Taking Space

The HabitatMap & AirCasting Blog

Evaluating Low-Cost Gas Sensors

One of the major challenges facing air quality scientists and citizen scientists is the dearth of information available regarding the performance of low-cost gas sensors. Though manufacturers provide data sheets that describe the performance of their wares, these data sheets are usually incomplete – lacking the information required to evaluate whether the gas sensor is suitable for mobile ambient air quality monitoring. Since the data sheets don’t provide the information we need, we decided to develop our own lab for evaluating the performance of low-cost gas sensors.

In support of New York Hall of Science’s Collect, Construct, Change (C3) youth environmental science program and in partnership with the New York City College of Technology, we evaluated the performance of three gas sensors: the Figaro TGS-2442 and the Alphasense CO-B4 carbon monoxide (CO) sensors and the SGX MiCS-2710 nitrogen dioxide (NO2) sensor. It’s important to note that in order to test the sensors, we first had to develop instruments for acquiring and interpreting their signal outputs. It’s possible that our instruments, the AirCasting Air Monitor and the AirGo, introduced errors that have not been accounted for. Therefore our results should not be considered the final word on the performance of these sensors. It should also be noted that the sensors we tested have warm up times approaching one hour. This compromises their utility for mobile air quality monitoring, where the ability to flip on an instrument and immediately begin recording measurements is an important feature.

Inside the Shinyei PPD42NS

Our lab for evaluating low-cost gas sensors. The gas blender is on the right and our first exposure chamber is on the left. Inside the exposure chamber is one AirGo and two AirCasting Air Monitors.

With an equipment budget of 5k we knew from the outset that our lab set-up would be bare bones. We had just enough money to purchase cylinders of “zero air”, NO2, and CO, a manually adjustable gas blender for mixing different concentrations of our target gases, and materials for constructing a DIY exposure chamber. We didn’t have sufficient resources to purchase reference analyzers to confirm our gas blending targets, scrub our zero air of CO, control for temperature and humidity, or test for response to interfering gases. The complete bill of materials for our lab can be downloaded here, including some preliminary sketches of the set-up authored by Charles Eckman, Lab Manager at the NorLab Division of Norco, Inc, along with some additional photos of our actual set-up.

Comparing the response of the TGS-2442 vs. the CO-B4

Comparing the response of the TGS-2442 vs. the CO-B4.

One thing we learned quickly was that the design for our initial exposure chamber, which was approximately two liters, was too large given the low flow rates that could be achieved by our gas blender. Although the volume of our initial chamber prevented us from being able to establish an equilibrium gas concentration, it could accommodate multiple instruments, making it suitable for running side-by-side comparisons. The above graph shows the results of one of these test runs, where we compared the performance of two TGS-2442s to one CO-B4, while changing the CO gas concentration inside the chamber. Note how the red line representing the CO-B4 response is moving up and down dramatically whereas the blue and green lines representing the two TGS-2442s rise slowly and then stabilize. Right off the bat we realized that the TGS-2442 was unsuitable for ambient air quality monitoring. It was extremely slow to respond to changes in CO concentrations and became saturated, rendering it incapable of responding when CO concentrations dropped.

Running step tests with the CO-B4

Running step tests with the CO-B4.

Upon discovering that our exposure chamber was too big, we crafted a smaller chamber that could fit only one instrument at a time. This made it impossible to run side-by-side comparisons but it did enable greater precision in generating gas concentrations. The above graph illustrates the results from one of our CO-B4 step tests.  We we’re impressed with the CO-B4s performance. It had a fast response time, a suitable lower-detection limit, and when we compared test runs between sensors, manageable out of the box variability. As mentioned earlier, we didn’t have the resources to scrub our “zero air” of CO and this can be seen in our results, with little to no change in the response between “zero air” and 100 ppb. However, we know from tests conducted at the New York State Department of Environmental Conservation’s Queens College air monitoring station that the CO-B4 has a lower detection limit that may match the manufacturers claim of less than 5 ppb. At approximately $90 this electrochemical sensor isn’t cheap, but depending on your budget and intended use it’s favorable performance may be worth the extra expense.

Running step tests with the MiCS-2710

Running step tests with the MiCS-2710.

After seeing the disappointing results from our TGS-2442 tests we were surprised to find that the MiCS-2710, also a metal-oxide sensor, performed as well as it did, see above graph. It’s multi-minute response times weren’t nearly as rapid as the CO-B4, which responded in seconds, but they were adequate and tests conducted by the EPA on the AirCasting Air Monitor indicated a lower detection limit approaching 10 ppb! A fairly good result considering it’s extremely low cost, retailing for approximately $5. However, a word of caution is in order as comparison tests, see below graph, revealed high out of the box variability. With no two sensors responding identically, each unit would need to be individually calibrated. Also, the sensitivity of the MiCS-2710 diminished as the NO2 concentration climbed above 100 ppb.

Comparing the response of one MiCS-2710 sensor to another MiCS-2710 sensor

Comparing the response of one MiCS-2710 sensor to another MiCS-2710 sensor.

Special thanks to Charlie Eckman who helped with our lab setup and protocol; Peter Spellane, Chair of the New York City College of Technology Chemistry Dept., who secured lab space for our experiments and served as an advisor; and Raymond Yap and Leroy Williams who ran all the performance tests. Funding for this project was provided by The Hive Digital Media Learning Fund in The New York Community Trust.

Make Your Own AirCasting Particle Monitor

Join us Tuesday, October 8th for the third installment of Make Magazine’s Urban Sensor Hack. The event takes place on Google Hangout and is free and open to the public. During our hack Michael Heimbinder, Tim Dye, Iem Heng, and Raymond Yap will guide you through a step by step process for making your own AirCasting particle monitor, discuss the challenges involved in achieving accurate air quality measurements, and detail our work with community based organizations and schools to conduct environmental monitoring and advance STEAM education. For those who can’t make the Urban Sensor Hack, we’ve published an illustrated step-by-step guide detailing how to make your own AirCasting particle monitor using the Shinyei PPD42NS optical particle counter.

Inside the Shinyei PPD42NS

Inside the Shinyei PPD42NS

As those who have been following the AirCasting project know, we’ve been conducting R&D on different air quality sensors for nearly two years in pursuit of a low-cost instrument appropriate for personal exposure monitoring. The Shinyei PPD42NS was one of the first sensors we evaluated. Our tests indicate that it responds to particles 2 microns in diameter and larger when air flow and light interference issues are addressed and signal filtering algorithms are in place. As you can see in the below test comparing the performance of the Shinyei PPD42NS with the more expensive Shinyei PPD60PV-T2, the Dylos DC1100 Pro, and the Thermo Scientific pDR-1500; the PPD42NS responds in line with the other instruments but the intensity (dynamic range) of it’s response is off and it’s unfiltered signal is noisy. For these reasons, we’re currently focusing our developments efforts on the PPD60PV-T2 which can detect smaller particles with substantially less noise.

Particulate Matter Instrument Comparison

Particle instrument intercomparison conducted by Tim Dye, Sonoma Technology, Inc. @timsdye. Click to enlarge.

For those looking to dig deeper on the PPD42NS have a look at this thorough breakdown of the product compliments of Tracy Allen at EME Systems. We’d also like to thank Chris Nafis, whose instructions and code for connecting the PPD42NS to an Arduino helped us get started with our own work, and David Holstius, whose blogging also informed our work.

AirCasting: Education Edition

In the spring we had lots of fun teaching students how to make AirCasting compatible air monitors and luminescent accessories.  For details on these educational programs, along with some really great videos, read on.

Making Air Monitors with High School Seniors

In 2012, the Newtown Creek Alliance, in partnership with HabitatMap, received an Environmental Justice Community Impact Grant from the New York State Dept. of Environmental Conservation to develop and teach a course on air quality monitoring at Queens Vocational & Technical High School, located in Sunnyside, Queens, New York City.

The 26 QVT students who participated in the AirCasting program during the spring of 2013:

  • • Built their own air monitors with instruction from a mechanical engineering professor from the New York City College of Technology and an electrical engineering professor from Manhattan College;
  • • Toured the neighborhood around their school with a historian from the Newtown Creek Alliance;
  • • Learned the basics of air quality from the Senior Vice President of Sonoma Technology;
  • • Developed and executed their own air quality monitoring plans using the AirCasting platform; and
  • • Presented their work and findings to their peers, members of the Newtown Creek Alliance, and staff from the New York State Dept. of Environmental Conservation.

To learn more about the program watch the above video and visit the Newtown Creek Alliance website.

Fashion Your Environment

Fashion Your Environment is a youth centered program focused on broadcasting environmental data through fashion and wearable technology. Youth participants were exposed to mentorship and training in design, prototyping, and fabrication of fashion accessories that visualize environmental data using AirCasting Luminescence. AirCasting Luminescence uses a microcontroller connected to the AirCasting app over Bluetooth to illuminate colored LED lights in response to sensor measurements received by the AirCasting app: green for low intensity, then yellow, then orange, and red for high intensity.

To learn more about the program, watch the above video and read our Tumblr blog.

The Fashion Your Environment Program is a collaboration between the New York Hall of Science, Parsons the New School for Design, and HabitatMap. Funding was provided by a grant from the John D. and Catherine T. MacArthur Foundation, administered by Mozilla, and in partnership with Hive Learning Networks.

More AirCasting Air Quality Monitors

It’s been eight months since we released the DIY guide for the first AirCasting compatible air monitor. Since then we’ve been busy evaluating the performance of a variety of different gas sensors and optical particle counters (more on this in later posts) to figure out just how well they work for our particular application: mobile ambient air quality monitoring to assess human exposures to air pollution in real world settings and scenarios. Along the way we were lucky to partner with Michael Taylor and Josh Schapiro from Carnegie Mellon’s CREATE Lab to develop another AirCasting compatible air monitor, the AirGo. This latest addition to the rapidly growing family of AirCasting compatible air monitors measures carbon monoxide (CO) using an Alphsense CO-B4 electrochemical sensor, particulate matter using a Samyoung DSM501A optical particle counter, and temperature & humidity using a Sensirion SHT15.


Michael Taylor & Josh Schapiro's AirGo monitor. Measuring carbon monoxide, particulate matter, temperature & humidity.

The AirGo does two noteworthy things that have improved the quality of the measurements we’re now collecting: 1) Josh Schapiro developed a custom printed circuit board that does a phenomenal job capturing the signal from the CO-B4, so we are now able to measure CO in the low parts per billion range. Of course, the sensor manufacturer also deserves kudos for creating a low-cost sensor appropriate for ambient air quality monitoring, and with it’s rapid response time, also appropriate for mobile monitoring. 2) Michael Taylor developed an algorithm for the Samyoung that reduces the noisiness of the measurements. Because optical particle counters are sampling such a small quantity of air (in this case .01 cubic feet) and the particle composition of air is so heterogeneous (the number of particles in one .01 cubic foot being different from the next) the measurements tend to go up and down dramatically from second to second. By implementing an algorithm that works somewhat like a moving average, he was able to iron out the spikes and valleys making it much easier to visually interpret the data while maintaining the integrity of the measurements. If you’re curious how it all works behind the scenes or want to make your own AirGo, Josh’s schematics and code are available on GitHub and the files for the 3D printed enclosure designed by Michael Taylor are available via Shapeways.

Gadgeteer AirCasting

Thomas Amberg's Gadgeteer AirCasting air monitor. Measuring alcohol, temperature & humidity.

We were also really excited to see this new design for an AirCasting compatible air monitor developed independently by Thomas Amberg using the Gadgeteer platform. You can make your own by following his Instructables guide. The most thrilling thing about Thomas’s design is that it illustrates first hand how flexible the AirCasting platform is. With minimal instruction from our team he was able to develop a novel sensor package on top of an entirely different electronics platform and connect with the AirCasting app to record, display, and share his data. This is the future of AirCasting: thousands of designs, millions of devices, all pushing data to the AirCasting CrowdMap.

AirCasting & HabitatMap in the News!

AirCasting & HabitatMap in the News!

The New York Times followed us around while we went AirCasting, then wrote a story and made a video about it. The story is excellent in that it covers not only AirCasting but a range of different citizen science air quality monitoring efforts. We also landed an article in Take Part that provides a nice overview of how HabitatMap got started and where we’re headed with AirCasting. And for the techies out there, have a look at the AirCasting coverage from the Arduino Blog.

My Air, My Health Challenge Update

We submitted our solution to the My Air, My Health Challenge judges a few weeks back. We won’t know who won until they make the official announcement at the Health Datapalooza IV. In  the meantime, we thought we’d share our solution (which can be downloaded as a PDF here) along with the instructional video we created  to introduce our pilot community health study participants to the AirCasting platform. Let us know what you think. Any and all feedback is appreciated.

Go With the Flow: Investigate Your Water System!

We are delighted to introduce Go With the Flow, part of HabitatMap’s MapThink Toolkit series. Go With the Flow is a short-term group research project that empowers high school students to investigate their water system through real-world learning and document their research findings using interactive web-based media.


Go With the Flow combines reading, writing, mapping, interviewing and local site visits. It blends traditional research techniques with innovative visual brainstorming exercises and uses maps for research, reflection and sharing. Our project gets students thinking about how they relate to the community around them and encourages them to examine how their water system and water use impacts the environment. This project aligns with the Common Core Standards, teaches effective research and communication techniques, and examines critical water issues from a geographical perspective.

Connecting Students to Community Through Research
MapThink positions students as democratic citizens who are active participants in their community and advocates for just and sustainable cities. Go With The Flow will guide students in investigating how homes and businesses are connected to water sources, how the facilities that provide clean water and dispose of dirty water are integrated into material flows and institutional networks that extend throughout and beyond the city, and how these flows and networks impact the geographic distribution of environmental burdens and benefits.


Why MapThink?
At the conclusion of the project, students will walk away with an understanding of:

• Real-world and Inquiry-based Learning
• The Value of Teamwork, Group Problem Solving & Brainstorming
• Innovative Research Techniques & How to Write Effective Narrative
• Professional Communication Skills & Presentation Techniques
• Technology & New Media Platforms + Best Practices
• How We Relate to and Can Connect With the Community Around Us
• The Importance of Water

Funding for the Go With the Flow toolkit was provided by the NYCEF Newtown Creek Fund of the Hudson River Foundation and HabitatMap donors.

Now Streaming Data from the Zephyr BioHarness 3

We’re happy to announce an update to the AirCasting app that includes connectivity for the Zephyr BioHarness 3. The BioHarness connects to the AirCasting app over Bluetooth, enabling AirCasters to record heart rate, heart rate variability, R to R interval, activity level, peak acceleration, breathing rate, and core temperature measurements. You can download the update from the Google Play Store.

Zephyr BioHarness 3

AirCasting from Williamsburg, Brooklyn

Mayeta Clark, an intern at the New York Times video desk, contacted me a few months back to discuss AirCasting. I suggested that rather than do a sit-down interview, she capture AirCasting in action, because context is everything, right?  If the air is always safe to breathe where you live then personal air quality monitoring isn’t particularly relevant. But it’s a different story when the air is frequently foul, as it is in some areas surrounding Newtown Creek in North Brooklyn. So after agreeing to a location shoot that’s where we headed.

I like watching the sunset behind the Manhattan skyline so I charted a route for us between two of my favorite Williamsburg sunset spots: the Scott Ave. pedestrian overpass and Grand Ferry Park. Both spots also happen to be located next to facilities that pollute the air. Allocco runs a busy construction and demolition debris waste transfer station on Scott Ave. and the New York Power Authority operates a peaker power plant on the waterfront parcel adjacent to Grand Ferry Park.

SThe CO+NO2+T+RH AirCasting Air Monitor in white and the PM monitor in grey. Both are attached to my backpack using carbiners and velcro straps

Ready to roll! The CO+NO2+T+RH AirCasting Air Monitor in white and the PM monitor in grey.


Looking west towards Manhattan from the Scott Ave. pedestrian overpass.  Allocco's facility can be seen in the foreground.

Looking west towards Manhattan from the pedestrian overpass. Allocco can be seen in the foreground.


An Allocco employee closing the gate to keep out prying eyes.  Allocco is one of the

An Allocco employee closing the gate to keep out prying eyes. Allocco is one of the "better" waste transfer stations in the area. Their Kingsland Ave. station exports material by barge, keeping trucks off the road.


While both these facilities contribute to poor air quality in the surrounding neighborhoods, the worst offenders are the thousands of diesel trucks that ply local roadways and the BQE.  As I’ve written previously, the section of East Williamsburg we traversed is plagued by a seemingly endless procession of trucks making their way up and down Grand St., Metropolitan Ave., and Vandervoort Ave. According to a Truck Study conducted by Organizations United for Trash Reduction and Garbage Equity, half these trucks are carrying waste. Not a surprise when you know this area handles nearly 40% of the 12 million plus tons of waste moving through waste transfer stations in New York City annually.

The industrial section of Metropolitan Ave east of the Grand St. Bridge

The industrial section of Metropolitan Ave. east of the Grand St. Bridge


West of the Grand St. Bridge Metropolitan Ave. is lined with 2 and 3 story homes. The exhaust and noise from the nearly constant truck traffic takes it toll on East Williamsburg residents

West of the Grand St. Bridge Metropolitan Ave. is lined with 2 and 3 story homes. The exhaust and noise from the nearly constant truck traffic takes it toll on East Williamsburg residents.


Unfortunately, I ran into some difficulties AirCasting the route I had selected. On the first attempt, with Mayeta accompanying, we ran out of time. During the second attempt my phone battery died. And on the third attempt my phone crashed. Aaaargh!  So on my fourth go, I decided to forgo stopping to attach photos and notes to my session or walk at a stable pace and just hopped on my bike and rode. Then of course it started to rain! but fortunately the rain didn’t last long.

The carbon monoxide (CO) measurements didn’t budge for the entire session and the nitrogen dioxide (NO2) measurements were also mostly stable. (When viewing the sessions, toggle the arrow on the bottom left to view the graph; hover your mouse over the graph to see the corresponding location on the map). I believe the measurements are stable because the gas sensors we’re using don’t provide high enough resolution to detect part per billion or even part per million changes in gas concentrations.

To address this shortcoming we have two projects underway. First, we’re working with Michael Taylor from Carnegie Mellon to develop a sensor package that uses an electrochemical CO sensor rather than a metal-oxide one. These sensors are more expensive but we’re hoping the expense will be justified by a lower detection limit and higher resolution. Second, we’re working with the New York Hall of Science and the New York City College of Technology to develop a low-cost calibration lab. This will allow us to place the AirCasting Air Monitors in an exposure chamber and test their response to known gas concentrations.

The particulate matter (PM) measurements are difficult to interpret because the data stream is very noisy, with measurements jumping up and down a lot. This sensor counts particles as they flow through a chamber and interrupt a tightly focused LED light beam. Normally, a sensor of this design would record measurements every second but display data in one minute increments or greater to average out the noise. However, when you’re walking, or riding a bike like I was, the spatial resolution provided by one minute readings is insufficient to identify sources. (I could be blocks away by the time a reading is displayed and never know where exactly my highest measurements were recorded).

To make sense of the noisy PM measurements, AirCasting provides average measurement values for defined areas via the CrowdMap.  But rather than average together measurements from a single stationary monitor, the CrowdMap averages together measurements from multiple monitors, carried by multiple AirCasters, during multiple AirCasting sessions. We believe this approach will compliment stationary air monitoring networks by increasing the spatial resolution of air quality data, helping to identify hot spots and sources, while also providing a better assessment of personal exposure to air pollution.

On my first AirCasting attempt the PM measurements were higher from the pedestrian bridge to the waste transfer station and declined as I walked towards Metropolitan Ave. On my final attempt I rode past the facility twice and each time the measurements were slightly elevated compared to the surrounding streets but not any higher than was generally the case as I cut west across Williamsburg picking my way towards the waterfront. One finding from this trip is that excessive shaking of the particle counter tends to falsely elevate the measurements. I attribute the spikes at the end of the session to taking off my backpack and dropping it down as I moved around the park. Michael Taylor is going to help us with this problem too, adding a fan to the exhaust port of the particle counter to regularize the air flow.

I also recorded temperature, relative humidity, and heart rate. Have a look if you’re curious. You can see when it started and stopped raining by looking at the RH stream and noting when I stopped and took cover under awnings.

My Air, My Health Challenge

My Air My Health Challenge

We’re delighted to be finalists for the EPA/NIEHS/HHS sponsored My Air, My Health Challenge. Together with researchers and engineers from Carnegie Mellon University and New York University, we will be using the AirCasting platform to link exposure to carbon monoxide and fine particulate matter with heart rate variability and blood oxygen levels. For those interested in learning more, our round one proposal can be downloaded as a PDF here. Note that round one of the Challenge required a “theoretical” proposal. We hope to accomplish everything in our proposal but with the limited time frame (5 months) and small budget (15k) we’ll need to leverage in-kind resources and build off existing hardware and software to the maximum extent possible. We’re already contemplating several changes on the hardware front from what was listed in our proposal, including using an electrochemical CO sensor rather than a metal-oxide one and using off the shelf commercially available hardware (the Zephyr BioHarness 3 and the CMS50F) to monitor heart rate variability, respiratory rate, and blood oxygen.