We can't get that low with confidence.  The performance specs for our CO2 sensor is it has an accuracy of ±40ppm, a calibration uncertainty of ±31ppm, and long-term stability of ±60ppm.  I'd say we can get down to ~100ppm reliably.  To properly design the scrubbing system, we'll need to know the chamber type and how long the low CO2 level needs to be held.