image credit: @donblair
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What I Want to Do
We are in the midst of a boom of low-cost air sensing devices, and open source particulate sensing projects have really taken off in the past year. In this note I’ll look at four in-development projects using almost identical sensors (Shinyei PPD42NS, Syhitech DSM501A, Shinyei PPD60PV ) and a low-cost commercial competitor with a similar design, the Dylos.
I'll also lay out a plan for comparing and calibrating these sensors to each other and to commercial reference equipment. Current efforts have correlated low-cost sensors with commercial reference sensors, but never to each other. AQICN, AirBeam, Sonoma Tech Gao et al. 2015, Holstus et al 2014. Thanks to @Willie for many of those links.
How the Sensors Work
There are issues calibrating optical sensors for not just water particles (humidity), but also for the albedo (or brightness) of the particles themselves, as Tim Dye succinctly documents. That said, the biggest differences between different devices is the way they control airflow. Passive monitors rely on convection to draw in air, while active monitors blow a controlled stream of air across the sensor’s path. Which of these strategies is best? I don't know, so I propose co-locating all of these sensors and testing them against benchmark equipment.
Airflow strategies comparison:
Passive airflow: a system that doesn't actively control for airflow, either using natural or thermal convection to drive particulates through.
Active airflow: The use of a fan or pump to control and/or meter airflow through a sensor.
The Dylos uses a small fan. A great teardown is here:
the Speck uses a small fan:
hooked up to their open source fluxtream platform and designed for home use, it's being used by the Southwest Pennsylvania Environmental Health Project.
There are a few more systems using these sensors too, see “other platforms” below.
The idea is to use a single Raspberry Pi to read all the sensors in their enclosures simultaneously, along with the reference system.
Currently there are only proprietary commercial services available for commercial sensors, such as Netronix, which costs around $100/month.
Filter system benchmark?
Both Dusttrak and pDR-1500 have a filter for "gravimetric analysis" (weighing the quantity of dust collected) as an additional calibration. There are extra questions about characterizing an optical sensors' response to local dust via fingerprinting the different dust particles. That may require an extra system, and I'm looking to source some low cost passive monitors as well.
[image of turtle pump + tube]