[Stoves] PM measurement with Speck

[Crispin Pemberton-Pigott]

Dear Dave

 

>Looking at the Speck product description and the Syhitech DSM501A
documentation (such as it is), I wonder if/how Speck can differentiate
several 2.5-micron-diameter particles from a solitary particle that is 10s
or 100s of microns in diameter.

 

The reflection off each particle is measured according to its strength with
a detector. Small particles give a very small signal. This is calculated to
give an effective diameter, as if all the particles are round, known as the
aerodynamic diameter.

 

The particles are fed through at a very high speed - perhaps 20 metres per
second in a thin air stream sheathed with a clear air stream for a total
diameter of about 1mm.  With a device as simple as the Speck the whole thing
is miniaturised but works on a similar principle at a low speed.

 

When only PM2.5 is wanted, a cyclone can be attached that removes particles
larger than PM2.5.  There are in fact very few particles in the range 2-3
microns which is why that size was chosen. Being a little off makes no
different to the total count.

 

Most particles from a solid fuel fire are below 0.5 microns unless there is
a fan blowing up ash in which case it tends to be PM4 or so, well away from
PM2.5.

 

Regards

Crispin

 

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