Perma Pure MRD Series gas sample dryer series (Monotube Dryer) is a variation of the MD-Series dryer with a number of important enhancements. Just like the MD Series, the MRD Series uses a single Nafion tubing selectively permeable membrane tube to continuously dry gas streams removing only water vapor. These dryers operate over a wide range of temperatures, pressures and flow rates, and dry to dew points as low as -60ยฐC.