Well, I've starting thinking again (that can be dangerous) about my charged-air project. It's very nice and stable now. So it's time to think about a next level. Probably a "secondary", turned on (optimally) when airflow rises in the filter compartment. I tried a couple very popular tach sensors a while ago, both showed garbage data alternating with good data. I could imagine getting data out of the mass airflow sensor...if that were practical, which I'm not convinced it is. Or a vacuum switch added with a T somewhere? Or conceivably, something like this:
The air flow switch is used to prove air flow in ducts. It has an adjustable trip point with a range from 200 to 1800 FPM (1 to 9.2 m/sec). The SPDT contacts are rated8A@250VAC.