This document provides a method for sampling compressed air and a guide for choosing suitable measuring equipment to determine its particle size and concentration by number (to be referenced as "concentration" throughout this document). It also describes the limitations of the various measurement methods and describes the evaluation and uncertainty considerations.This document will report the particle size and concentration of all types of particle combined and does not aim to be able to segregate the separate solid and liquid particle fractions. When it is required that the concentration of a specific fraction is to be determined then recourse to the relevant standard method from the ISO 8573 series is recommended.NOTE 1 The test methods described in this document are those suitable for determining the purity classes given in ISO 8573-1.NOTE 2 Particle content determined as concentration by mass is dealt with in ISO 8573-8.NOTE 3 This document does not address instances where non-isothermal conditions exist, and separate arrangements should be made where particles may be formed by vapour condensation or lost through evaporation.