The Met One 831 sensor measures particulate matter (PM) by counting and sizing individual particles using scattered laser light. The unit then converts the count data to mass measurements in micrograms per cubic meter (µg/m3). The Met One 831 counts particles in four different PM size ranges simultaneously (PM1, PM2.5, PM4, and PM10). Each of these represents a maximum particle size. Thus, PM10 measures all particles 10 micrometers (µm) in diameter or less and PM1 measures all particles 1 µm in diameter or less. This handheld device was designed for use in applications such as remote sampling, indoor air quality monitoring, and industrial and occupational hygiene. It has an internal battery pack that provides 8 hours of continuous operation, and it can store up to 2,500 data points that can be either viewed on the Met One 831 display or exported to a computer using a USB cable. This operating procedure explains what you need to do to collect quality data using the Met One 831 for your monitoring project.