The setup looks like this: (look at that wire forest!)
The gas sensors are wired in parallel. Nothing has changed in the individual wiring of each sensor. They just link to a common ground and power source (except the CO sensor - it gets its own power from pin 11). The code I used reads out all the "raw" numbers for each of the sensors, and also monitors the PWM wave for the CO sensor.
The numbers on each sensor are all fairly consistent. After repeated runs, I got these numbers:
The only problem is that the sensor's don't seem to respond at all to changing gas levels.... I've checked my connections twice already, but there doesn't seem to be a mistake. So I'm assuming it has something to do with either my code or a bad connection. I suspect it might be a connection issue, actually, because the H2 sensor and the LPG sensor are both really hot, while the other two sensors are ice cold (as if I didn't even plug them in). Interestingly, while "burning in" the H2 sensor, the sensor was ice cold. But the power source I used to burn the sensors in displayed that the H2 sensor had a current... So I'm not sure what the temperature means in terms of connections.
All 4 sensors together use a total current of 0.43 A max at 5.0 V.