ISO 8573-9:2004 specifies test methods for determining the liquid water content in compressed air, expressed as the liquid water mass concentration. The methods' limitations are also specified. One of a series of standards aimed at harmonizing air contamination methods, it identifies sampling techniques, specifies measurement procedures and also gives requirements for evaluation, uncertainty considerations and reporting for the air purity parameter liquid water. The test methods are suitable for determining the purity classes in accordance with ISO 8573-1. (Water vapour content is dealt with in ISO 8573-3.)