How to measure gases

[kecked]

I made a long path detection system using a leaky fiber optic in a coil. take a pair of fibers and scrap one side so it leaks. Run a reference signal down one and the IR in the other. Measure the absorption. The IR is wideband from lamp and fiber launch. The other fiber picks up the signal and goes to a monochrome/ccd detector.

[kecked]

Zirconium based O2 sensor

Kind of risky in compost? The running temp is way high and could combust the compost. Why not electrochemical? Granted you need a good hydrocarbon filter.
Sputtered Teflon works well. I made an optics based oxygen sensor but can't share the details.