Membrane thickness is important for several reasons (10). First, thickness, in combination with the porosity (the % air in a porous structure), allows prediction of how much liquid is necessary to fill the pore structure. Second, thickness variations can affect strip manufacture if the processing steps cannot accommodate the variation. Third, the thickness of the membrane has to be matched to the thickness of the other strip components and the housing to avoid overcompression in the assembled device. Thickness is straightforward to measure using gages that do not crush the membrane. If the membrane is unbacked, only the membrane is measured. If the membrane is cast onto a polyester film, however, the reported thickness is that of the composite structure. Variations can result from the membrane, the film, or both. Manufacturers have specifications on the thickness of the polyester film as a raw material; its thickness is normally more consistent than that of the membrane.

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