Maintaining a 40% CO₂ aqueous environment?

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](http://www.theplantedtankfaq.com/CO2_FAQ.html#)) because they rely on a [pH/KH chart](http://www.theaquatools.com/co2-calculations).


Atmospheric $\ce{CO2}$ levels are measured based on [IR absorbency](http://www.esrl.noaa.gov/gmd/ccgg/about/co2_measurements.html). 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](http://airs.jpl.nasa.gov/story_archive/Measuring_CO2_from_Space/History_CO2_Measurements/)."


Is there a reliable way to measure the $\ce{CO2}$ concentration that is relatively inexpensive (under $1,000)? Is the [drop checker method](http://www.aquaticplantenthusiasts.com/co2/4-drop-checker-revealed.html) reliable?


