Compressed-air quality testing to ISO 8573-1

What does a compressed-air purity test actually measure?

“Air quality” is not one number. A dryer display showing a good dew point says nothing about oil carry-over, and a clean filter element says nothing about whether the dryer is coping with summer inlet temperatures. A proper purity test treats each contaminant as a separate question with its own answer.

• Solid particles – rust and scale from pipework, desiccant dust, wear debris and atmospheric dust drawn through the intake. Measured as a count per cubic metre in defined size bands, or as a mass concentration for coarser classes.
• Water – vapour, expressed as pressure dew point (PDP), and liquid water where a dryer has failed or condensate is not being drained. PDP is the temperature at which vapour in the compressed air would condense at line pressure.
• Oil – liquid oil and aerosol from oil-injected compressors, plus oil vapour. Vapour also comes from the intake air and from hydrocarbons in the surroundings, so even an oil-free compressor does not guarantee oil-free air.
• Microbiological contamination and gases – relevant for food, pharmaceutical and breathing-air duty, and tested to separate methods when the specification calls for it.

The aim is not to collect impressive numbers. It is to show whether the air at a defined point meets a defined class, and if it does not, which part of the treatment chain is responsible.

ISO 8573-1 purity classes explained

ISO 8573-1 expresses air quality as three digits in the order particles, water, oil. A specification of Class 1.4.1, for example, means particle class 1, water class 4 and total oil class 1. The table below summarises the commonly quoted limits. Always check the current edition of the standard for the full particle tables before writing a specification.

In practice the water class tells you what dryer you need. A refrigerated dryer typically delivers a PDP around +3 °C (class 4). A desiccant dryer typically delivers −20 °C or −40 °C (class 3 or 2), and −70 °C needs a molecular-sieve desiccant dryer. Class 0 is a contract term, not a product label: it means nothing until the limits are written down.

Which test method suits each contaminant?

The parts of ISO 8573 that follow part 1 describe how each contaminant is measured. Matching the method to the class you need to prove is the first decision in any test plan.

Some tests give a result on site in minutes; others give a result only after a laboratory has processed the sample. A test plan should say which is which, so nobody treats a site indicator as a certified laboratory result.

Where should air samples be taken?

A result only means something for the point it was taken from. Air leaving a dryer can be dry and clean while the same air, 200 metres downstream in an old galvanised header, picks up rust and condensate. Choose sampling points that answer the question you are asking.

• Compressor discharge, before treatment – establishes the raw load on the dryer and filters, including oil carry-over from the separator.
• After the dryer and each filter stage – proves each item of treatment equipment is doing its job.
• Main header or ring main – shows what the distribution system adds or removes.
• Critical points of use – the point where product contact, instrument air or breathing air demands a class. This is the point that matters for compliance.
• After point-of-use filters – where local treatment is used to upgrade general plant air to a stricter class.

Sample from the top or side of a horizontal pipe, never the bottom, where liquid collects. Use a dedicated sampling valve rather than a quick coupling that has been on and off a dozen hoses. Flush the point before sampling, and record line pressure, temperature and flow condition at the time of the test.

How a compressed-air quality test is carried out

1. Agree the requirement. Confirm the purity class each point must meet, and who set it – process owner, equipment supplier or a product standard.
2. Map the system. List compressors (oil-injected or oil-free), dryers, filters, drains, receivers and the routes to each point of use.
3. Select points and methods. Choose sampling points that separate treatment performance from distribution effects, and pick a method for each contaminant.
4. Record operating conditions. Note load, pressure, ambient and inlet temperature and which machines are running. A test during a cool morning at half load may not reflect an afternoon at full load in May.
5. Prepare and flush. Clean sampling fittings, purge lines and allow instruments to stabilise at line pressure.
6. Take the samples. Measure particles, dew point and oil as planned, with controlled sample volumes and clean handling for anything sent to a laboratory.
7. Analyse and compare. Compare each result with the required class, stating the method and its limitations.
8. Trace the cause. Where a point fails, compare upstream and downstream results to locate the source – compressor, dryer, filter, drain or pipework.
9. Act and retest. Fix the cause, then repeat the test at the same point under similar conditions to verify the correction.

Common causes of poor air quality

When a result misses its class, the cause is usually ordinary and findable. These are the faults we see most often.

Hyderabad summers deserve special mention. High ambient temperatures raise compressor discharge and dryer inlet temperatures, and a refrigerated dryer sized on a nameplate at 25 °C ambient may struggle well before 40 °C. Dust also loads intake filters faster. A test in winter can pass and a repeat in May can fail with nothing else changed.

How to decide what purity your plant needs

Over-specifying purity costs energy. Every extra filter stage adds pressure drop, and pressure costs power – roughly 7% more compressor power per extra bar of discharge pressure. Desiccant dryers also use purge air or heat. Under-specifying risks product rejects, instrument faults and corrosion. The right answer is usually a sensible general plant class with stricter treatment only where it is needed.

• General workshop and plant air: typically a refrigerated dryer and general-purpose filtration.
• Instrument air and pneumatic controls: typically lower dew point and finer filtration to protect valves and positioners.
• Painting and coating: low oil and particle content to avoid surface defects.
• Food, beverage and pharmaceutical product contact: strict oil and particle classes, often with microbiological controls, set by the plant’s own quality requirements.
• Outdoor lines in cold conditions: dew point below the lowest pipe temperature, not just below room temperature.

Write the class down for each point of use, with the reason. That single document makes every later test, purchase and service decision easier.

How Reddy Compressors helps

Reddy Compressors has served industry since 2002 from our office at Tarnaka, Secunderabad. As an independent multi-brand specialist we work on Atlas Copco, Ingersoll Rand, ELGi, KAESER, Chicago Pneumatic, Gardner Denver, Sullair, Doosan, Kirloskar, Bobcat and many other makes, so our view of your air treatment is not tied to one manufacturer’s catalogue.

For air quality, we survey the compressor room and treatment chain, help you define the purity class each point needs, plan the sampling points and methods, and take site measurements such as dew point. Where a certified laboratory analysis is needed for oil or micro-organisms, we say so and plan the sampling around it. When a point fails, we trace the cause through the separator, dryer, filters, drains and pipework, then carry out the repair, service or parts supply and retest. Our service teams cover the whole of Telangana and Andhra Pradesh, and we supply equipment and spare parts across India.

What to send us

• The process or product the air serves and any purity class already specified
• Compressor make, model and type (oil-injected or oil-free), with nameplate photos
• Dryer and filter make, model and grade, and their position in the line
• A simple sketch of the compressor room and main pipework
• The symptom you see: water, oil, rust, product rejects or instrument faults, and when it happens
• Recent service history for separators, filters and drains

Frequently asked questions

What is ISO 8573-1 class 1.4.1 compressed air?

Class 1.4.1 means particle class 1, water class 4 and total oil class 1 under ISO 8573-1. In practical terms it calls for very fine particle filtration, a pressure dew point of +3 °C or lower – typically what a well-sized refrigerated dryer delivers – and total oil content of no more than 0.01 mg/m³. It is common where air is not in direct contact with product; direct-contact air usually needs a stricter water class such as 2.

How do you test compressed air for oil?

Oil aerosol and liquid are usually measured by passing a known volume of air through a filter membrane or sampling device, then analysing it in a laboratory. Oil vapour needs a separate method, such as an adsorption tube or a photo-ionisation detector, because vapour passes through coalescing filters. To claim a strict total-oil class, both aerosol and vapour must be measured at the point of compliance.

Why is there water in my compressed air lines after the dryer?

The usual causes are a dryer overloaded by high inlet temperature or flow, a fouled condenser, a failed condensate drain, an open bypass valve or exhausted desiccant. Water can also come from wet air entering through a cross-connection or a standby machine without treatment. Measure the dew point immediately after the dryer and at the point of use to tell a dryer fault from a distribution problem.

How often should compressed air quality be tested?

There is no single interval for every plant. Test when a purity class is first specified, after any change to compressors, dryers or pipework, and after a quality complaint. Plants with product-contact or regulatory requirements usually set a periodic schedule in their own quality system. Continuous dew-point monitoring between tests gives early warning of dryer problems at low cost.

Does an oil-free compressor guarantee oil-free air?

No. An oil-free compressor does not add lubricating oil to the air, but the intake can draw in oil vapour and hydrocarbons from vehicle exhaust, nearby processes or the atmosphere. Those pass into the system and can also condense in pipework. If a strict oil class is required, the air still has to be treated as necessary and tested at the point of use.

What is the difference between dew point and pressure dew point?

Dew point at atmospheric pressure and pressure dew point describe the same air in different conditions. Pressure dew point is measured at the line pressure and is the figure that matters for condensation inside pipework. When air is expanded to atmosphere its dew point falls, so an atmospheric figure looks better than the real condition in the line. Always specify and report pressure dew point.

Related pages

• Dew-point measurement
• Compressed-air quality and treatment
• Compressed-air dryer service
• Filter lifecycle and differential pressure
• Compressed-air audit and measurement hub