Part 2: Oil aerosol content

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:

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

 

Method Type of flow Detection limit typical sampling time
A Full flow > 1 mg/m3 50 – 200 h
B1 Full flow 0,001 – 10 mg/m3 0,2 – 10 h
B2 Partial flow 0,001 – 10 mg/m3 0,2 – 10 h

 

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)