PurpleAir sensors use PMSX003 laser counters to measure particulate matter in real time, with each laser counter alternating 5-second readings averaged over 120 seconds. Each laser counter uses a fan to draw a sample of air past a laser beam. The beam from these class IIIa/3R lasers will reflect light from any present particles onto a detection plate, like dust shimmering in a sunbeam. The reflection is measured as a pulse by the detection plate, and the length of the pulse determines the size of the particle while the number of pulses determines the particle count. These particle counts are used to calculate the mass concentrations of PM1.0, PM2.5, and PM10 for standard indoor (CF_1) and outdoor particles (ATM). (https://www2.purpleair.com/community/faq#hc-how-do-purpleair-sensors-work)
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https://www.purpleair.com/sensorlist?key=C21YVYSKZCPT6KTN&show=35053
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