Healthy Indoors Magazine - USA Edition

HI Feb 2019

Healthy Indoors Magazine

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Healthy Indoors | 29 ing through the chamber). We can see that the bulk of the particle pulses are found between the yellow lines with the red line roughly indicating the median height of these cali- brated particles. Without getting bogged down in the actual detail we can then generally discuss the requirements for the standard in view of the above figure. In the text below I'll use clean- room applications to refer to the industry the standard was intended to address and then discuss thoughts as to how these might also be applied to more general air quality ap- plications (which might include both indoor and outdoor air quality monitoring). So with the above in mind here are the requirements for calibrating particle counters with ISO 21501-4. 1. Size calibration This requirement states that the signal to noise ratio of a calibration channel be at least 2:1. In the above figure that means that the number of counts for the height under the red line be twice the number of counts for the height under the green line. What we're really attempting to do here is to ensure that we can discriminate particles from noise on the smallest channel and correctly identify particulates by size on the other channels. For the cleanroom industry being able to size particulates accurately is a critical requirement since many manufactur- ing processes are highly dependent on the size of particu- lates in the environment and to the quantities of particulates at various sizes. In air quality applications if we're estimating mass based on particulate sizes and concentration then our estimate can only be as good as the accuracy of the sizing and count- ing in the instrument. Instruments that measure aggregate mass must rely on estimates of size distributions in the en- vironment in order to attempt to attribute the total scattered light to estimate of particulate mass. 2. Verification of size setting This requirement states that we be able to setup intermedi- ate measurement sizes based on our calibrated sizes and that these new sizes be reasonably accurate (+/- 10% of the actual threshold). If we calibrate our instruments for a handful of sizes, the user should be able to sort particulates by sizes we have not calibrated, with the instrument calculating the expected thresh- old of these sizes based on the sizes that were calibrated. For cleanroom applications, the heart of a particle count- er is its ability to discriminate particles by size. So, it's critical that the sizing of all channels be reasonably accurate when calculating an intermediate size threshold. estimate of mass. The most expensive of these have some means of excluding certain particles by size (in subsequent samples) in order to refine their distribution estimate and im- prove mass estimates. To calibrate a particle counter using ISO 21501-4 the manufacturer uses very special particulates with a tightly controlled size, shape and refractivity (typically polystyrene spheres from a NIST traceable source). I'll omit discussing the calibration system in detail (since such discussion could easily result in several articles by itself), suffice it to say that these particles are aerosolized and presented to the instru- ment mixed with clean air such that these are the predomi- nant particles passing through the instrument. As these particles pass through the instrument they cross the light beam scatter light as described. Scattered light reaching the optical detector is amplified and the result- ing pulse height recorded. In this case instead of binning it into a reasonably small number of channels (as we would when operating) a large number of bins are used to provide a lot of resolution to the sizing. We record a large number of particle heights during a sampling period and create a histo- gram of these (plotting the # of particles seen in each bin). This histogram looks something like the simplified plot in the image to the right with the blue line representing the histo- gram values with the counts in each bin on the Y axis and the height bins on the X axis. So, that as we move from right to left on the graph the particle pulses increase in height, from very tiny noise pulses on the far left to a few very large pulses (typically multiple particles passing through the beam together) on the far right. The short green line indicates roughly where the noise threshold is. Generally height bins to the left of this line are considered to be "noise" (very small pulses and these will also typically be present in clean air). Height bins to the right of the green line are considered to be signal (these will only be present when the calibration particles are pass-

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