I have to +1 this if you're working with a sensor that has any potential life safety applications. Arduino and the MQ-2 are not rated for life safety applications. It would be impossible to calibrate over time (you should bump test daily in a perfect world). The very best would be rough go/no go advisory in an environment that's constantly monitored where life safety isn't a factor, and that would still require a fresh air zeroing right before you anticipated the presence of your target gas.