The hydro store has single use testers on the cheap. It's a kit... The device that administers the test and 3 single-use tests for $20, I believe. Additional tests were $3-4 each test. Probably not economical in the long run, but good for figuring how effective your CO2 dosage is. I might get one for my yeast CO2 setup to decide on a frequency for remixing bottles.
Does anybody have an opinion on the link the spammer posted? $120 for a basic ppm meter sounds nice. Accuracy? Reliability?
C2C