What is a Constant Power Circuit?
The following is an example application circuit to power the heater for the solid state gas sensor:



,where V1 is our 5 V DC source, Rserial is a resistor, and RH is the heater on the gas sensor.

In this example we are using an Rserial value equal to our nominal powered heater resistance value.  This means, by way of the maximum power transfer theorem, that most power will be transferred when the heater resistance is equal to the serial resistor, Rserial.  If the heater resistance, Rh, varies in either direction from nominal the power dissipated in the heater will fall off quadratically.

The following graph shows when the simulation is run.  The X-axis is the Rh value being swept from its minimum to its maximum operating resistance value.  The plotted curve is showing the power dissipated by the heater resistor Rh as its value deviates.



Notice that even if the heater value resistance changes on us by +- 15%, the power dissipated in the heater resistor changes by only ~1%, thus maintaining a relatively constant temperature on the sensing element.

Aren't solid state gas sensors fun!

Edit1:
If your budget allows it and you are going to be required to become proficient with these technologies, I find that this book has a lot of useful information about the history, characterization and performance of solid state gas sensors.  The circuit applications presented in the book, however, are lacking.