In one case, oil droplets were generated and classified to efficiently generate a wide range of size distributions of aerosolized particles to use for calibration. and such methods have been suggested previously in the literature. ISO, Determination of particle size distribution-Single particle light interaction methods-Part 4: Light scattering airborne particle counter for clean spaces (21501-4), 2018.
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Alternative calibration methods are allowed under ISO 21501-4, however, as long as their uncertainties are evaluated and described, 4 4. The procedure for doing so recommended in ISO 21501-4 requires the ability to read out the detector response using either an internal Pulse Height Analyzer (PHA) of the LSAPC or by a certified technician using an external PHA. Instrument calibration should be performed on a regular schedule, e.g., annually. Important features of the ISO standard include estimation of the size setting error (SSE, i.e., where the particle size bin boundary lies relative to the set value), the size resolution (SR, the ability of the instrument to distinguish between particles of similar size, which is limited due to phenomena such as voltage peak broadening), and counting efficiency (the ability of the LSAPC to determine particle number concentration accurately vs a reference counter). This standard requires a variety of certified reference particles (CRPs), e.g., polystyrene (PS) particles of known mean diameter, x C, and standard deviation, σ C, with a refractive index of 1.59 at 589 nm.
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Ĭurrent procedures of calibration and verification of LSAPCs in order to comply with cleanroom standards (e.g., ISO 14644-1) are detailed in ISO 21501-4: 2018 4 4. as well as simultaneous determination of size and refractive index. In this simple picture, the light is only collected at a single angle, but more elaborate calibration schemes have also been realized to better determine the particle size, 2 2. A sketch of the working principle of an LSAPC is depicted in Fig.
By restricting the flow of particles in an airstream of a known volumetric flow rate, the LSAPC can quantify the number of particles as particle number concentration per volume of air in different size range categories. Light scattering airborne particle counters (LSAPCs) operate on the principle that airborne particles scatter light (e.g., from a laser) and by the fact that a nonlinear relationship between the scattered light intensity and particle size exists. In addition, our calibration method provides results that are closely related to the instrument’s properties during its application, contrary to the ISO method’s indirect calibration procedure that requires voltage readouts from internal terminals. In a direct comparison, both methods perform in compliance with the requirements in ISO 21501-4:2018, although our direct approach generally yields lower uncertainties. In this paper, we present an alternative method that allows direct in-use calibration of a particle counter without the need for information, which is typically only available to the instrument manufacturer. The ISO standard 21501-4:2018 describes a calibration method, yet this method requires detailed technical insider knowledge. Critical parameters for particle sizing are therefore an instrument’s size resolution and size setting, and these need regular calibration. This is essential for quality assurance, as different particle sizes have various associated risks connected. In addition to the amount of airborne particles, these instruments also measure the size of each registered particle. In cleanroom environments, light scattering airborne particle counters are typically employed to monitor particle contamination of production lines and for general classification purposes.
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