I went back through this thread and I didn't see that you calibrated this thing. What does it output? A mV reading? Are you applying an offset or multiplier to get the numbers you posted in ppm?
I would flow a pure gas other than CO2 through the sensor to set a zero point or derive an offset. Once you know that, whatever it reads in ambient air outside away from any local CO2 sources should equal approximately 0.00034 atm CO2 (don't know what that's supposed to be in ppm). What it reads in the head space of your test chamber should be the partial pressure (P) of CO2 above your tank. The solubility in air multiplied by 0.8317 should give you the solubility in aqueous solution. I've only ever done it in mol/L. Don't know anything about converting to ppm.
Another thing to consider is that it looks like you are supposed to flow the gas through this sensor.