SO 8573-1 is the international standard for compressed air purity, and the reference framework used by audit bodies, certification schemes and industry customers to assess compressed air purity. While not legally required in every jurisdiction, it has become the de facto standard across sectors such as electronics and automotive, medical devices, food and beverage, and pharmaceuticals. ISO 8573-1 defines compressed air purity in
three contaminant categories, each one being assigned a class number based on either particle concentration per size bin (particles), pressure dew point (water) or total oil concentration (oil). A lower class number corresponds to a more stringent contaminant limit. Specifying compressed air against a defined ISO 8573-1 class enables a shift from reactive intervention to condition-based maintenance. Each contaminant category contributes
to different failure mechanisms, although effects often interact in practice. Particulate classes are selected based on downstream sensitivity, ranging from general pneumatic applications (typically Class 2–3) to more critical instrumentation and actuation systems (potentially Class 1). Moisture-related failures depend on the
pressure dew point relative to the lowest temperature in the system. ISO 8573-1 defines dew point classes which are used to specify and verify dryer performance. Oil contamination can affect elastomer seals and can also contribute to deposit formation. Differential pressure across filter stages is the primary operational indicator for particulate filter loading and is commonly used to schedule filter element replacement. Monitoring pressure drop against manufacturer limits enables planned maintenance and helps avoid both energy losses and filter performance degradation. Specifying the correct purity class also helps to avoid two opposite economic penalties.
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Over-specification imposes unnecessary pressure drop, energy cost and element replacement expenditure across the lifetime of the installation, while under-specification generates unplanned repair costs, scrap and quality non-conformities that typically exceed the saving on filtration hardware. The required class must therefore be defined by the end user against application risk. While ISO 8573-1 provides the classification framework, it does not prescribe which class applies to which use case. Maintaining the required purity class turns
compressed air from an imprecise utility into a measurable and controllable part of the manufacturing process. The standard’s classification therefore helps manufacturers to confirm that every product batch is exposed to the same level of air purity, helping to remove any compressed air variables that can cause defects.
Selecting the correct dryer and filter combination is a balancing act between achieving the necessary air purity and minimising the total cost of ownership through energy and maintenance considerations. Dryers should be installed early in the system to reduce moisture before air reaches filters and downstream equipment. A common mistake is to over-specify a dryer, which leads to unnecessary energy consumption. Filters address the A (solid particles) and C (total oil) parts of the ISO code. High-efficiency particulate filters are critical at two key points in the treatment chain: immediately after adsorption dryers, where they capture desiccant fines released by the adsorbent
Louis Cottaz
bed, and at point of use, where they provide the final barrier protecting sensitive processes and equipment from any residual particulate carry-over from the distribution piping. Without high-efficiency filtration at these stages, an otherwise compliant system can easily fall short of its target ISO particle class. Coalescing filters are deployed at multiple
stages. However, they do not remove oil in vapour form – that requires dedicated activated carbon filtration further downstream. They are installed in the most demanding industries where even trace hydrocarbon vapour can compromise product integrity, contaminate a batch or pose a health risk.
Before making changes to a compressed air system, engage with an expert to conduct a comprehensive system evaluation against ISO 8573-1. The most critical stage is the gap analysis, where an expert compares an organisation’s measured ISO class against the required ISO class for a specific application, and provides a roadmap for optimisation. With the right combination of dryers,
filters, point-of-use protection and ongoing measurement, design engineers can move compressed air from being an unmeasured inexact utility to a fully characterised, controlled and auditable input to production. ISO 8573-1 provides a common global framework for a measurable definition of ‘clean air’ that delivers consistent product quality and reliable operations – and the ability to prove it.
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