This part of ISO 8573 provides a guide for choosing a suitable method to determine the solid particle concentration in compressed air, expressed as the number of solid particles in respective size classes. It describes the limitations of the various methods. This part of ISO 8573 identifies sampling techniques and measurement methods based on the counting of particles, and describes the evaluation, uncertainty considerations and reporting of the air purity parameter, solid particles. NOTE 1 The test methods described in this part of ISO 8573 are those suitable for determining the purity classes given in ISO 8573-1. NOTE 2 Particle content determined as mass concentration is dealt with in ISO 8573-8.