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.