Optical PM sensors count light scattered by particles moving through a sensing volume. The instrument sees optical behavior, then estimates mass concentration for size fractions such as PM1, PM2.5, PM4, and PM10. That conversion depends on assumptions about particle density, shape, refractive index, and size distribution.
This is why a published PM accuracy number should be read as a datasheet capability under specified test aerosols, not as a universal field guarantee. Cooking oil aerosol, wildfire smoke, mineral dust, pollen fragments, and humidified particles scatter light differently. The sensor can still be very useful, but the correct interpretation is trend, event, recovery, and relative burden.
Precision and accuracy are different. Precision asks whether repeated readings under the same condition agree. Accuracy asks whether the estimate matches a reference method. A building-monitoring workflow needs both, plus enough context to know when the aerosol has changed.
Terrestream treats PM data as measured telemetry with explicit limits. The page at Measurement Evidence: Precision labels the PM charts as datasheet capability or illustrative scenario so the chart cannot be mistaken for field validation.
