I have an MQ131 gas sensor. In principle when you hook it up it correctly, it forms a voltage divider. So does the Resistance of a gas sensor increase or decrease when the gas concentration increases?
The datasheet of the MQ131 says that the resistance is lower if more ozone is detected.
Thanks! I read the data sheet for what felt like an hour and still couldn't find it. But I think different manufacturers have different data sheets, so I guess it may not have (explicitly) been said on mine...
I used that one: http://www.sensorsportal.com/DOWNLOADS/MQ131.pdf
The first paragraph tells you how the sensor reacts on changing the ozone concentration.