I picked up my AirPi at the London MiniMaker Faire. Good times. It's now built and running.
However, it was showing a CO reading of around 11ppm. In the same room my CO alarm was reading 0.
Now my guess is that the resistance in pure air is not very fixed between devices and that's the calibration requirement needed. I'm assuming that the Resistance field in the configuration file defines this so I tweaked it to be 0.
11ppm could be acurate for inside a house have a look at the wiki page it states:
0.1 ppmv Natural atmosphere level (MOPITT) 0.5–5 ppmv Average level in homes 5–15 ppmv Near-properly adjusted gas stoves in homes, modern vehicle exhaust emissions 17 ppmv Atmosphere of Venus 100–200 ppmv Exhaust from automobiles in the Mexico City central area 700 ppmv Atmosphere of Mars 5,000 ppmv Exhaust from a home wood fire 7,000 ppmv Undiluted warm car exhaust without a catalytic converter
Well ,after some consideration I stuck the AirPi next to an open window (it was where the lock is) and left it running.
Over last night it showed a drop down to zero then it's slowly climbed up during today.
I'm very interested to see what happens over the next few days. If the climb is steeper tomorrow (Monday) at around 7/8 AM then it's looking very much like car exhausts!
I have my doubts about the 11ppm because that's for being near a working gas appliance and right now all my are certainly off! I'm in Ipswich in fairly rural Suffolk so a PPM of 0 overnight is believable.