Perma Pure Nafion Gas Sample Dryers operate by transferring moisture from the sample gas stream to a counter-flowing purge gas stream, much like a shell-and-tube heat exchanger transfers heat. Water molecules travel through the Nafion® tube wall in its chemical structure, dispersing into the purge gas stream. It is important to note this is not permeation in the tradition sense. In contrast to typical permeation membranes, the water molecules do not pass through “holes” in the membrane. The water molecules become a part of the materials’ chemical structure, and are transported in a first order chemical reaction from one side of the membrane to the other. This transport mechanism is highly selective towards water and it is this selectivity that makes Nafion so valuable for many applications.