A Student Air-Filter Demo Whose Sensor Measured the Wrong Thing

A student team at Cal Poly Humboldt built a tabletop air-filter demo with fans, interchangeable filters and a smoke-filled box. Its air-quality sensor turned out to be the wrong kind: it read carbon dioxide, and the filters catch only particles, not gas. The project is documented on Appropedia, and the team's own account of what went wrong is the most useful part.

Final design. Photo: Kalin D. on Appropedia, CC BY-SA 4.0.

The brief

The client was the university's School of Engineering, which was growing, with new Mechanical Engineering and Energy Systems Engineering programs, and wanted to attract prospective students. At its spring preview and admissions events it had only stickers, flyers and handouts on one or two tables. The team, working through the spring 2024 semester, had to build something small, portable and hands-on that shows what air quality engineering is. The listed requirements included pieces no bigger than 4 by 4 feet and none over 50 pounds, a cost of no more than $500, and setup in under ten minutes.

Three prototypes

1. A small cardboard-and-hot-glue model to show the client the design, plus a to-scale working "bridge" with a functional filter cartridge. These showed the need to test fan positioning and to use more than one fan.
2. A full-scale model that mostly reused the bridge and added acrylic, so the client could judge its size. It allowed full testing of the fans and ventilation.
3. A new design for the client's request to take up less table area: it grew upward instead of being scaled down, reused materials from the second, and allowed full testing of the fans, ventilation and a CO₂ monitor.

Prototype 3 with the fans installed. Photo: Kalin D. on Appropedia, CC BY-SA 4.0.

The finished demo

• Contamination box: a 1 ft by 1 ft vented box with an airtight rear door and an acrylic front window to hold the smoke.
• Chimney: 2 ft by 5 in by 5 in, housing two fans, the air sensor, an Arduino display, a slide-in filter cartridge and a backup filter. The display sits outside at eye level.
• Filters: cotton, fiberglass and HEPA, in 3D-printed cartridges that can be swapped quickly.

Filter materials, left to right: cotton, fiberglass, HEPA, with a 3D-printed cartridge in front. Photo: Kalin D. on Appropedia, CC BY-SA 4.0.

It was built mainly from quarter-inch plywood, with small cubes of scrap wood to screw into, silicone to seal the cracks, and gold spray paint with the school logo traced from an image. The final product cost $205.74, and the team saved money by reusing material from the prototypes.

How it works

A visitor places the demo flat and switches on the fans and the Arduino display. Incense is lit inside the box so that as much smoke as possible goes up the chimney. A prospective student reads about each filter material, picks one and swaps the cartridge quickly so little smoke escapes, then compares the readings before and after to see which filter is better. Afterward the incense is put out, the fans run until the display falls, and the box is wiped clean. Upkeep is routine cleaning, charging the batteries and replacing worn filter material.

What the team learned

• The sensor monitored carbon dioxide, but the chosen filters remove only particulate matter. The team had not researched which sensor they needed.
• They would use clear silicone next time.
• Because the interior was painted before assembly, the L brackets and some silicone stayed unpainted; touching these up is the next step.

Sources

• Appropedia contributors, “Humboldt SoE air filtration demo” (https://www.appropedia.org/Humboldt_SoE_air_filtration_demo), licensed under CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/). Photos by Kalin D. on Appropedia, CC BY-SA 4.0. Changed here: rewritten and condensed in hubnx's own words; the cost table, team names, testing appendix and troubleshooting template are left out.