing charcoal

Figure 8: The completed habitat with the air-lock door removed to show the interior.

Observations

Once the two habitats are planted with seeds they should be observed closely for several weeks.

Make detailed notes (keep a journal), take digital photographs or perhaps set up a stationary video-camera and make a time-lapse video over the duration of the experiment. Record your observations, comments and any questions you have.

Pay close attention to the differences between the two habitats as the seeds germinate and grow.

Extension Activities

  1. The system that has been constructed will maintain a stable level of carbon dioxide and water vapour for a very long period of time. However there has been no provision to control the internal temperature of the habitat. Design a modification to the system to accomplish this.
  2. Although the humidity inside will remain relatively constant, it may become too damp and moulds will form. Design a modification to extract water vapour from your habitat.
  3. Your habitat will probably be temperature controlled by the fact that it is sitting comfortably inside a temperature controlled room. On Mars it is incredibly cold. The Sun is only one-third as bright as it is on Earth. Design a modification to provide temperature control of your habitat as if it were the basis of a design to be used on Mars.