The RAE Systems 45 mm External Filters are made of PTFE Teflon membrane with a 0.2 micron pore size to prevent water from being sucked into the sensor manifold of your RAE Systems gas monitor. They will also prevent dust and other particles from entering the monitor, thereby prolonging the operating life of the sensors. To install the water trap filter, twist it tightly into the Luer connector on the inlet probe. If the water trap filter changes color, traps dust or other particulates inside it, or if water has been sucked in, the filter needs to be replaced. The water filter should not be used for some of the gases that could be absorbed by the filter material. These gases include: CL2, PH3, NH3, and HCN. Package of 10 filters.