Some sampling procedures require that an inert support disk be used in the cassette. Polypropylene support disks are impervious to most solvents. The disks have been designed to provide a smooth support for delicate membrane filters.
Cellulose support pads provide a smooth
surface and allow for a uniform air flow distribution through the sampling
cassette. These support pads are the standard material (cellulose fibers) which
meet the requirements of all NIOSH and OSHA standard air sampling methods.