What two measurement methods does ISO 8573-2 specify for compressed air oil content?

ISO 8573-2 specifies two complementary methods for measuring total oil content in compressed air. Method 1 (Aerosol oil): compressed air is drawn through a membrane filter at controlled flow rate; oil aerosol droplets collect on the membrane; gravimetric weighing before and after collection gives oil aerosol mass in mg/m³ at reference conditions (20°C, 1 bar abs). Method 2 (Oil vapour): compressed air passes through an activated carbon sorbent tube; adsorbed hydrocarbon vapours are extracted with solvent and measured by GC-FID (gas chromatography, flame ionisation detection) or gravimetry, giving oil vapour in mg/m³. Total oil content reported per ISO 8573-1 = oil aerosol + oil vapour.

What oil aerosol concentrations does ISO 8573-2 typically measure upstream and downstream of compressed air filters?

Upstream of the first coalescing filter stage (at compressor outlet), oil aerosol concentrations from lubricated rotary screw compressors are typically 5–30 mg/m³ at normal operating conditions — primarily oil from compressor lubricant carryover through the separator stage. A high-efficiency coalescing pre-filter (40–70% efficiency) reduces this to 2–10 mg/m³ (ISO Class 4–3 oil). A second-stage high-efficiency coalescing filter (ISO 12500-1 tested) reduces to <0.1 mg/m³ (ISO Class 2 oil). Activated carbon adsorption removes oil vapour to <0.003 mg/m³ (ISO Class 1 oil). ISO 8573-2 measurements at each stage confirm that each treatment element is functioning within its rated efficiency.

Why must ISO 8573-2 measurements be taken at the point of use rather than at the dryer or filter outlet?

Compressed air recontamination from distribution pipework is a significant and frequently underestimated source of point-of-use oil contamination. Rust scale and scale deposits in carbon steel pipework contain absorbed compressor oils from years of service — these oils desorb as aerosols and vapours when warm, dry compressed air flows past. Biological films can produce hydrocarbon vapours. Flexible hoses and fittings may leach plasticisers into the air stream. ISO 8573-2 measurements at the filter outlet may show Class 1 oil while point-of-use measurements at the application show Class 3–4 due to distribution system recontamination — particularly in older facilities with legacy carbon steel pipework. Specification compliance must be verified at the actual point of use.

How does filter element aging affect ISO 8573-2 oil aerosol measurements?

Coalescing filter element performance degrades progressively with service life from two mechanisms. (1) Media saturation: as the glass fibre coalescer media fills with coalesced oil and solid particles, flow velocity through the loaded media increases — reducing oil droplet residence time in the media and decreasing coalescence efficiency. (2) Liquid carryover: at end of service life, the liquid oil accumulated in the sump of a loaded coalescing element may be re-entrained into the outlet air stream if the sump drain is inadequate or if flow velocity exceeds element re-entrainment velocity. ISO 8573-2 measurements on aged elements consistently show higher oil aerosol output than on new elements — service intervals must be based on end-of-life performance, not initial efficiency, to guarantee continuous compliance with the specified ISO 8573-1 oil class.

What is the reference condition for ISO 8573-2 oil content measurements?

ISO 8573-2 expresses oil content in mg/m³ at standard reference conditions: 20°C and 1 bar absolute (atmospheric pressure). This normalisation is essential because compressed air at system pressure (typically 7 bar gauge = 8 bar absolute) contains the same total mass of oil in a much smaller volume than at atmospheric pressure. An oil aerosol concentration of 0.01 mg/m³ measured at 8 bar absolute corresponds to 0.08 mg/m³ at 1 bar reference conditions — an 8× difference. ISO 8573-2 measurements taken at system pressure must be converted to reference conditions before comparison to ISO 8573-1 class limits, which are specified at 20°C and 1 bar absolute.

How does ISO 8573-2 relate to ISO 12500-1 for compressed air coalescing filter performance?

ISO 8573-2 is the field measurement standard — it specifies how to measure oil aerosol and vapour in the compressed air stream at point of use or at treatment stage outlets. ISO 12500-1 (Filters for Compressed Air — Test Methods — Part 1: Oil Aerosol) is the laboratory filter performance test standard — it specifies how to measure the oil aerosol removal efficiency of coalescing filter elements under controlled test conditions in a test rig. ISO 12500-1 generates the manufacturer's published filter efficiency data (inlet oil concentration → outlet oil concentration); ISO 8573-2 verifies that the installed filter achieves the expected performance under actual service conditions (flow rate, temperature, pressure, inlet oil concentration) that may differ from ISO 12500-1 test conditions.

What is the minimum ISO 8573-2 measurement duration for reliable oil content results?

ISO 8573-2 specifies minimum collection times for the aerosol membrane filter method to ensure statistically reliable gravimetric results. At low oil concentrations (Class 1: <0.01 mg/m³), collection must continue until a minimum detectable mass accumulates on the membrane filter — typically 0.1–0.5 mg for reliable gravimetric accuracy. At 0.01 mg/m³ and 10 L/min sample flow, this requires 10–50 minutes of collection time. At higher concentrations (Class 3–4: 1–5 mg/m³), collection times of 5–15 minutes at 10 L/min are sufficient. Inadequate collection time produces high relative measurement uncertainty (>±50%) that makes compliance determination meaningless — laboratory analytical balance sensitivity and minimum detectable mass determine the minimum valid collection volume for each Class level.

Does ISO 8573-2 address total hydrocarbon content or only compressor-derived oil?

ISO 8573-2 oil measurement methods capture all carbonaceous hydrocarbon aerosols and vapours — not only compressor-derived lubricant oil. Atmospheric inlet air contamination (vehicle exhaust fumes, solvent vapours near painting or cleaning operations, industrial process emissions) drawn into the compressor inlet and carried through the treatment system contributes to total oil content in the compressed air. In facilities near petroleum storage, automotive service bays, or industrial paint shops, atmospheric hydrocarbon concentrations may be 0.1–1.0 mg/m³ — significant relative to Class 1 (0.01 mg/m³) and Class 2 (0.1 mg/m³) limits. High-quality compressed air specification for sensitive applications must therefore also control the compressor inlet air quality, not just the treatment efficiency.

What are the consequences of exceeding ISO 8573-1 oil class limits in specific applications?

Consequences of oil contamination exceeding the specified ISO 8573-1 class depend on the application. (1) Pneumatic instrumentation (Class 1:4:1): oil above 0.01 mg/m³ coats positioner diaphragms and 4–20 mA transmitter orifices with sticky deposits, causing drift and hysteresis in control loops — plant upsets and SIS degradation. (2) Food contact packaging (Class 1:2:1): oil above 0.01 mg/m³ contaminates packaging surfaces — potential food safety regulation violation and product recall. (3) Pharmaceutical parenteral filling (Class 0): any detectable oil constitutes a contamination incident requiring investigation under EU GMP Annex 1. (4) Spray painting (Class 3:4:3): oil above 1 mg/m³ causes fish-eye paint defects and adhesion failure — product quality rejection. Regular ISO 8573-2 verification prevents these high-consequence contamination events.

How does ISO 8573-2 oil measurement differ from VOC (volatile organic compound) measurement in compressed air quality?

ISO 8573-2 oil measurement specifically targets hydrocarbon oil aerosols and vapours from compressor lubricant — petroleum-derived C15–C40 hydrocarbons with vapour pressures at or below atmospheric conditions. VOC measurement (ISO 8573-8) covers volatile organic compounds with significant vapour pressure at ambient conditions — C4–C12 aromatic hydrocarbons, ketones, esters, alcohols from atmospheric pollution, solvent carry-through, or biological growth. VOC contamination is outside the scope of ISO 8573-2 but is equally important for food, pharmaceutical, and laboratory applications. A compressed air quality specification addressing both oil and VOC contamination must reference both ISO 8573-2 (oil) and ISO 8573-8 (VOC) measurement methods independently.

Knowledge Center›Standards›ISO 8573-2

INDUSTRIAL STANDARD · 2018 (3rd edition)

ISO 8573-2

Q: Why must ISO 8573-2 measurements be taken at the point of use rather than at the dryer or filter outlet?

Compressed air recontamination from distribution pipework is a significant and frequently underestimated source of point-of-use oil contamination. Rust scale and scale deposits in carbon steel pipework contain absorbed compressor oils from years of service — these oils desorb as aerosols and vapours when warm, dry compressed air flows past. Biological films can produce hydrocarbon vapours. Flexible hoses and fittings may leach plasticisers into the air stream. ISO 8573-2 measurements at the filter outlet may show Class 1 oil while point-of-use measurements at the application show Class 3–4 due to distribution system recontamination — particularly in older facilities with legacy carbon steel pipework. Specification compliance must be verified at the actual point of use.

Q: How does filter element aging affect ISO 8573-2 oil aerosol measurements?

Coalescing filter element performance degrades progressively with service life from two mechanisms. (1) Media saturation: as the glass fibre coalescer media fills with coalesced oil and solid particles, flow velocity through the loaded media increases — reducing oil droplet residence time in the media and decreasing coalescence efficiency. (2) Liquid carryover: at end of service life, the liquid oil accumulated in the sump of a loaded coalescing element may be re-entrained into the outlet air stream if the sump drain is inadequate or if flow velocity exceeds element re-entrainment velocity. ISO 8573-2 measurements on aged elements consistently show higher oil aerosol output than on new elements — service intervals must be based on end-of-life performance, not initial efficiency, to guarantee continuous compliance with the specified ISO 8573-1 oil class.

Q: What is the reference condition for ISO 8573-2 oil content measurements?

ISO 8573-2 expresses oil content in mg/m³ at standard reference conditions: 20°C and 1 bar absolute (atmospheric pressure). This normalisation is essential because compressed air at system pressure (typically 7 bar gauge = 8 bar absolute) contains the same total mass of oil in a much smaller volume than at atmospheric pressure. An oil aerosol concentration of 0.01 mg/m³ measured at 8 bar absolute corresponds to 0.08 mg/m³ at 1 bar reference conditions — an 8× difference. ISO 8573-2 measurements taken at system pressure must be converted to reference conditions before comparison to ISO 8573-1 class limits, which are specified at 20°C and 1 bar absolute.

Q: How does ISO 8573-2 relate to ISO 12500-1 for compressed air coalescing filter performance?

ISO 8573-2 is the field measurement standard — it specifies how to measure oil aerosol and vapour in the compressed air stream at point of use or at treatment stage outlets. ISO 12500-1 (Filters for Compressed Air — Test Methods — Part 1: Oil Aerosol) is the laboratory filter performance test standard — it specifies how to measure the oil aerosol removal efficiency of coalescing filter elements under controlled test conditions in a test rig. ISO 12500-1 generates the manufacturer's published filter efficiency data (inlet oil concentration → outlet oil concentration); ISO 8573-2 verifies that the installed filter achieves the expected performance under actual service conditions (flow rate, temperature, pressure, inlet oil concentration) that may differ from ISO 12500-1 test conditions.

Q: What is the minimum ISO 8573-2 measurement duration for reliable oil content results?

ISO 8573-2 specifies minimum collection times for the aerosol membrane filter method to ensure statistically reliable gravimetric results. At low oil concentrations (Class 1: <0.01 mg/m³), collection must continue until a minimum detectable mass accumulates on the membrane filter — typically 0.1–0.5 mg for reliable gravimetric accuracy. At 0.01 mg/m³ and 10 L/min sample flow, this requires 10–50 minutes of collection time. At higher concentrations (Class 3–4: 1–5 mg/m³), collection times of 5–15 minutes at 10 L/min are sufficient. Inadequate collection time produces high relative measurement uncertainty (>±50%) that makes compliance determination meaningless — laboratory analytical balance sensitivity and minimum detectable mass determine the minimum valid collection volume for each Class level.

Compressed Air — Test Methods for Aerosol Oil Content

Measurement of oil aerosol and vapour contamination in compressed air at the point of use, for compressed air treatment system qualification and purity class verification per ISO 8573-1.

KEY PARAMETERS

Oil aerosol + vapour (mg/m³)

Measurement

<1 mg/m³

ISO Class 3 oil limit

<0.01 mg/m³

ISO Class 1 oil limit

Membrane filter (aerosol) + sorbent tube (vapour)

Methods

01 /

Oil Content Measurement Scope

ISO 8573-2 specifies two complementary test methods for measuring total oil content in compressed air: the aerosol method (membrane filter collection and gravimetric measurement for liquid oil mist) and the vapour method (activated carbon sorbent tube collection and solvent extraction for oil vapour). Total oil content = aerosol oil + vapour oil, reported in mg/m³ at reference conditions (20°C, 1 bar). ISO 8573-1 oil classes are verified using the total oil content measurement from ISO 8573-2.

02 /

Compressed Air Treatment Verification

ISO 8573-2 measurement is performed downstream of each treatment stage to verify that the installed equipment achieves the specified oil class. A coalescing pre-filter reduces oil aerosol from compressor lubricant carry-over (typically 5–40 mg/m³ at compressor outlet) to <1 mg/m³ (ISO Class 3 oil). A high-efficiency coalescing stage reduces further to <0.1 mg/m³ (ISO Class 2). An activated carbon adsorber removes oil vapour to <0.01 mg/m³ (ISO Class 1). Point-of-use measurement confirms each stage is functioning correctly under actual operating pressure and flow conditions.

03 /

Relationship to ISO 8573-1

ISO 8573-2 is the test method standard referenced by ISO 8573-1 for the oil contamination class parameter. ISO 8573-1 specifies what purity class is required; ISO 8573-2 specifies how to measure whether that class is achieved. Additional ISO 8573 parts cover other contaminants: Part 3 (water and humidity measurement), Part 4 (solid particles by mass), Part 6 (gaseous contaminants), and Part 9 (liquid water measurement). Together these parts form the complete measurement framework for the ISO 8573-1 purity class system.

COMMON ENGINEERING MISTAKES

Using ISO 8573-2 oil aerosol measurement as a proxy for total hydrocarbon content. ISO 8573-2 measures oil aerosol and liquid oil — it does not detect oil vapour, which requires activated carbon treatment and separate measurement.

Not recognising that oil aerosol content in compressed air increases with downstream filter element aging even within the rated service life. Verification measurements must be taken at end-of-life conditions, not only on new elements.

ENGINEERING REFERENCES

STANDARD

ISO 8573-2:2018, Compressed Air — Part 2: Test Methods for Aerosol Oil and Particle Content, ISO Geneva

Measurement method standard for oil aerosol and vapour content in compressed air; specifies membrane filter (aerosol) and sorbent tube (vapour) methods for ISO 8573-1 oil class verification.

STANDARD

ISO 8573-1:2010, Compressed Air — Part 1: Contaminants and Purity Classes, ISO Geneva

Parent classification standard defining oil content classes verified by ISO 8573-2 measurement; the purity class system that DRYCORE™ compressed air treatment systems are designed and certified against.

STANDARD

ISO 12500-1:2007, Filters for Compressed Air — Test Methods — Part 1: Oil Aerosol, ISO Geneva

Laboratory filter performance test standard for coalescing filter efficiency; provides manufacturer-certified oil aerosol removal efficiency data validated against ISO 8573-2 field measurement conditions.

STANDARD

ISO 8573-4:2019, Compressed Air — Part 4: Test Methods for Solid Particle Content, ISO Geneva

Companion test method standard for solid particle measurement in compressed air; used alongside ISO 8573-2 oil measurement for comprehensive ISO 8573-1 purity class verification at point of use.

FREQUENTLY ASKED QUESTIONS — ISO 8573-2

ENGINEERING ARTICLES REFERENCING ISO 8573-2

Compressed Air Dryer Selection: Refrigeration, Desiccant, and Membrane Drying Technologies vs ISO 8573-1 Dew Point Classes

12 min read

Compressed Air Quality Verification — ISO 8573-4, -5, and -7

12 min read

ISO 8573-2

Oil Content Measurement Scope

Compressed Air Treatment Verification

Relationship to ISO 8573-1

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