Speaking of DIYing a pH probe: I have a Milwaukee portable pH meter that seems to still work ok, even after I neglected it for several months. The business end has a cap on it, where you are supposed to keep some water to keep that end wet at all times. I wonder if I could cut a hole in the end of the cap, cover the hole with semipermeable membrane, add the reference KH solution in the cap, reinstall the cap and use the assembly as a CO2 probe? See my crude sketch below:

What is the major downside to cutting a hole in the cap that way? I could continue to store the probe with water in the cap, but it would let atmospheric CO2 enter the water.

This post, this morning on APC, DIY Drop Checker - Page 13 - Aquatic Plant Central- aquascaping...a living art started me to thinking more about making a DIY CO2 probe. So, this afternoon I visited my local Home Depot, found their bin of Tyvek, helped myself to a 3" square of it (so, sue me, I swiped it!), and now I'm chomping at the bit looking for progress!