Pore sizes are based on liquid filtration ratings. Filters will retain much smaller particles in air. For example, a 0.8 µm liquid rated filter will retain > 50% of particles as small as 0.08 µm (1 order of magnitude smaller).

It depends upon the application. Generally:

  • Collect on the rough/dull side for gravimetric analysis. The rough side minimizes loss of dust.
  • Collect on the smooth/shiny side for microscopic analysis. The smooth side enhances viewing.

At higher flow rates, a small spot may appear in the center of the filter when a closed-face cassette is used: the 4 mm inlet works as an impactor with a very large 50% cut-point. Adding an additional piece will not necessarily solve the problem entirely when sampling at very high flows (5 to 10 L/min) and large particles are present. In addition to extending the length of the cassette, one can remove the inlet piece and sample with an open-face cassette.

This filter consists of a PTFE membrane filter laminated onto a second PTFE membrane for extra support and strength. It is not necessary to use a support pad when sampling with laminated PTFE filters.