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ISO 8573 Series

... the „mother“ of all compressed air treatment standards

The subject of the ISO 8573 series is the compressed air purity at any specific point of a compressed air system (downstream after-cooler). The ISO 8573 series defines standardized and manufacturer independent measuring methods for contaminations in compressed air and methods to express the required or measured compressed air purity.

The improper use or maintenance of components outside their specification, the failure of components or improper or changed ambient conditions are not subject of and thus are not considered by ISO 8573.

The ONLY THING that matters is the measured compressed air purity during the measurement period.

However, according operating and marginal conditions are elementary for compressed air treatment components in order to achieve the required compressed air purity.

Thus any assurance for a compressed air purity according to ISO 8573 SHALL BE RELATED to according operating and marginal conditions!

Compressed air — Part 1: Contaminants and purity classes

Third edition (2010)


ISO 8573-1 is the key element of the ISO 8573 series of documents and specifies

  • Terms and definitions
    • in reference to ISO 3857-4 – Compressors, pneumatic tools and machines - Vocabulary - Part 4: Air treatment
  • Reference conditions for gas volume statements
    • 1 bar(a) ; 20°C ; 0% relative humidity
  • 3 major contaminants in compressed air
    • Particles
    • Liquid water / Humidity
    • Total oil (see info-box)
  • Purity classes (for details see dedicated page) for these 3 major contaminants and reference to the according standardized test methods
  • Designation (for details see dedicated page) for compressed air purity statements
  • further descriptions and guidance (Annex)

For application of ISO 8573-1 purity classes the according standardized test methods of ISO 8573 part 2-9 must have been applied.

These standardized test methods, developed by a team of world-wide experts, offer the most robust and precise measurement available and additionally

  • offer the required sensitivity (resolution and detection limits)
  • offer the required selectivity (in order to measure just what shall be measured)
  • offer minimized cross-influences (disturbing influences by other substances or conditions)
  • are independent of any manufacturer (worldwide applicable by everyone)

Class 0 is a very special matter with its definition

«As specified by the equipment user or supplier and more stringent than class 1»

means for any class 0 statement there is the requirement to additionally state an according maximum allowable contaminant level, lower than the contaminant level defined by the according class 1 (see also designation).

Furthermore, in case the maximum allowable contamination level is lower than the detection limit of the according test method defined in ISO 8573 part 2-9, a suitable test method has to be agreed and stated.

The reference condition for humidity of 0% and its according conversion of volume flows only has marginal effect and thus is typically neglected.

Compressed air — Contaminant measurement — Part 2: Oil aerosol content

Third edition (2018)


ISO 8573-2 specifies test methods to determine the content of liquid oil and oil aerosols in compressed air. Depending on the expected oil content level, desired measurement period and sampling capabilities different methods can be applied:

Method    Type of flow         Detection limit       Typical sampling time  
AFull flow> 1 mg/m³50 - 200 h
B1Full flow0,001 - 10 mg/m³0,2 - 10 h
B2Partial flow0,001 - 10 mg/m³0,2 - 10 h

Applying one of the test methods of ISO 8573-2 is mandatory for the allocation of a purity class according to ISO 8573-1.

Most common is method B2 and thus a partial flow measurement, i.e.

  • for a defined time period a sample flow is taken from the turbulent compressed air main flow applying isokinetic sampling and is guided to a sampling disc holder
  • liquid oil and oil aerosols are collected on sampling discs inside the disc holder
  • at the end of the sampling period the collected oil is extracted from the sampling discs using a solvent, followed by a quantitative analysis using gas chromatography (respectively IR spectroscopy)
  • the determined oil mass is referred to the volume flow during the sampling period, resulting in the according mass concentration

The isokinetic sampling procedure, the sampling disc holder, the requested sampling discs and the analytical procedures are comprehensively and precisely described, thus ensuring robust and comparable measurement results all over the world.

ISO 8573-2 test methods require specialized technical competences and professional analytical skills and thus should be carried out by according specialized institutions.

A gas chromatograph requires calibration which shall be done with compressor oil(s) used in the compressed air system. Ideally, an oil probe is taken directly out of the compressor after the test. Alternatively, a sample of “fresh”, unused compressor oil may be used. The application of generic GC oil calibration standards may result in a substantial measurement error.

Oil detector tubes are allowed for a preliminary test in order to decide for the required test method and test period, typically applied with heavily contaminated compressed air. However, oil detector tubes cannot be used to align with any purity class. The methods described in ISO 8573-2 shall be used to identify the (liquid) oil contamination! (see Annex E)

Compressed air —Part 3: Test methods for measurement of humidity

First edition (1999)


ISO 8573-3 introduces several test methods to determine the content of humidity in compressed air, describing the typical range of application and limitations of each method.

Applying one of the test methods of ISO 8573-3 is mandatory for the allocation of a purity class according to ISO 8573-1.

ISO 8573-3 doesn’t demand a specific method, but the proper selection of a suitable method for the intended measurement task as well as related measuring device according to the manufacturer’s declaration.

Beside a lot of useful information about humidity measurement, ISO 8573-3 provides a professional method for the calculation of the saturated water vapor pressure in compressed air in its Annex B.

ISO 8573-3 defines the reference condition for humidity statements in compressed air as

«7 bar(e) and 20°C, unless otherwise agreed».

In practice, humidity statements are typically referred to the actual working conditions (pressure, temperature) during the measurement period – without any further reference statement and thus actually non-compliant to ISO 8573-3.

Compressed air —Part 4: Test methods for particle content

Second edition (2019)


ISO 8573-4 introduces several test methods to determine the content of particles in compressed air, describing the typical range of applications and limitations of each method. The result is a concentration by number.

Applying one of the test methods of ISO 8573-4 is mandatory for the allocation of a purity class according to ISO 8573-1.

Most common is the partial flow method using an optical particle sizing and counting instrument, i.e.

  • for a defined time period a sample flow is taken from the turbulent compressed air main flow applying isokinetic sampling and is guided to another downstream sampling line or directly into a diffusor
  • selection of a suitable measurement device (optical aerosol spectrometer (OAS) according to ISO 21501-1 respectively optical particle counter (OPC) according to ISO 21501-4)
  • ensuring a valid calibration (using certified and NIST traceable polystyrene latex microspheres (PSL) or requested by ISO 21501-1 or ISO 21501-4, respectively)
  • ensuring the avoidance of coincidence errors (possibly further dilution required offered by an additional sampling line with isokinetic sampling)
  • using a suitable diffusor in combination with typically non-pressure resistant particle sizing and counting instruments

ISO 8573-4 test methods cannot differentiate between solid and liquid particles and thus measure both – solid and liquid particles.

ISO 8573-4 test methods require specialized technical competences and professional measurement devices and thus should be carried out by according specialized institutions. There are countless sources of error to be avoided and several device-specific features to be considered – especially when a class 1-3 for particles according to ISO 8573-1 shall be measured.

Any kind of particle measurement shall just be carried out when no more liquid particles or condensable vapors are present in the compressed air. Both contaminants in the compressed air may result in a substantial measurement error.

Pressure regulators SHALL NOT BE USED for pressure expansion.

Within ambient air there are much higher particle concentrations than those that can be measured in the compressed air. Thus, any kind of contamination from the ambient air results in a substantial measurement error.

Compressed air —Part 5: Test methods for oil vapour and organic solvent content

First edition (2001) – to be revised soon


ISO 8573-5 specifies a test method to determine the content of (gaseous) oil vapor in compressed air.

Applying the test method of ISO 8573-5 is mandatory for the allocation of a purity class according to ISO 8573-1.

Most common is the partial flow method, i.e.

  • for a defined time period a sample flow is taken from the turbulent compressed air main flow
  • the sample flow is guided first to a high efficiency oil removal filter (1) and finally into the 2-zone activated carbon tube (2) (see Annex A)
  • oil vapors are collected on the activated carbon
  • at the end of the sampling period the collected oil is extracted from the activated carbon using a solvent, followed by a quantitative analysis using gas chromatography
  • the determined oil mass is referred to the volume flow during the sampling period, resulting in the according mass concentration

The resulting measurement range of this method is stated as 0,001 mg/m³ to 10 mg/m³.

The ISO 8573-5 test method requires specialized technical competences and professional analytical skills and thus should be carried out by according specialized institutions.

Oil detector tubes are allowed for a preliminary test in order to decide for the required test period, typically applied with heavily contaminated compressed air. However, oil detector tubes cannot be used to align with any purity class. The method described in ISO 8573-5 shall be used to identify the (gaseous) oil contamination! (see Annex A.2)

Within ambient air there may be higher oil vapor concentrations than those that can be measured in the compressed air. Thus any kind of contamination from the ambient air may result in a substantial measurement error.

Compressed air —Part 6: Test methods for gaseous contaminant content

First edition (2003)


ISO 8573-6 provides test methods to determine the content of the (gaseous) contaminants carbon monoxide, carbon dioxide, sulphur dioxide, nitrogen oxides and hydrocarbons in the range of C1 to C5 in the compressed air.

ISO 8573-6 describes several sampling methods, offering further details in each according Annex.

ISO 8573-6 is mainly informative rather than specifying.

The actual edition of ISO 8573-6 is out of date and thus no longer presents the state of the art.

Compressed air —Part 7: Test methods for viable microbiological contaminant content

First edition (2003) – to be revised soon


ISO 8573-7 specifies a test method to determine the content of viable microbiological organism in compressed air. The result is a concentration by number stated as CFU/m³.

 

--- Revision will bring up an entirely new document ---
Compressed air —Part 8: Test methods for solid particle content by mass concentration

First edition (2004)


ISO 8573-8 specifies test methods to determine the mass concentration of solid particles in compressed air.

Applying the test methods of ISO 8573 part 8 is mandatory for the allocation of a purity class according to ISO 8573-1.

Most common is the partial flow method, i.e.

  • for a defined time period a sample flow is taken from the turbulent compressed air main flow applying isokinetic sampling and is guided to a sampling disc holder
  • solid particles are collected on sampling discs inside the disc holder
  • at the end of the sampling period the collected particles are weighted (difference of sampling disc mass before and after the sampling period)
  • the determined particle mass is referred to the volume flow during the sampling period, resulting in the according mass concentration

Modern compressed air systems supply compressed air purities with only few, primarily aerosol-type solid particles. Their minor weight cannot be determined by weighting any more, thus, in order to determine the particle concentration, ISO 8573-4 rather than ISO 8573-8 is typically applied.

Nevertheless, the sampling method of ISO 8573-8 allows – in a wider sense – the identification of solid particles in compressed air. Analyzing the collected particles with a Scanning electron microscope (SEM) the type of particle and thus its source in the compressed air system may be detected.

Any kind of particle measurement shall just be carried out when no more liquid particles or condensable vapors are present in the compressed air. Both contaminants in the compressed air may result in a substantial measurement error.

Within ambient air there are much higher particle concentrations than those that can be measured in the compressed air. Thus, any kind of contamination from the ambient air results in a substantial measurement error.

Compressed air —Part 9: Test methods for liquid water content

First edition (2004)


ISO 8573-9 specifies test methods to determine the content of liquid water in compressed air.

Applying the test methods of ISO 8573 part 9 is mandatory for the allocation of a purity class according to ISO 8573-1.

Most common is the gravimetric method, i.e.

  • compressed air is guided to a combination of water separator and fine filter (full flow measurement)
  • the separated condensate is collected in both units
  • at the end of the test period the weight of the collected condensate and difference of mass of the fine filter element is determined
  • the determined condensate mass is referred to the volume flow during the test period, resulting in the according mass concentration

A measurement according to ISO 8573-9 shall just be carried out when the compressed air is fully saturated with humidity (100% r.h.). Compressed air, which is not saturated, may result in a measurement error due to vaporizing water.


Applying the test methods of ISO 8573 part 2-9 is mandatory for the allocation of a purity class according to ISO 8573-1.

The “contaminant oil” is often source of a misinterpretation of ISO 8573.
ISO 8573-1 defines the total oil content and its according purity classes as one of the major contaminants in compressed air. Oil itself is defined as a mixture of hydrocarbons composed of six or more carbon atoms (C6+). Thus the

«total oil content = liquid oil + oil vapor + according hydrocarbons»

Therefore, the total oil content according to ISO 8573-1 includes – beside the “original” hydrocarbons of the liquid compressor oil or the generated oil vapor – additionally all kinds of hydrocarbons within the suction air of the compressor, e.g. vapors of fuel, kerosene, diesel or their according ashes and flavors based on hydrocarbons.

Sometimes the ISO 8573 series is misleadingly used for the declaration of validated performance figures of compressed air treatment products. Since ISO 8573 just describes test methods for contaminants in the compressed air but neither defines any working nor inlet conditions there is no reference to any performance figures.


Powerful, useful, worldwide valid, widely accepted and well known standard. The definition of compressed air purity classes unitizes, simplifies and specifies the linguistic usage of a compressed air purity. The standardized and manufacturer independent test methods are robust, representative and allow a reliable and repeatable determination of a compressed air purity at any place in the world.

Misuse and misinterpretation of ISO 8573 for advertisement purposes.


Standards are protected by copyright .
Thus they are distributed exclusively, e.g. in Germany by Beuth-Verlag ( www.beuth.de ).