Thin, hydrophobic microporous membranes can lend unique structural capabilities while protecting sensors in harsh environments. Pall offers a wide range of membranes that can be specifically matched to unique sensor applications.
Hydrophobic membranes are typically formed from a single polymer or combination of polymers and have a three-dimensional, sponge-like structure. Depending on materials of construction, some membranes are inherently hydrophobic, while others can be rendered hydrophobic by chemical treatment. These membranes can be coated with chemicals and/or layered with catalysts to impart custom features and performance.
Base membrane polymers for sensor applications include polytetrafluoroethylene (PTFE), glass fiber composites (cellulose-glass fiber and non-woven polypropylene), polyethersulfone (PES) and a modified acrylic copolymer. These membranes allow airflow or gas exchange while excluding fluid and/or particle contamination. Engineering design considerations regarding the choice of material and its respective sealing requirements are crucial to performance in a sensor assembly.
Hydrophobic Emflon PTFE, for example, is chemically and thermally resistant, and offers the best combination of airflow, water intrusion pressure and pore size characteristics. Pall Emflon PTFE membranes are used to protect sensors that measure hydrogen in the reactor core cooling water at a Canadian nuclear power facility. Prior to this application, plant operators were replacing several of the critical hydrogen sensors daily. The PTFE material was chosen because it repels low surface tension liquids and vents gases in a high-temperature, high-radiation environment.
Chemical fuel cells, flow meters, carbon monoxide detectors and smoke alarms are other examples where hydrophobic membranes have been matched to the sensor to enhance reliability. The membrane protecting a sensor in a carbon monoxide detector, for example, must allow the free flow of carbon monoxide while blocking the intrusion of environmental dust and contaminants for the life of the detector, typically about five years.
Membranes can also be used as a substrate when coated with chemicals and/or catalysts that render the membrane an active component in a sensor assembly. Gas sensors, specific to a certain gases like oxygen, contain fluid and two electrodes. The presence of oxygen induces a voltage between the two electrodes. A membrane substrate for an oxygen sensor can be fashioned into an electrode by screen-plating a thin layer of precious metal catalyst like platinum directly on the membrane. As an alternative to using platinum foil, plating onto the membrane saves the precious metal and provides the mechanical strength needed for the electrode.
Industrial oxygen sensors used in mining, household gas burners for furnaces, and car engines commonly employ this technology. Membranes used as substrate can also control the rate of diffusion within a sensor assembly. This can serve as a low cost alternative to expensive mechanical solutions.
Membrane characteristics, including pore size, thickness, airflow rate, water intrusion pressure, and bubble point (KL), determine the amount of contaminant protection provided by a given material in specific applications (see Table 1). Thickness is expressed as microns or mils. Airflow rate is measured by calculating the time it takes a volume of air to pass through a defined membrane area at a defined pressure (expressed as airflow per unit area/per unit time/per unit pressure or sccm/cm2/psi). Water intrusion pressure measures the pressure required to force water through a hydrophobic membrane, expressed in units of pressure such as psi. KL is the pressure at which a sudden rise in airflow occurs as pressure is raised and corresponds to the liquid expelled from the pores of a wetted filter medium. This pressure is proportional to pore size.
Particles retained by the membrane are dictated by pore size, and membranes with larger pore sizes have higher airflow rates. The relationship between pore size, particle size, and retention is usually defined for a liquid system. In an airstream, however, a Pall membrane can capture particles smaller than expected based on the size of the pores due to tortuous paths followed by pores in the three dimensional structure. This characteristic is reflected in the titer reduction of the membrane, which quantifies the ability to remove particles or aerosolized bacteria from an airstream.
Microporous materials can be attached to housings and finished devices via adhesives, thermal sealing or sonic welding. Thermal sealing involves pressing a heated die directly against the membrane. Ultrasonic welding also uses pressure but generates its heat using high frequency vibrations. In addition, most medical and some industrial applications require that the device be sterilized to eliminate infective contaminants. Hence, the hydrophobic material must also be able to withstand sterilization via ethylene oxide, gamma irradiation, or autoclaving in addition to the sealing process. Sealing method and housing material are important contributors to membrane/device compatibility.
There is a trend in the sensor market toward solid-state technology where multiple variables can compromise sensor readings. However, when membrane is incorporated into sophisticated chemical sensors, the device has a more vigorous response time and is more specific because access to the sensor can be restricted to selected molecules. Not only does the design prevent component breakdown within the sensor assembly, it also allows the use of larger gas sensors that are capable of sampling a much larger volume, producing a more accurate result.
