I was looking at find a sensor that would let me monitor radon levels in my home. While it did lead me to the very cool uRADMonitor, the model D was a bit above my budget for this science project. Instead, I decided it would be much easier to just monitor the mitigation system (eg. a fan) to ensure it didn’t fail.
I’ve posted a parts list an instructions here, but it’s all pretty simple. Take a door/window sensor with terminals (Monoprice 15270, Linear WADWAZ-1, Schlage RS100, etc), connect it up to a differential pressure sensor, T off the tubing from the manometer (that’s the thing with the blue water it in), and connect it to the low pressure connection on the pressure switch. I chose the Dwyer 1910‑00 because it was solid (almost all metal), the most sensitive of the 1900-series line, and can be picked up for ~$20 on eBay, but any other sufficiently sensitive switch should work.
The device handler, a parts list, and basic instructions on how to put it together are here:
Hopefully someone else finds this useful!