The polyethersulfone (PES) membranes used in the TECH INC membrane filters have asymmetric pore structure.  The pore structure varies within the thickness of the membrane such that the largest openings occur on one side and the smallest openings occur on the opposite side.  When viewing the membrane with reflected light at low incidence angles, each side has a somewhat different visual appearance.  The side with the largest pores will appear more dull  than the side with the smallest pores which will appear shinier.  With a little bit of experience, most users can easily identify the sides.  The membranes can be used with either surface oriented upstream without affecting retention.  However, orienting the dull side
upstream increases total throughput while orienting the shiny side upstream allows for better analyses of the retained particles.
The empty field is calculated using the formula: Volumetric flow rate (ml/min) = Flux LMH (Liter/m2/h) * Filter area (m2) * 1000/60