ISO 8573-1 Explained: Compressed Air Purity Classes

Compressed air specifications often say "Class 1", "Class 0" or "[2:2:1]" with little explanation. These labels come from ISO 8573-1, the international standard that gives buyers, compressor suppliers and test laboratories a shared way to describe air quality. This article explains what the standard covers, how its classes are organised, how to read a designation, and the most common misunderstanding about Class 0. It deliberately stays at the level of concepts. For the exact limit of each class, consult the current edition of the standard itself. ISO has a revision of this standard under development, so check which edition your specification refers to.

What ISO 8573-1 is

ISO 8573-1 specifies purity classes of compressed air with respect to three contaminants: particles, water and oil. The classes apply independently of where in the system the air is specified or measured, so the same language can be used at the compressor outlet, after a dryer or at the point of use. The standard also identifies gaseous and microbiological contaminants, but it does not define purity classes for them.

It is the first part of a series. The other parts do not set limits; they describe how each contaminant is measured. For example, separate parts cover oil aerosol, humidity, solid particles and oil vapour, and further parts cover gases and microorganisms.

Three contaminants, three separate scales

Each contaminant has its own scale of classes, and the three are chosen independently.

Particles. This scale measures solid particles in the air, typically from atmospheric dust, wear and pipe scale. The lower classes are defined by how many particles of different size ranges are allowed in a given volume of air; the higher classes describe particle concentration by mass instead. A class is only achieved when every size range it contains is met.

Water. This scale measures moisture in two forms. Water vapour is expressed as the pressure dew point, which is the temperature at which the air at system pressure would begin to condense. Liquid water is expressed as a concentration. A drier air stream has a lower pressure dew point and therefore a lower class number.

Oil. This scale measures total oil, meaning liquid oil, oil aerosol and oil vapour added together. The classes are defined by the maximum concentration of oil in a given volume of air.

For all three scales the rule is the same: a lower class number means cleaner air, and each step down in number is a tighter limit. Each scale also has a final "X" class for air that is more contaminated than the highest numbered class.

How to read a designation

A purity requirement is written by giving the standard and its edition year, followed by the three class numbers in brackets, always in the order particles : water : oil, separated by colons.

• A designation such as ISO 8573-1:2010 [A:B:C] means particle class A, water class B and oil class C. The three numbers are independent of each other, so a specification can ask for a very clean oil class together with a modest water class.
• If one contaminant is not specified, a hyphen replaces its number, which shows that this contaminant is not being classified.
• Always include the edition year, because class definitions are tied to a particular edition.

What Class 0 really means

Class 0 is the most misunderstood entry in the standard. It does not mean that there are no contaminants in the air. In the standard, Class 0 is defined as a level that is specified by the equipment user or supplier and is more stringent than Class 1. Three consequences follow:

• Class 0 is agreed, not fixed. Unlike the numbered classes, it has no limit of its own. The customer and the supplier agree a limit and write it down.
• A Class 0 claim needs its limit. In practice a bare "Class 0" is incomplete: ask for the agreed limit, which must be tighter than Class 1.
• Class 0 statements are not automatically comparable. Two documents that both say "Class 0" may describe different limits, test methods and test conditions.

When you receive a Class 0 statement from any supplier, ask for the stated limit, the test standard and method used, the test conditions, and which products the test report covers. For oil in particular, the standard treats oil vapour measurement as optional only for Classes 3, 4 and X, so for the cleaner classes the vapour has to be considered as well as aerosol and liquid oil.

A compressor type is not a purity class

It is easy to assume that an oil-free compressor automatically delivers a particular oil class. A purity class describes the air at a defined point in the system. That air depends on the compressor, the quality of the intake air, filters, dryers, piping and the point of use. The Compressed Air & Gas Institute (CAGI) notes that an oil-free compressor delivers air with an oil content no greater than what it takes in at its intake, and that oil-free air still needs filtration for particles. Our article Oil-Free vs Oil-Injected Screw Compressors: How to Choose looks at the compressor side of this question.

How to use the classes when you specify air

1. Identify critical points of use and decide whether the air touches the product directly.
2. Choose a class for each contaminant separately. Particles, water and oil need different equipment: filters mainly address particles and oil, and dryers address water (see our products).
3. Name the edition and the measurement point in the specification.
4. Plan testing. Air quality should be verified by sampling over time and not assumed from the type of equipment.
5. Use sector guidance where it exists. The standard describes classes; it does not decide which class an industry must use. Customer requirements and sector guidelines do.

Talk to HUADA

HUADA is the flagship industrial air power brand of Fujian Huade Group, founded in Quanzhou, Fujian, China in 1985 and exporting to 20+ countries. If you have a purity requirement to meet, send us the class you need for each contaminant and the point of use. To start, request a quote through the inquiry form on our Contact page.