Low-Cost PM2.5 Monitor at Make-Think-Code
PM2.5,
fine particulate matter, is the air pollutant most associated with damage to health, yet
many aspects of its distribution are not yet understood. So, when the
Make-Think-Code Lab
announced a "Sensing the Environment"
hackathon we were excited, as Make-Think-Code is:
...a technology-focussed lab at
the Pacific Northwest College of Art that brings together members of Portland's vibrant
creative, tech, civic, and educational communities to explore the powerful role that creativity
and technology play in the search for imaginative and impactful solutions to complex and
urgent problems.
So, the hackathon provided a perfect opportunity to collaborate on the development
of a low-cost PM2.5 outdoor air quality monitor.
At the hackathon, we set ourselves the goal of designing - and prototyping - a monitor that would be easy for citizen science groups, individuals, and researchers to assemble and deploy. Since several PM2.5 monitors are already available on the market (e.g. Dylos, Purple Air, and Shinyei), we focused on designing a sensor that would not need to depend on nearby A/C power or internet connectivity, allowing us to monitor under cloudy conditions, in shaded areas, as well as in parks and open spaces located away from a power source or internet connection. Designing and prototyping the monitor meant working on two aspects: the monitor itself, as well as a design and prototype of a shelter for the monitor.
MONITOR DESIGN
For the
monitor assembly, we chose a
Plantower 5003 PM sensor; mainly because it showed good performance in tests conducted by the South Coast Air Quality District in California. We used
a Teensy 3.2 Arduino controller, a
DS1037 real time clock from Sparkfun
for getting the observation timestamp,
and a Teensy audio shield
for holding the SD card (used for logging the data).
In the very first prototype, we used 3 AA batteries to power the monitor assembly.
However, the monitor ran for only 6 hours on the 3 AAs.
In our current prototype, we use an 8000mAh power bank which powers the monitor for ~30 hours.
Under this design, a week to 10-day long measurement campaign will
require several power bank changes.
We expect soon to test a 26000mAh solar-powered power bank in the hope that it can run
for a 1-week campaign without a battery change.
SHELTER DESIGN
The
shelter design has some interesting constraints: it needs to be reasonably
weatherproof as it may have to
sit out in the sun or rain for weeks. At the same time, it must also allow for
battery changes and data downloads from the datalogger. Cost is also concern, as is
simplicity of manufacture and assembly,
since our goal is to have large numbers of sensor kits available for assembly by interested
community members. Again, our designs have shifted over time, from a Stevenson Screen
design based on the
Canary
through two acrylic versions.
Our current design is drafted in Fusion 360, which lets us
parameterize to allow for easy changes in material
thickness or battery compartment size. The door (in gray) lifts out to give access for
changing the battery or the SD data card.
The shelter is designed to be laser-cut out of acrylic or other weatherproof material.
The laser-cutting pattern is first generated as a CAD drawing from the Fusion 360 model, as shown here.
A second pass in Illustrator or the equivalent may be necessary to get the parts
arranged efficiently for the particular piece of material being cut. Once the parts are
cut out, the circuit board and sensor get clipped to the center divider and the whole
thing will slot together in moments with no glue or fasteners required.
We are looking forward to completing and deploying three to five prototype PM2.5 monitors and beginning to design the back-end database and visualizations by January 2018.