How to use smart air quality sensors for environmental monitoring
a mass concentration number is more complex. The software used for the conversion needs to consider the particles’ optical parameters like shape and refractive index. As a result, OPCs can suffer from greater inaccuracy compared with other PM sensing methods such as direct, weight- based, gravimetric technologies. Not all OPCs are the same. Highly accurate and expensive, laboratory-grade OPCs can count every particle in the measurement cell. Lower-cost commercial-grade OPCs are available that sample only about 5% of the aerosol particles and use software-based estimation techniques to arrive at an overall ‘measurement.’ In particular, the density of large particles like PM10 is typically very low, and they can’t be measured directly by low-cost OPCs. As particle size increases, the number of particles in a given particle mass drops dramatically. Compared with an aerosol of PM1.0 particles, an aerosol with PM8 particles has about 500 times fewer particles for a given mass. To measure larger particles with the same accuracy as small particles, a low-cost OPC has to integrate data over several hours to arrive at an estimate. Fortunately, aerosols have fairly consistent distributions of small and large particles in real-
world environments. With properly designed algorithms, it’s possible to accurately estimate the number of larger particles, such as PM4.0 and PM10, using measurements of PM0.5, PM1.0, and PM2.5 particles. Amperometric gas sensors Instead of measuring particle counts, amperometric sensors measure gas concentrations. They are electrochemical devices that produce a current linearly proportional to the volumetric fraction of the gas being measured. A basic amperometric sensor consists of two electrodes and an
electrolyte. Gas concentration is measured at the sensing electrode, which consists of a catalytic metal that optimizes the reaction of the gas to be measured. The gas reacts with the sensing electrode after entering the sensor through a capillary diffusion barrier. The counter electrode acts as a half- cell and completes the circuit (Figure 2). An external circuit measures the current flow and determines the gas concentration. In some designs, a third ‘reference’ electrode is included to improve the stability, signal-to-noise ratio, and speed the response time of the basic amperometric sensor.
Figure 2: Amperometric sensors use two electrodes separated by an electrolyte to measure the concentrations of gases. (Image source: Spec Sensor)
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