1496 2. MATERIALS AND METHOD 2.1 Instrumentation The OPC N3 sensor (Make: Alphasense, United Kingdom) is a small (75 mm x 60 mm x 6 5mm) and light (105 g) optical particle counter that can identify particles with diameters ranging from 0.38 to 40 µm in 24 bin sizes (Figure 1). The particles are counted using an elliptical mirror and a dual-element photodetector. Figure 1 - Portable aerosol spectrometer (PAS), OPC N3 sensor, sensor assembly, and portable weather station The manufacturer of the OPC N3 sensor provides software and the necessary drivers to run the sensor from a computer or PC. The sensor is integrated with a Raspberry Pi microprocessor to create a compact and portable monitoring device suitable for monitoring in mines. The overall sensor assembly weighs less than 220 g and can be operated continuously with a power bank. For monitoring of important weather parameters like temperature, relative humidity, and wind speed that can alter the sensor response, a portable weather station (Kestrel) is deployed along with the PAS. This compact device can monitor parameters in real-time with a resolution of up to 1 second and store the data simultaneously with its builtin memory system. The accuracy and precision of the sensor was determined by conducting a colocation experiment along with a research grade reference instrument. The performance of the sensor was comprehensively evaluated in terms of linearity, precision, error, and bias based on the testing protocol and metrics for non-regulatory supplemental and informational monitoring applications recommended by the United States Environmental Protection Agency (US EPA).
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