[aprssig] APRS Air Quality Monitoring

[Scott Miller]

> My guess is this sensor works differently than the others you have worked with. I believe it is a laser that measures the light scattered at different angles. The bigger the particle the more light will be scattered at larger angles. It has quite a large chamber (3 inches on a side) and uses a small fan to exhaust the chamber.

I haven't seen one with multiple detectors, but that sounds like a 
nephelometer - they measured reflected off-axis light.  Apparently you 
calibrate them with Arizona road dust...

> I'm curious about the sensors you are talking about. While I could understand methods that depend on a calibrated flow, e.g you could get particle concentration by counting particles passing a point, I would expect that increasing the flow would increase the count, raising the sensitivity of the sensor rather than decreasing it. I don't see how increasing the flow could decrease the sensitivity. What am I misconceptualizing?

The moving air isn't what's measured - it's just a laminar flow over the 
optics to keep the dust off.  That seems to be the tricky part.

Scott
N1VG