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Gasser

Gasser is a self-contained mobile sensor for sensing NO2, O3, and SO2. Currently we run it only with NO2 as that is the most present pollutant in Paris.

Gasser has essentially three main parts:

  • The sensor (any analog or digital sensor can work) -- top left in the main box.
  • The ADC & computer (analog to digital converter) -- main board inside the box.
  • The communication medium (3G, ethernet, or write to an SD card) -- white dongle on the left side of the image.
  • Power -- black box on the left side of the image.

The Sensor

After testing an ensemble of various sensors and holding many long-winded discussions with experts in the field, we have currently settled with the Alphasense B4 series of sensors. These sensors, with their control board, run in the ~100eur, but their precision is unmatched by anything else on the market.

Output of the sensor is in the sub-mV range, and therefore proper grounding and filtering of the lines to and from the sensor is essential. Our first version of gasser (not documented here, but basically the same idea just worse :P) was able to achieve precisions in the ~10ppb range when tested in a laboratory setting.

The ADC & Computer

We currently use a Raspberry Pi with a Delta-Sigma ADC. The Raspberry Pi hardware is very nice (and cheap) for rapid prototyping, and allows us to send sensor information to anything from a GPRS dongle to a megaphone (both have been done). The software on the raspi is currently quite trivial, taking in information from the open-source delta-sigma ADC's library and simply sending it on either to the internet or to the SD card in a file. As our software framework becomes more complex we will begin pushing it to github or somesuch.

The Communication / Storage Medium

We quickly fell in love with the Huawei E220 GPRS dongle, it has proven to be extremely stable, cheap, and have rather good reception. Interfacing with this dongle is quite easy, as it is fully supported by linux, so simple configuration of wvdial allows the raspi to have a full internet connection most anywhere in France. We use a SIM card from Free (2eur/month), which comes out to be a rather good deal if you're not sending inordinate amounts of data.

Power

We currently use an 8Ah battery often billed as something to "recharge your iPad and iPhone" with. This works quite well as it allows for the sensor system to stay on all the time, whether plugged in or not. According to our calculations and trial runs, this should allow the system in its current state to stay on for about 5-6 hours. It also allows the system to stay on perpetually if given a 5v micro-usb input source such as a tablet charger or other >1.5A 5v wall-wart.

Future Plans

Although there isn't much specific software to be run on the raspi, the full system configuration has been rather thought out, and everything is relatively foolproof (lost internet connections go back up, dead daemons get restarted, etc.) Although it probably won't happen right away, we are hoping to release a custom raspbian image with all the scripts and daemons configured that would allow anyone to quickly and easily set up their own monitoring system with this wonderfully cheap hardware.

BOM

  • Raspi
    • ~30 eur
  • Huawei E220
    • ~35eur (amazon has them)
  • Delta-Sigma ADC for RasPi
    • ~30eur
  • AlphaSense B4-series sensors
    • ~110 eur
  • 8Ah phone charging battery (optional)
    • 30eur.
  • Wonderful plastic box
    • ~10eur

Total: 255eur for a high-precision highly-customisable mobile pollutant sensing platform, and this is only the v2 prototype :-)

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Page last modified on December 10, 2012, at 10:53 AM