The purpose of ISO 21501-4 is “to provide a calibration procedure and verification method for particle counters, so as to minimize the inaccuracy in the measurement result by a counter, as well as the differences in the results measured by different instruments.” ISO 21501-4 sets minimum performance standards for airborne particle counters and defined methods for calibration and verification of performance. While standards existed previously that covered these
topics, none were as comprehensive as ISO 21501-4.
Prior to ISO 21501-4, no international standards existed that required all critical parameters on airborne particle counters to be tested and meet specific performance requirements. The implementation of this standard forced improvements in calibration systems and, often, improvements in airborne particle counter design and performance. Many older instruments are not capable of meeting the performance requirements of ISO21501-4.