Part 9: Test methods for liquid water content

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.