Why is counting efficiency checked at each calibration interval?

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Counting efficiency is a measure of how accurately the particle counter is counting (i.e., a calibration verification). Many things can impact counting efficiency during the lifetime of an optical particle counter [OPC]—for example, a slight optomechanical misalignment of the illumination source might go undetected causing a given OPC to correctly size but potentially undercount particles. 

ISO 21501 requires (among other elements) counting efficiency be checked following calibration adjustment. Once calibrated for sizing characteristics using NIST size standards, the OPC must be operated to compare counting efficiency to an Electrostatic Classifier or an OPC instrument with higher sensitivity than the instrument under test (considered a secondary standard, formally tested against an Electrostatic Classifier and verified at 100% counting efficiency for the particle size of interest).