Re: Water quality testing and dissolved carbon dioxide

It is more common for instrumentation to fail than for abrupt changes to occur spontaniously in aquatic environments. The first thing I would check is the instrumentation. Try exhaling through a straw in to a glass of water and then sampling the glass of water for CO₂. Or try using a lime water indicator solution. You may be able to get some agricultural lime from the people that stripe your athletic fields.

If you find that your readings are accurate, you need to check the sensitivity of your test. Zero can mean lees than 10, less than .01, less than .000001, etc. depending on sesitivity. In real systems, zero almost never means absolute zero. I think your school might like to forge a former partnership with EPA's Volunteer Monitoring Program. They can help with questions that come up, and your school would be contributing to the continued water quality monitoring and cleanup efforts of the US Environment & Ecologyal Protection Agengy.