Direct approach to determine the size setting error and size resolution of an optical particle counter

In cleanroom environments, light scattering airborne particle counters are typically employed to monitor particle contamination of production lines and for general classification purposes. In addition to the amount of airborne particles, these instruments also measure the size of each registered particle. This is essential for quality assurance, as different particle sizes have various associated risks connected. Critical parameters for particle sizing are therefore an instrument's size resolution and size setting, and these need regular calibration. The ISO standard 21501-4:2018 describes a calibration method, yet this method requires detailed technical insider knowledge. 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. 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 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.