The sys admin's daily grind – Airsensor

Exactly How Smelly?

Article from Issue 197/2017

If the air in a room smells stale, somebody will get up and open the window to let in some fresh air. Charly, however, wanted to measure its exact staleness, so he set off to find out armed with USB hardware and Linux.

Stale air results from the accumulation of various gases in an enclosed room. Carbon dioxide is the classic case – just imagine 16 people in a small, windowless conference room. Add to this, a mixture of volatile organic compounds, known to the world of science as VOCs. Among other things VOCs are alcohols, volatile deodorant components, aldehydes from furniture, detergent fumes, nicotine, briefly: carbon-based emissions of all kinds.

You could use human noses to sniff VOCs, but sensors in the form of a USB stick are not only more suitable but also immune to attacks of nausea. The Rehau sniffer [1], for example, uses an LED with a color changer to indicate the state of the air in the room.

Pure Air

A tool helps me to read the measured values from the stick. Before you build the software [2], you first need to install the libusb-dev and build-essential packages. After unpacking the zip file to /usr/local/, the change directory to usb-sensors-linux/trunk/airsensor/. When you get there, do this:

gcc -o airsensor airsensor.c -lusb

which will compile the small C program, creating an airsensor binary. When launched, the sensor gives you a new air reading approximately every 10 seconds:

2016-12-09 11:27:26, VOC: 557, RESULT: OK
2016-12-09 11:27:37, VOC: 540, RESULT: OK
2016-12-09 11:27:48 AM, VOC: 542, RESULT: OK
2016-12-09 11:27:59 AM, VOC: 563, RESULT: OK
2016-12-09 11:28:11 AM, VOC: 515, RESULT: OK
2016-12-09 11:28:22 AM, VOC: 505, RESULT: OK

The unit of measurement for the VOC concentration is parts per million (ppm); my sensor is specified for values between 450 and 2000 ppm. It delivers even higher values, but an algorithm has probably interpolated these. For values below 1000 ppm, the built-in LED lights up green, yellow between 1001 and 1500 ppm, and red above that.

Using the -o parameter, I told the software to output a measured value and then terminate. That makes it easy to write the values to a database as datapoints and draw history graphs. Figure 1 shows what happens when you cook ham and eggs for breakfast.

Figure 1: The event that led to the dramatic deterioration of Charly's indoor climate at 6:45 a.m. will be revealed at the end of the article.

Charly Kühnast

Charly Kühnast manages Unix systems in the data center in the Lower Rhine region of Germany. His responsibilities include ensuring the security and availability of firewalls and the DMZ.

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