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I have bought a MQ135 gas sensor. I have read online from multiple sources that a calibration/warm-up period of between 12-24hrs is required for the sensor to give its most accurate readings. I currently have it attached to one of my UNO's just spitting out Analog readings to the Serial monitor and seems (just after an hour!) to have settled at a reading of 50 which it has stayed at now for a period of around 20 minutes. My questions surrounding this process really are:

  1. What is the point in doing this process if when a project that uses this sensor is powered off for some number of hours? Will you have to let it warm-up/calibrate again before having faith in the readings?
  2. How do I actually identify PPM for a gas such as C02? Is there some form of constant that I should be comparing the reading obtained to or something similar?
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  • Welcome to Arduino SE. Feel free to take the tour at arduino.stackexchange.com/Tour that will help you get the most from this site. The warm-up period has to do with the physics of how the sensors work. This is a device primarily used with Arduino so I believe it is OK here. But you might want to edit your post to indicate which kind of Arduino you are using and show some of your code so you can pose an Arduino-specific question. – SDsolar Apr 20 '18 at 20:27
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By my opinion, it is impossible to calibrate these solid electrolyte based sensors. I achieved best results when I measured the CO2 level in 5 minute intervals, by heating the sensor for 100 seconds. I cant remember these times precisely, but by heating and allowing it to cool down I was able to get quite reliable results. But then I realized that the sensor is getting some offset. If you want to do a two-point calibration, you can put it to fresh air, it should read about 0.04%, or you can breathe directly to the sensor for a short while and it shoud read 4%. But I highly recommend using NDIR CO2 sensor, those are used in home air monitoring devices.

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