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</o:shapelayout></xml><![endif]--></head><body lang=EN-CA link="#0563C1" vlink="#954F72"><div class=WordSection1><p class=MsoPlainText>Dear Dave<o:p></o:p></p><p class=MsoPlainText><o:p> </o:p></p><p class=MsoPlainText>>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.<o:p></o:p></p><p class=MsoPlainText><o:p> </o:p></p><p class=MsoPlainText><span style='color:black'>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 <i>aerodynamic diameter</i>.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>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.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>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.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>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.<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p><p class=MsoPlainText><span style='color:black'>Regards<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'>Crispin<o:p></o:p></span></p><p class=MsoPlainText><span style='color:black'><o:p> </o:p></span></p></div></body></html>