@epierre Thanks for a rapid response!

Yes I've been trying to calibrate it assuming out door air is at 399ppm. I understand each one has variances but would like to know how you guys worked it out.

I'm using

ppm = 116.6020682 (Rs/Ro)^-2.769034857

from davide's blog http://davidegironi.blogspot.ch/2014/01/cheap-co2-meter-using-mq135-sensor-with.html#.VinvWX4rIuU

Since the board show RL, I took that to calculate Rs. This lets me calculate Ro, thus calibrate it.

Not quite sure where I'm going wrong. I had it working 8 months ago, and came back to it and couldn't figure out how i had it working so well.. I was able to get pretty nice graphs and now its very unstable.

I'm having a play around with it, you can see my current points here https://thingspeak.com/channels/31162