ISO 8573-8 specifies test methods to determine the mass concentration of solid particles in compressed air.
|Applying the test methods of ISO 8573 part 8 is mandatory for the allocation of a purity class according to ISO 8573-1.
Most common is the partial flow method, i.e.
- for a defined time period a sample flow is taken from the turbulent compressed air main flow applying isokinetic sampling and is guided to a sampling disc holder
- solid particles are collected on sampling discs inside the disc holder
- at the end of the sampling period the collected particles are weighted (difference of sampling disc mass before and after the sampling period)
- the determined particle mass is referred to the volume flow during the sampling period, resulting in the according mass concentration
Modern compressed air systems supply compressed air purities with only few, primarily aerosol-type solid particles. Their minor weight cannot be determined by weighting any more, thus, in order to determine the particle concentration, ISO 8573-4 rather than ISO 8573-8 is typically applied.
Nevertheless, the sampling method of ISO 8573-8 allows – in a wider sense – the identification of solid particles in compressed air. Analyzing the collected particles with a Scanning electron microscope (SEM) the type of particle and thus its source in the compressed air system may be detected.
Any kind of particle measurement shall just be carried out when no more liquid particles or condensable vapors are present in the compressed air. Both contaminants in the compressed air may result in a substantial measurement error.
Within ambient air there are much higher particle concentrations than those that can be measured in the compressed air. Thus, any kind of contamination from the ambient air results in a substantial measurement error.