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SENSORS & DATA CONVERSION the cycle, which can ultimately lead to corrosion and electronic failure. External temperature changes can be dramatic, whether experienced throughout the course of a typical day or during a sudden thunderstorm. Consider sensors in a car that has been sitting outside in the winter. As soon as the car is turned on, the engine begins to warm up, quickly reaching temperatures between 100 and 120°C. The transition between the initial cold temperature and the eventual hot temperature causes the air inside the sensor housing to expand, increasing the pressure on the seals. Or, consider the opposite extreme — the air inside sensor housings on a solar energy panel is extremely hot during the day; high-pressure sprays used to clean the panels cause the internal temperature to drop suddenly, potentially generating pressure drops greater than 137.9 mbar (2.0 psi) inside the housings, which can cause even the most robust seals to fail over time. Protecting against pressure differentials Over the years, engineers have developed a variety of methods for eliminating pressure differentials. Drilling a hole in the housing or creating a hole with a torturous path eliminates the pressure differential, but this open system concept allows contaminants, including dust, dirt, water, and even insects, to enter the enclosure and compromise the electronics. Many engineers consider hermetically sealing the enclosure as the most secure method to protect electronics. However, hermetic seals require that only non-permeable materials be used in the device, which means no plastic components. This option is usually unrealistic for most applications because they would be extremely heavy and very expensive. In addition, wires entering the housing would defeat the purpose of hermetic seals because the rubber grommet becomes a potential entry point for moisture and contaminants as it degrades due to environmental factors, aging, and pressure differentials. Another option is the use of potting compound. However, potting materials can put tensile stress on printed circuit boards and cause them to warp. And like hermetically sealing, potting compound is not always feasible because it eliminates the ability to service or replace the electronics, it is expensive and heavy, and specialized equipment must be added to the manufacturing process. Other common options include larger, more robust seals, stronger gaskets or more nuts and bolts. These solutions will remedy the immediate leakage and contamination issues, but they are short-term and may ultimately fail because the fundamental problem of pressure differentials has not been addressed. The device is simply more airtight and more expensive to manufacture without having a solution for the root cause. Equalizing pressure without compromising performance Vents made of expanded polytetrafluoroethylene (ePTFE) allow continuous pressure equalization while still maintaining an environmental seal. Expanded PTFE is a uniquely microporous, chemically stable membrane that is inherently waterproof. Its unique node-and-fibril microstructure is open enough to allow gas molecules including vapors to pass through it easily, but the openings are so small that liquid and other particulates are repelled. Expanded PTFE also withstands UV rays and functions reliably in a broad temperature range, which makes it suitable for the challenging environments in which sensors are installed. Figure 1 compares the different options for protective housings during a vacuum. The dotted green line at –75 mbar (–1.09 psi) is the estimated point at which a vacuum is sufficiently strong to cause a leak in a sealed enclosure. The purple line represents a hermetically sealed metal enclosure, which is able to maintain the seal despite the pressure differentials. The red line represents a typical metal or plastic enclosure with IP67-rated gaskets. This enclosure initially withstands vacuum pressures up to about –70 mbar (–1.02 psi); however, when this pressure is exceeded, the seals are compromised and leaks begin to occur. Additionally, the aging of the gasket causes its compression set to increase gradually over time, which compromises the gasket at lower pressure differentials. Finally, the light blue line represents a vented enclosure, which quickly dissipates the pressure differentials, preventing the pressure in the enclosure from exceeding –35 mbar (–0.51 psi), thereby minimizing the stress on the enclosure’s seals. The unique structure of ePTFE improves the reliability of your electronic devices by equalizing pressure continuously, thus eliminating the issue of pressure differentials. Vents made of ePTFE reduce manufacturing costs because they require fewer bolts and eliminate the need to over-engineer housings, gaskets, and o-rings. Because the seals are not continuously being stressed by pressure, they require less maintenance and repairs, improving the long-term reliability of the product and reducing the total cost of ownership. Selecting the right vent Product reliability is crucial to the success of any product. Selecting the right vent requires more than just choosing a part from a catalog. Vents come in a variety of forms, material sets, and sizes, all of which have different levels of airflow and durability. Therefore, the microstructure of the ePTFE membrane should be engineered based on the requirements of a specific application — such as maximum airflow, IP rating, temperature extremes, standards the housing needs to pass, and size of the enclosure. The type of IP-rating specified for a device dictates the amount of protection against water pressure and solids that the venting membrane must provide, while the severity of the surrounding environment can affect the method and placement for integrating the vent into the housing. Once you select a potential venting solution, you should work with the manufacturer’s engineering staff to verify that the vent works reliably in your specific application. The process starts with modeling the pressure change in the enclosure based on the heat and mass transfer of air in the enclosure under the worst environmental conditions and different vents. Real-life performance testing can include airflow through your device, water entry pressure, climate chamber cycling, and installation compatibility. After this type of testing, the manufacturer should be able to assist your design team in determining the type of installation method such as welding, adhesive, screw-in, or snap-in, as well as the vent size and construction, such as plastic, metal, etc. If a vent is not tested and integrated into the enclosure carefully, it can become the weakest point of an enclosure, causing failure of the enclosure and the electronics inside. However, selecting the right ePTFE vent for pressure equalization can improve device performance by preventing particulate and water ingress while allowing diffused moisture to escape. The end result is longer product life, lower total costs, and higher brand integrity. 42 Electronic Engineering Times Europe June 2015 www.electronics-eetimes.com


EETE JUN 2015
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