[aprssig] APRS Air Quality Monitoring

[Steve Dimse]

> On Nov 11, 2018, at 8:35 PM, KF4LVZ <aprssigZbr6 at acarver.net> wrote:
> 
> You can scale the output by changing the airflow.  Swap the fan out for
> something with 10x the flow (as small diaphragm vacuum pump with
> throttle valve so you can tune it) and the 0-999 will actually be
> 0-9990.  It reduces the resolution by 10x but you'll have your range.
> Just put a filter in front of the vacuum pump to keep the dust from
> fouling the diaphragms and valves.
> 
> I've done this at work with commercial detectors to tweak the range as
> needed without swapping out for other sense heads (expensive).

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.

The first test I did of this was to blow out a match right next to the inlet. By removing the match quickly I give a pulse on input, and it reacts just as one would expect if it were based upon the average number of particles inside the chamber, with a gradual (meaning ~15 seconds) fall after removing the smoke input. If this were based upon something that was flow dependent (number of counts past the laser) I would expect the cutoff to be sharp. 

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? 

Steve K4HG