[aprssig] APRS Air Quality Monitoring

KF4LVZ aprssigZbr6 at acarver.net
Sun Nov 11 23:41:45 EST 2018

On 2018-11-11 20:20, Scott Miller wrote:
>> 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...

Yes, this just looks at the intensity of the reflected light and
correlates the intensity to a particle size.  It bins the range of
reflected light intensities (which is why there's an LNA on the
photodiode).  Nothing overly fancy and certainly no multiple detectors.
Whatever crosses the light path gets counted.  But it's easy to confuse
if multiple particles cross at once.

>> 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.

Yeah, it's not measured but it's assumed as part of the calculation to
convert a count-per-unit-time to a particle-per-unit-volume or
mass-per-unit-volume (#/L or ug/L).  Any change to the flow will shift
the calibration.

My sensors here are going crazy with the SoCal fires.  I have very old
windows (I rent) and they're drafty.  The smoke is very much obvious
when looking at the history graphs.

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