I wish to grow algae in a 40% $\ce{CO2}$ (water under air with 40% $\ce{CO2}$ content) aqueous environment.
Simple DIY methods are available, but they are generally unreliable (For more info on DIY read here) because they rely on a pH/KH chart.
Atmospheric $\ce{CO2}$ levels are measured based on IR absorbency. However, such measurements are difficult and "complicated by the temperature signal generated by carbon dioxide as well as by the signatures of ozone and water vapor."
Is there a reliable way to measure the $\ce{CO2}$ concentration that is relatively inexpensive (under $1,000)? Is the drop checker method reliable?