So, working with the BME680 has left several odd impressions.

1) The gas sensor

No idea what it is measuring, but it does measure something.  For example, a Sharpie, immediately picked up.  It reads in Ohms.  Range seems to be around 10k - 250k.  

When you set up the sensor, they suggest doing a "burn in" of the gas sensor for up to 5 minutes before taking readings.  I've found this really needs to be more like 10-20 minutes before it actually stabilizes.

The sensor is activated, by basically turning on a little internal heater.  This heater does something or other, and then the sensor produces values.  Side effect?  The builtin thermometer is affected by this!  Once I set my heat cycle up, I noticed that the thermometer read high by 2C!  This is ok, as long as you know this up front.  My code simply subtracts 2.

2) Pressure:

Because I have 2, I can put them right next to each other, and see how consistent they are.  The temperature is pretty consistent between the two, however, the pressure, not so much.  They are off from one another by about .89mb.  This isn't to say they are inaccurate.  For example, they both move together perfectly.  So if one gains 5mb, the other does too.  The curves don't diverge up and down, they stay exactly .89 apart.  I guess without a calibrated test environment, I will never know the actual pressure.  However, for my experiment, this is irrelevant.  They are accurate to moves up and down, so as long as I subtract the appropriate 0.89, I get the difference readings I need.