For those of you running the Datalogit/DIY combo:

When I checked the free-air value for the sensor, it read 4.17V on AN1 and the ground value for AN2 was 0.15 or so. Therefore the AN1-AN2 value was the predicted 4.0V the unit should return.

Did I check the calibration correctly, or should AN1 equal 4.0V when the sensor is in the air?

 

That is the way I understand it also and is exactly how my Datalogit behaves. Apparently the datalogit records some floating ground problems and the Delta AN1-AN2 resolves the conflict.

You should, however, calibrate the sensor independently of the datalogit using an accurate DVOM. Attach the Vout (pin 7) to the positive lead and the GND (pin 6) to the negative lead. In free-air and after the warmup period (DVOM should read 2.5 during the warmup), you should see ~4.0V .