Compressed Air Purity Class

Canonical Definition

A three-number quality specification defined by ISO 8573-1 in the format [S:W:O], where S is the solid particulate class (1–9), W is the water/humidity class (1–9), and O is the oil aerosol and vapour class (1–4). Lower class numbers indicate higher purity. Example: ISO 8573-1 Class 1:2:1 specifies: S1 (≤20,000 particles ≥0.1 µm per m³; no particles >0.5 µm), W2 (PDP ≤ −40°C or liquid water: 0 g/m³), O1 (≤0.01 mg/m³ total oil). Each class is specified independently, allowing different purity requirements for different contaminant types.

Engineering Context

ISO 8573-1 purity classes guide selection of compressed air treatment train components. General industrial pneumatics (actuators, cylinders): Class 5:4:3. Instrument air (flow meters, control valves, analysers): Class 2:4:2. Food contact and pharmaceutical: Class 1:2:1. Point-of-use filters, coalescing pre-filters, desiccant dryers, and activated carbon beds are selected and sequenced to achieve the specified class at the application pressure and rated flow rate. Purity class must be verified at the point of use, not at the compressor outlet.

Applicable Standards

Related Technologies

Related Terms