2-3 hours is a long time...............
The day is only 10 hours so you miss 2-3 hours on either end.

also, what is fine at 1-2 hours of the CO2 being on, is quite different 3-4 hours into the day cycle.

You are playing catch up the entire day.

The time constant can be reduced a number of ways though..........

1. Increase the surface area to Kh ref solution ratio. This would allow more gas to equilibrate faster based on Fick's 1st law.

2. Decrease the air gas distance, this based on Fick's 1st law will also increase the Flux

3. If you wanted to be very critical, temp can also be added into Fick's 1st law.

4. Reduced KH solution around probe, less volume of ref solution for the CO2 to diffuse evenly into.

5. There's more but I gotta run to a seminar

Regards,
Tom Barr