A requirements specification might contain a line like “ISO 8573-1:2010 [2:4:2]”. In production, however, it is often the case that no one can say whether the existing system actually delivers this quality—or where it should be measured. This is exactly where the most expensive wrong decisions regarding compressed air quality occur.

If the requirement is set too low, water, oil, or particles enter processes where they don’t belong: sticking valves, corrosion in the network, rejects in coating, or quality risks in food and pharmaceutical environments. If it is set too high, you pay permanently for treatment that no one needs.

This article explains how the standard series is structured, how to read a three-digit specification, what Class 0 actually means, and which questions must be clarified before specifying.

What ISO 8573-1 regulates—and what it doesn’t

ISO 8573 is a series of standards. Part 1 is the classifying document: it defines purity classes for compressed air regarding particles, water, and oil, regardless of where in the system the air is specified or measured. This allows you to either define the required quality at a specific point or—after testing according to Parts 2 to 9—classify the actual quality present.

The other parts describe the testing procedures. Which part is to be applied depends on the contaminant to be evaluated:

• Part 2: Oil aerosol content. Oil vapor is explicitly excluded here and is regulated in Part 5.
• Part 3: Water content in vapor form.
• Part 4: Solid particle content.
• Part 5: Oil vapor and organic solvents.
• Part 6: Gaseous contaminants.
• Part 7: Viable microbiological contaminants.
• Part 8: Solid particle content by mass concentration.
• Part 9: Liquid water content.

This distinction is more important than it sounds: a class specification required in a requirements specification is a specification. If a class is promised in a quote as an achieved property, it is a performance commitment; it is verified by testing according to the applicable part of the standard series.

Part 1 also names gaseous and microbiological contaminants. These are not part of the three-digit specification: a microbiological requirement is not a fourth digit, but a separate definition with its own testing procedure.

Edition and Validity

ISO 8573-1:2010 is published in its third edition from April 2010. It replaces the 2001 edition, including its technical corrigendum. Following systematic review, it was confirmed in 2017; on December 4, 2025, a new project for the fourth edition was registered at ISO, and the 2010 edition has since carried the status “under revision.”

In practice, this means: as long as no new edition is published, the 2010 edition remains the authoritative published version. A draft does not replace it. However, a party can contractually agree on a different document or a different requirement level—in which case, what is stated in the contract applies. Because older editions continue to be used in practice, the year belongs in every specification.

The Three Contaminants and Their Metrics

Solid Particles

Particles originate from ambient air, wear and tear, and the piping network itself, such as rust and scale in older steel pipes. The standard series provides two testing paths for solid particles: Part 4 deals with particle content, while Part 8 deals with particle content by mass concentration. Which path applies to your class can be found in the class table in the standard text.

Water

For water, the relevant metric depends on the defined class. For water classes 1 to 6, the pressure dew point is used. In contrast, classes 7 to 9 refer to the liquid water content. Consequently, the testing procedure also differs: Part 3 applies to water content in vapor form, while Part 9 applies to liquid water, expressed as mass concentration.

The pressure dew point describes the temperature at which the compressed air would precipitate condensate under operating pressure. For assessing the risk of condensation, the lowest temperature to which the pipe is actually exposed is therefore decisive—not the room temperature in the compressor room. A pipe routed outdoors in winter may significantly fall below a pressure dew point that is sufficient in summer.

However, preventing condensation is only the lower limit. Process, product, or customer requirements may demand a lower pressure dew point than would be necessary to avoid condensation—for example, due to product protection, hygiene requirements, or specifications from a requirement sheet.

Oil

The oil class refers to the total oil content from liquid oil, oil aerosol, and oil vapor. Those who only consider aerosols underestimate the value—and may measure according to the wrong part of the standard series. It should also be noted that the intake ambient air also contains hydrocarbons.

Limit Values: Where They Are Bindingly Stated

The specific limit values per class—particle size ranges and counts, pressure dew points, liquid water contents, and total oil contents including units and comparison symbols—are to be taken exclusively from the standard text of ISO 8573-1:2010. Summaries from manufacturer documentation differ in details and are not sufficient for a specification.

Therefore, work with the licensed standard text. The following overview shows which information a reliable specification must contain—regardless of which classes you choose.

Class 0 – The Most Common Misunderstanding

Class 0 is regularly read as “guaranteed oil-free” or “no contamination.” That is not what the standard says. Class 0 means: the user or the equipment supplier sets a requirement for a specific contaminant that is stricter than Class 1 for exactly that contaminant—and documents the associated value in writing.

A Class 0 requirement is initially a specification. It may only be presented as a proven property if a test report matching the scope of the statement documents the relevant contaminant, the agreed limit value, the measurement point, the operating conditions, and the testing procedure used.

In procurement, it helps to consistently distinguish between four levels:

• Requirement or specification: What is to be achieved?
• Measurement result: What was measured at a defined point under defined conditions?
• Validated system property: Does the system demonstrably and repeatably meet the requirement at the defined measurement point?
• Product or series statement: What can be generally claimed about a device or a series?

The appropriate follow-up questions for a quote are therefore simple: For which contaminant does the statement apply, what value was defined, at which measurement point, under which operating conditions, and according to which testing procedure?

“Oil-free compressed air” – A Term in Need of Clarification

The expression “oil-free compressed air” is not unique without a defined total oil limit, measurement point, and testing procedure. Depending on the risk assessment, a system can be designed for the required total oil content using different compressor and treatment concepts. Suitability must be validated for the specific application. This means neither “zero oil” nor automatically Class 0.

Therefore, the following should be kept clearly separate:

• oil-free compression: no oil entry from the compression process itself;
• oil-injected compression: oil entry from the process, which is separated downstream;
• downstream treatment: separation and filtration, depending on the form of oil;
• total oil content at a defined measurement point: the actual result;
• Class 0: a written, agreed requirement stricter than Class 1;
• suitability for food, pharmaceuticals, or other sensitive processes: the result of a risk assessment and validation.

These terms are not synonymous. Oil-free compression is a design feature, not a measurement result; a low total oil content is a measurement result, not a class commitment; and neither of the two statements alone proves suitability for a specific sensitive process.

From Requirement to Treatment

The class describes a result, not a setup. It is achieved through a coordinated chain: aftercooler, separator, compressed air receiver, filter stages, dryer, and depending on requirements, an adsorption stage, plus condensate drains and condensate treatment. The complete chain must be validated at the defined measurement point.

It is worth taking a close look at energy and maintenance costs instead of treating every stage as a flat cost factor:

• Active treatment stages may require electrical power, purge air, or heat.
• Passive filter stages do not consume energy themselves but create an indirect energy demand through their pressure loss.
• Every stage generates maintenance effort.
• Therefore, treatment should only be carried out to the quality that is actually required.

Oil Separation: What Each Stage Achieves

On the oil side, the form of oil is decisive:

• Coalescing filtration addresses liquid oil and oil aerosols.
• Oil vapor may require a suitable adsorption stage, for example with activated carbon.
• Which arrangement is necessary depends on the contamination source, oil form, intake air, compressor principle, operating conditions, and the specified class.

A blanket statement like “low oil class always means activated carbon” therefore falls short. Whether and in which design an adsorption stage is necessary depends on the specific case and is confirmed by measurement at the defined point.

Drying: Multiple Procedures, a Question of Design

Refrigerant drying and adsorption drying are the most common, but not the only procedures. Also in use are dryers with consumable desiccant, dryers with regeneration via heat of compression, and membrane dryers. Which procedure is suitable depends on:

• the required pressure dew point,
• the inlet and ambient conditions,
• operating pressure and volume flow,
• the manufacturer’s correction factors for deviating conditions,
• the resulting pressure loss,
• the purge air or energy requirement,
• the maintenance effort.

Dryer performance is based on defined inlet conditions. A higher inlet temperature or a lower inlet pressure reduces capacity; manufacturers provide correction factors for this. Stricter drying than required can—depending on the dryer principle—cause additional energy or purge air requirements as well as higher investment costs.

Application Examples for Orientation

The following overview is explicitly illustrative. It describes possible components based on application-related risk assessment and validation—not requirements and not classes generally valid for an industry.

Measurement, Sampling, and Ongoing Monitoring

A class specification is not complete without a measurement point. Between the compressor outlet, treatment, and point of use, there are pipes, fittings, and often further filter stages; quality can change in both directions. It must therefore be determined at which point the class must be maintained.

Regarding testing: sampling and measurement procedures depend on the contaminant to be tested. The measurement point, extraction procedure, flow conditions, number of samples, and measuring device must correspond to the respective applicable part of ISO 8573-2 to ISO 8573-9. Without this methodological consistency, the result must not be presented as validation of a purity class.

Ongoing monitoring during operation should be separated from this. Pressure dew point and filter differential pressure are operational status indicators. They prove neither the particle nor the total oil class and do not replace validation with the applicable measurement procedure. The testing interval should be derived from process criticality, customer and industry requirements, system changes, and the risk assessment.

Typical mistakes

• Specifying the class without stating the edition. Without the year, the requirement is not unique.
• Interpreting Class 0 as “oil-free” without asking for the affected contaminant, the defined value, and the test certificate.
• Treating “oil-free compressed air” as a clearly defined technical state instead of defining the limit value, measurement point, and testing procedure.
• Only considering aerosols for oil. The total oil content, including oil vapor, is decisive.
• Generally assuming pressure dew point for water. In classes 7 to 9, the liquid water content is decisive.
• Aligning the pressure dew point solely with condensation avoidance, even though the process requires a lower value.
• Specifying the class at the compressor outlet, even though it must be maintained at the point of use.
• Treating pressure dew point and differential pressure displays as proof of compliance with a class.
• Adopting limit values from secondary sources instead of working with the licensed standard text.

Frequently Asked Questions

Which edition of ISO 8573-1 is currently valid?

The third edition, ISO 8573-1:2010, is currently published. It was confirmed in 2017 and has been marked as “under revision” since December 4, 2025; at the same time, the fourth edition was registered as a project. As long as this is not published, the 2010 edition remains the authoritative version. Always include the year in every specification.

Does a low class number always mean better compressed air?

Within a single contaminant, a lower number stands for a stricter requirement. The numbers cannot be compared between the three contaminants: a 1 for water and a 1 for oil describe different metrics and different testing procedures.

Do I strictly need an oil-free compressor for oil-free compressed air?

Not necessarily. The decisive factor is the required total oil content at the defined measurement point, not the type of device. Depending on the risk assessment, this value can be achieved with different compressor and treatment concepts. Oil-free compression avoids oil entry from the compression process; whether it is required depends on the assessment of process risk and the follow-up costs of any entry.

How often should compressed air quality be tested?

The testing interval should be derived from process criticality, customer and industry requirements, system changes, and the risk assessment. Pressure dew point and filter differential pressure provide indications of changes in between, but do not replace testing.

Next step

If you would like to classify which purity classes your application requires and what your treatment contributes to this, we would be happy to review your documentation and measurement results together. Helpful information includes the required class specification, volume flow and operating pressure, inlet and ambient conditions, existing treatment components, the intended measurement point, and existing test reports. Depending on the region, further processing will be handled by SCC air compressors or an SCC service partner. You can get started via consultation from SCC air compressors.