The RAE Systems External Filters are made of PTFE (Teflon) membrane with a 10 micron pore size to prevent water from being sucked into the sensor manifold, which would cause extensive damage to the monitor. They will also remove any dust and other particles from entering the monitor and prolong the operating life of the sensor. To install the water trap, simply attach it to the front of the inlet tubes using a short (approximately 1 in.) length of Tygon tubing. Package of 5 filters.