Bad air

At some point I noticed that my office air was getting bad, especially in late afternoon. As this building is somewhat old, and doesn't have good air exchange now that old fireplace isn't used anymore this wasn't very surprising. How bad it was though was a bit more unknown.

I managed to get my hands on a carbon dioxide sensor for some time and was surprised to see co2 levels reaching nearly 2000 ppm during afternoons. No wonder I was getting really tired towards end of days. So I got some improvements made (added intake vents, and wind-powered vacuum at top of old chimney) to improve situation. After this I did notice improvement, but at that time didn't have tools to find out how much the situation really improved.

This was close to a year ago. Out of curiosity I now got a few sensors that should be able to determine this. Specifically, Sensiron SCD30 carbon dioxide sensor and SGX Sensortech MiCS-VZ-89TE air quality sensor.

MiCS measures VOC levels (Volatile Organic Compounds, measured in ppb or parts per billion) by using semiconductor sensor. Datasheet doesn't exactly tell how that is done, but in many similar sensors (as far as I know) this involves hot plate (very tiny one, but nevertheless) on which chemicals and compounds react and change the resistance of this plate which in turn can be measured. This technology isn't very specific, but to put it bluntly, is is still useful to detect many kinds of, well, volatile compounds (most, if not all of which can be considered to be less than healthy to breath in.)  Although datasheet mentions co2, it doesn't really directly measure it and instead gives "co2 equivalent" reading mostly based on VOC reading. Many times these correlate, but not always. But even then, the good side is that these sensors give out a relative reading of air quality and are relatively cheap to be embedded in many consumer-level goods. Think of your phone signalling you that "yeah, you might want to get out of here" when you enter area with less than great air. That at least is target for many of these sensors.

SCD30 on the other hand uses nondispersive infrared sensor (NDIR) to measure actual carbon dioxide level in air, and pretty much nothing else (although I guess some gases/chemicals with similar absorption characteristics as co2 might somewhat affect the reading.)  So this doesn't detect nasty solvents, cigarette smoke or other such things that might be in air. These sensors tend to be a bit more expensive too so they're not that often seen in consumer equipment.

I left these sensors running for some time, below being graph from one day period (noon-ish to noon-ish next day). Sensors actually had been running much longer at this point but I managed to screw up logging (several times, actually) so previous data was lost. Ah well.

Here the co2 level measured by SCD30  is on blue. The level drops when I leave office, eventually falling to current outside air levels (400ppm), and on next day starts to rise immediately when I come in, despite improved air circulation. At least it no longer gets anywhere close to previous 2000ppm levels, instead maxing out at 1200ppm. Not great, but much better than previously.

Red graph shows "equivalent co2" as measured by MiCS. At start of graph it follows SCD30 results relatively closely (not bad!), but veers wildly off at start of next day for some reason. Huge spike you can see in that and VOC reading are because I put small amount of denatured alcohol on tissue and left it near the sensor, to which it reacted immediately. Note that actual (blue) co2 levels weren't changed; only now there was volatile compounds in air that this sensor detected, resulting severe degradation in reported air quality.

I now hope I didn't damage the sensor by leaving alcohol too close to it. Hot plate sensors shouldn't be that sensitive (we're talking about table spoon's worth of alcohol total here) but who knows how this one reacts. Its behavior at end of graph doesn't look too good though. I need to follow its behavior some more to see how it behaves now.