ISO 29461 specifies methods and procedures for determining the performance of particulate air filters used in air intake filter systems for rotary machinery such as stationary gas turbines, compressors and other stationary internal combustion engines. It applies to air filters having an initial particle efficiency up to 99,9 % with respect to 0,4 μm particles. Filters with higher initial particle efficiencies are tested and classified according to other standards (e.g. EN 1822). These procedures are intended for filters which operating at flow rates within the range 0,25 m3/s (900 m3/h) up to 1,67 m3/s (6000 m3/h). This part of ISO 29461 refers to static (barrier) filter systems but can be applied to other filter types and systems in appropriate circumstances. Two methods of determining the efficiency are used in this part of ISO 29461: — particulate efficiency (measured with respect to particle number and size); — gravimetric efficiency (percentage weighted mass removal of loading dust. Also a flat sheet media sample or media pack sample from an identical filter is conditioned (discharged) to provide information about the intensity of the electrostatic removal mechanism. After determination of its initial particle efficiency, the untreated filter is loaded with dust in steps until its final test pressure drop is reached. Information on the loaded performance of the filter is then obtained. The performance results obtained in accordance with this part of ISO 29461 cannot be quantitatively applied (by themselves) to predict performance in service with regard to efficiency and lifetime. Other factors influencing performance to be taken into account are described in the annexes.