Your ISO 8573-5 and Residual Oil Vapour Monitoring Questions Answered

What is “residual oil vapour” really?

In discussing compressed air purity, residual oil vapour refers to the remaining oil based content after the water, particles, liquid oil and aerosol oil have been filtered out. Oil vapour is regarded as any gaseous hydrocarbon based on 6 carbon atoms and can also include volatile organic compounds(VOC’s) and organic or inorganic solvents.


Will installing an oil-free compressor eliminate oil contamination in our compressed air?

The older reciprocal design compressors are subject to oil carryover where the lubricating oil passes the piston rings and is entrained into the airstream as additional contamination. Changing to an oil-free design will only remove the compressor as an origination point of oil contamination. Any existing contamination in the system will still be present and the new compressor will continue to compress oil vapour as part of the intake charge.


How are liquid and aerosol oils removed from the compressed air?

These liquid forms of oil contamination are captured in successively finer filtration stages from mechanical to coalescent to activated carbon filters in the air treatment plant and also at point of use.


Will an oil aerosol test be enough to account for the vapour content?

If you are only looking to achieve Class 3 or lower for oil content, there is no need to measure vapour. For Class 0, 1 or 2, vapour has to be measured separately and the result added to the aerosol concentration to give the total oil contamination according to ISO 8573-1. The vapour concentration could be vastly different compared to the aerosol and making decisions based on aerosol testing alone could cause big problems in terms of product quality, plant reliability and safety.


Where does the oil vapour come from?

Oil vapour is present in the air around us at all times and at higher concentrations in industrial and urban areas. The consequence of this air pollution is that it passes through the inlet filters of air compressors, increasing the vapour concentration and is then distributed around the compressed air system. Unfiltered oil aerosols can collect in air systems to form liquid oil and this can form more vapour due to changes in temperature. With the right conditions of pressure and temperature, oil vapour can condense to form liquid hydrocarbons inside the air system which can then promote more oil vapour as the temperature rises. Maintenance and modification of the air distribution ring or connected systems can directly introduce additional hydrocarbon vapours from chemical processes, solvents or combustion exhausts.


How is oil vapour removed from compressed air?

Activated carbon filters will help reduce the overall oil content but to really remove hydrocarbon vapours, a specific oil vapour filter system has to be employed.


Where should oil vapour filters be employed?

Having a correctly sized oil vapour filter as the last element of the post compressor treatment chain should remove the intake component prior to the air distribution system but the cost of this and the resultant pressure loss would need to be considered against your risk assessment process and measurements of the actual oil vapour levels in the delivered air. It may be more effective to install smaller vapour filters at critical points of use as these would be necessary anyway to account for in-system generated oil vapour.


How can the presence of oil vapour be detected quickly?

There are two common methods of vapour detection, chemical indicator tubes and electronic PID sensors. Neither method is compliant with ISO 8573-5 for oil vapour measurement but the indicator tubes will prove the presence of the vapour and PID sensors will give an accurate measurement of it.


How does ISO 8573 apply to oil vapour measurement?

ISO 8573-1 lists the compressed air purity classes and oil vapour only needs to be measured if you are looking to achieve oil purity classes 0, 1 or 2. In that case, the only approved measurement method (taking a sample for gas chromatography) is described in ISO 8573-5.


Where should we measure for oil vapour?

All critical points of use identified in your risk assessment are obvious measurement points but you may also wish to measure at the end of the compressor air treatment chain to see what the vapour levels are prior to the air distribution system. This will give an indication how efficient the treatment system is and also reflect on how this differs from the levels at point of use, which will point to the level of oil contamination in the distribution system. Other measurement points could be sections of the distribution system where the ambient temperature increases sharply as this can give rise to an increase in oil vapour.


How can we tell if our air purity testing has been done in compliance with ISO 8573?

Firstly, be familiar with the standard and the approved measurement methods in it. Ask your contractor, technician or quality manager for a method statement describing the measurement process for each of the 3 main contaminants: particles, water and oil. Check your test report to see if it correlates to these methods and if it is unclear, ask for clarification. If the quality audit requires the fitting of membranes into the air stream and sending these and other samples off to a laboratory for testing, it is most likely to be compliant. If you don’t have membrane holders and isokinetic sampling points already installed on your plant and there is no need to shut the air system down prior to the start of the audit so that they can be fitted, your testing may not be fully compliant.


What are the approved methods of testing for oil vapour to ISO 8573-5?

The approved method is to sample the air into a stainless steel tube containing specific amounts of charcoal. The method and the sample tube specification are stated in ISO 8573-5 and once the sample is complete it is sent for gas chromatography in a laboratory to reveal the results. There are no other approved methods of testing. The standard does mention chemical indicator tubes in that they can be used for indication of the presence of hydrocarbon vapour but not for the measurement of it.


Do we need to measure oil vapour contamination strictly according to ISO 8573-5 methods?

If your company quality regime includes maintaining records of compressed air purity according to ISO 8573 then yes. However, the standard is there to provide an international  framework of comparison for air purity measurements, it is not a legal requirement for it to be met in full. Alternative measurement methods could certainly be used as long as they are not claiming to be ISO 8573 compliant and they are acceptable to your site quality systems.


Should we be monitoring oil vapour more regularly?

Your own site risk assessments should guide decisions as to what is prudent or acceptable and it may be decided that compliant audits are combined with regular or continuous oil vapour monitoring in order to catch and deal with any issues promptly before they become major problems.


How can oil vapour contamination be measured continuously?

Electronic PID sensors are ideal for continuous monitoring of hydrocarbon vapours as they respond quickly to changes and provide very accurate measurements. These units can be operated independently or combined with the pant control systems to flag changes beyond a predetermined threshold. PID sensors are also incorporated into units that can measure dew point and particle concentrations, providing an easy solution for continuous and quick monitoring of delivered compressed air purity.



Related Links:

Compressed Air Purity Measurement:


How to Overcome the Practical Problems of ISO 8573 Oil Measurement in Compressed Air:


S120 PID Oil Vapour Sensor:


S600 Portable Combined Particle, Dew Point & Oil Vapour Air purity Analyser:


S601 Combined Particle, Dew Point & Oil Vapour Air purity Analyser for Continuous Measurement:



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