Do you test for Carbon Monoxide in your inspections? I know some inspectors carry a detector tool around during the inspections for their own safety, some clip on your belt. Which detectors are you using?
I have the personal CO monitor pictured. It always reads 4 PPM even when the ambient air is 0 ppm I would not recommend it. Sensit P100 seems to be what most use. They come in 2 year, 3 year, and maybe 4 year. As soon as you turn it on the first time the clock starts ticking and it stays on until the time is up. Then you buy a new one. There are many others brands just check around. The things that I think are important are no calibration for a set period of time, digital readout that shows the ambient air ppm, have a clip to attach to you so it doesn’t fall off and get lost of you don’t have to hold it.