The US EPA conducted a comparison of eight low-cost (<$2500) particulate matter (PM) sensors by collocating them with PM2.5 (particulate matter with aerodynamic diameter < 2.5 microns) Federal Reference Method (FRM) instrumentation with 5-minute resolution readings for a month. Here is a link to the report.
The key take-aways from this work are summarized below (written by Prateek Shrestha, PhD student in the Miller air quality lab).
- Most low-cost PM sensors are designed for indoor environments, but this study compared outdoor readings as well after weather-proofing the instruments.
- 1 to 5 minute sampling resolution proved to work the best.
- Instruments which report mass concentration based on in-built algorithms are pretty accurate as well and the major portion of disagreeing data is attributed to the base light scattering related error.
- The response of various sensors vary widely depending upon the relative humidity and temperature.
- Only MicroPEM accounts for RH in the mass-concentration conversion algorithm.
- AirBase CanarIT ($1500) : Once the AirBase CanarIT has been set up, all it requires is power and the occasional reboot when it loses connection to the server. Even in the event connection is lost, the sensor continues recording and saving data for transmission once connection has been reestablished. The requirement that it be furnished with a GSM SIM card and data plan adds a recurring expense to operations.
- CairPol CairClip PM2.5: The prototype CairClip sensor does not appear to function at temperatures below 19 °C. As a result, there is very limited data with which to draw any further conclusions. The operator urges further testing in a warmer environment. The long battery life, simple software, and simple operation contribute to its high scores in uptime, ease of use, and mobility.
- Carnegie Mellon Speck ($150): The 1-second time resolution causes file sizes to get large and cumbersome very quickly. As a result, download times are very long. While the use of UTC seconds for time stamps likely saves memory, it also inhibits the ability of the operator to verify correct operation in the field.
- Dylos DC1100 ($300): The low mobility score is because the Dylos DC1100 does not record time stamps internally. It must therefore be connected to a computer at all times via RS-232 to collect meaningful data.
- Met One Model 831 ($2050): All calculations were performed on the PM1 size fraction. Larger size fractions are highly prone to outliers and do not match Grimm FEM data nearly as well even after concerted efforts have been made to remove those outliers.
- RTI MicroPEM ($2000): The MicroPEM is a comparatively high-maintenance instrument. Time stamps sometimes malfunction when battery power is low, even if the device is operating on external power.
- Sensaris Eco PM: The sensor to tablet via Bluetooth to website via WiFi method of data recovery is a highly questionable design decision. The Bluetooth connection clearly has problems, and it is possible the WiFi connection does as well. Data are apparently not stored at any point until it reaches the server. As such, dropped packets at any point in the process will result in lost data. This sensor required a substantially greater level of effort than any of the other PM sensors at every stage of the project and yielded the least amount of data. Given the small volume of data collected, the quantitative measurements reported above should be considered highly suspect. The mobility score is low because it requires a location that has WiFi and no iOS devices present.
- Shinyei PMS SYS-1 ($1000): This sensor required extensive waterproofing in preparation for field placement and modest electrical knowledge in making the necessary signal/power connections. It was observed to be extremely light sensitive and extraordinary precautions had to be performed to collect data useable for the evaluation.
This was a study conducted by Alion Science and Technology under contract EP-D-10-070 for the Human Exposure and Atmospheric Sciences Division of U.S. EPA.