The Carbon Dioxide Sensor is only for measurement of CO2 in dry gasses.  

Between readings, gently wave the sensor through air (if possible) to refresh the air in the sensing section of the sensor. 

Carbon dioxide is denser than the other components of air. It will sink to the bottom of any collection vessels. Absolute measurement values should take account of this. You can see the effect of this by pouring CO2 into a bucket and slowly lowering the sensor into the bucket. A dramatic increase in recorded CO2 will take place as the sensor reaches the bottom half of the bucket.  

The sensing snout of the sensor is lightly tapered to allow it to be gently pushed into an appropriate sized hole. The opening of the included Nalgene bottle will demonstrate this. 

The included Nalgene bottle has a rubber ringed hole to one side that is designed to create an airtight seal around the oxygen and carbon dioxide sensor. It is recommended that in designing the practical, you ensure the CO2 sensor is at the bottom of the collecting vessel.  

There is a disparity between the range of the CO2 sensor and the Oxygen sensor, CO2 is present in significantly lower concentrations than O2, you may find that attempts to measure both CO2 and O2 changes around plants will not be successful, this is not a fault of the accuracy of the sensors, it is a consequence of the relative concentrations. We would recommend that for photosynthesis studies measure the change in CO2 only for good data.