ISO 8573-1 Compressed Air Quality Classes (Particles, Water & Oil)
ISO 8573-1 classifies compressed air purity as three digits—particles : water : oil (for example 2:4:1). Use the tables below to set pressure dew point (PDP) and filtration goals, then size the dryer—not a substitute for process validation or Class 0 contracts.
Best for: plant engineers mapping applications to purity classes and treatment trains.
Not ideal for: claiming “ISO Class 0 guaranteed” without a written user/supplier specification and measurement plan.
Quick answer: How do ISO 8573-1 classes work?
Notation: ISO 8573-1:2010 [A:B:C] where A = particles, B = humidity/water (pressure dew point for Classes 1–6), C = total oil.
Water shortcuts: Class 4 ≤ +3°C PDP (typical refrigerated dryer) · Class 3 ≤ −20°C · Class 2 ≤ −40°C · Class 1 ≤ −70°C (desiccant). Class 0 means stricter than Class 1 and must be defined by the user/supplier—not a marketing synonym for “oil-free.”
Size dryer for +3°C PDP (Class 4 water) → Convert PDP ↔ ADP →
What is ISO 8573-1 (and what it is not)?
ISO 8573-1 defines purity classes for compressed air contaminants. It does not by itself size equipment, certify a plant, or replace industry-specific food/pharma rules. Measurement methods sit in other parts of the ISO 8573 series (for example humidity in Part 3). Always specify the measuring point (after dryer, at point of use, etc.).
How are particle classes defined?
Particle counts are per cubic metre in size channels 0.1–0.5 µm, 0.5–1.0 µm, and 1.0–5.0 µm (ISO 8573-1:2010). Class 6 switches to mass concentration.
| Class | 0.1–0.5 µm | 0.5–1.0 µm | 1.0–5.0 µm | Notes |
|---|---|---|---|---|
| 0 | As specified by user/supplier and more stringent than Class 1 | Contractual | ||
| 1 | ≤20,000 | ≤400 | ≤10 | Count / m³ |
| 2 | ≤400,000 | ≤6,000 | ≤100 | Count / m³ |
| 3 | Not specified | ≤90,000 | ≤1,000 | Middle channel required |
| 4 | Not specified | Not specified | ≤10,000 | — |
| 5 | Not specified | Not specified | ≤100,000 | — |
| 6 | Mass concentration: 0 < Cp ≤ 5 mg/m³ | Not particle counts | ||
How are water (humidity) classes defined?
For Classes 1–6, water is limited by pressure dew point. Liquid-water Classes 7–9 use concentration methods (ISO 8573-9)—fold them into specs only when liquid is present.
| Class | Pressure dew point | Typical dryer |
|---|---|---|
| 0 | Specified (stricter than 1) | Per contract |
| 1 | ≤ −70°C | Desiccant (deep) |
| 2 | ≤ −40°C | Desiccant |
| 3 | ≤ −20°C | Desiccant |
| 4 | ≤ +3°C | Refrigerated |
| 5 | ≤ +7°C | Refrigerated |
| 6 | ≤ +10°C | Refrigerated / mild |
Convert PDP ↔ atmospheric dew point with the compressed air dew point converter when charts or sensors disagree on which dew point they report.
How are oil classes defined?
Oil class is total liquid + aerosol + vapor (mg/m³) for Classes 1–4. Class 5 is not defined the same way in the vapor/aerosol table; Class X is above 5 mg/m³.
| Class | Oil (mg/m³) |
|---|---|
| 0 | Specified (stricter than 1) |
| 1 | ≤ 0.01 |
| 2 | ≤ 0.1 |
| 3 | ≤ 1 |
| 4 | ≤ 5 |
| X | > 5 |
Which ISO class do I need for my application?
These are typical planning maps—always verify against paint, food, or pharma process specs. Use the dryer deep links after you lock the water class / PDP.
- General pneumatics (cylinders / valves) — typical
3:4:3or4:4:3: refrigerated dryer ≤ +3°C PDP plus particulate/coalescing as needed. Size refrigerated dryer @ +3°C PDP → - Blow-off / cleaning — typical
3:4:3: similar to plant air; watch open piping if ambient < PDP. Size dryer → - Spray / paint — typical
2:3:1to2:2:1: often desiccant (≤ −20°C or colder) plus coalescing / carbon. Size @ −20°C → · or −40°C → - Food-contact packaging air — often ~
1:2:1(verify plant standard): desiccant + filtration; not a food-compliance guarantee. Size dryer @ −40°C PDP → - Pharma / electronics — often
1:2:1+; Class 0 only if written into the user/supplier agreement. Size dryer → · Hub pharma / cleanroom card
| Application | Typical [A:B:C] | Dryer tendency |
|---|---|---|
| General pneumatics | 3:4:3 or 4:4:3 | Refrigerated |
| Blow-off / cleaning | 3:4:3 | Refrigerated |
| Spray / paint | 2:3:1 – 2:2:1 | Desiccant (+ filters) |
| Food packaging (contact) | Often ~1:2:1 | Desiccant + filtration |
| Pharma / electronics | Often 1:2:1+ | Desiccant; validate |
What treatment chain reaches the class?
- Aftercooler / condensate drains — drop inlet temperature before the dryer.
- Pre-filter — protect dryer from bulk solids/liquids.
- Dryer — refrigerated for water Class 4–6; desiccant for Class 1–3 (see refrigerated vs desiccant guide).
- Coalescing filter — aerosols / oil mist toward oil class targets.
- Activated carbon — oil vapor / odor when Class 1 oil or paint/food specs require it.
Size capacity with correction factors in the air dryer sizing calculator; compare technologies in the dryer guide.
Example 1: Plant air for cylinders → Class 4 water
Target: 4:4:3 planning. PDP: +3°C. Action: refrigerated dryer + particulate/coalescing as needed. For 215 SCFM at reference conditions with 15% margin → about 247 SCFM rated dryer (open calculator with these inputs →).
Example 2: Paint booth → Class 2–3 water
Target: water Class 3 (−20°C) or Class 2 (−40°C) depending on paint. Action: desiccant dryer; include 10–15% regeneration air in compressor margin. Size desiccant @ −40°C →
Example 3: Spec says “Class 0”
Meaning: stricter than Class 1 for the contaminant(s) named—and the numeric limits must be written by the parties. “Oil-free compressor” alone does not equal ISO Class 0. Document measurement points and methods before purchasing.
6 common ISO 8573 mistakes (and fixes)
- Using atmospheric dew point in a Class 4 water spec → Spec pressure dew point; convert with the PDP converter.
- Treating Class 0 as a product badge → Require a written [A:B:C] or numeric limits.
- Copying particle Class 6 as “1,000,000 counts” → Class 6 is mass concentration (mg/m³), not that count.
- Ignoring Class 3 particle middle channel → ≤90,000 in 0.5–1.0 µm still applies.
- Sizing dryer = compressor SCFM with no derate → Apply inlet/ambient/pressure factors.
- Forgetting point-of-use vs after-dryer sampling → Class applies at the agreed measuring point.
How do I verify the class on site?
- Water: calibrated dew-point instrument reporting pressure dew point at line pressure.
- Particles: optical particle counters per the applicable ISO 8573 part / method.
- Oil: sampling for aerosol + vapor as required by the oil class and method standard.
This guide does not replace accredited laboratory procedures.
FAQ
What does ISO 8573-1 [2:4:1] mean?
Particle Class 2, water Class 4 (≤ +3°C PDP), oil Class 1 (≤ 0.01 mg/m³). The three numbers are independent—improving water does not automatically meet oil Class 1.
Is refrigerated drying enough for ISO water Class 4?
Usually yes when the dryer can hold ≤ +3°C PDP at your flow and inlet temperature. Confirm rated SCFM with correction factors in the dryer calculator.
Do I need a separate “air quality class calculator” tool?
For most users the application map + deep links on this page are enough. An interactive selector tool is only warranted if search demand stays calculator-heavy after this guide ranks—see change CP-0141 gate (default: absorb here).
Where does treatment sit in the pneumatic workflow?
Compressor → dryer/filters → pipe → actuators. Start from the Air Treatment hub section.
Next step: Size the dryer for your water class
Pick the PDP from the water class table, then enter compressor SCFM and site temperatures in the dryer calculator. Use the dew-point converter if your sensor reads atmospheric dew point.