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controlling lighting in parking and pedestrian areas, evaluating energy consumption, and monitoring weather and air quality. These intelligent devices are usually equipped with sensors to gather data in real time — from industrial humidity sensors and gas leak detectors to accelerometers in seismic equipment. In fact, the industry estimates that more than a trillion sensors will be used for this purpose by 2020. However, the data from an electronic device can only be as reliable as the sensors gathering that data. What can compromise a sensor’s performance, or even cause it to fail? Most intelligent devices are installed in challenging environments that can compromise the sensor’s performance. The sensors can be suffer from ingress of moisture, particulates, and other contaminants; condensation; high-pressure sprays; shock; and vibration. If the sensor is mounted on the device housing, it must be able to withstand these challenges. If the sensor is integrated inside the electronic device, it may be more protected from the environment, but it is still exposed to the temperature and pressure variations within the device housing. For example, sensors inside automatically control LED luminaires are increasingly vulnerable to the demands of this evolving technology: • advanced components that are more sensitive to internal temperatures and humidity; • transparent lenses that cause condensation to have a greater impact on the light sensor and warranty claims more numerous; and • LED light sources that demand longer life of the entire system, requiring more robust protection from both contaminants and condensation. To protect sensitive electronics, most engineers design sensor housings with robust materials, durable seals, and strong bolts to ensure a tight seal. The enclosure is effectively air-tight and waterproof, particularly if it must pass Ingress Protection (IP) or National Electrical Manufacturers Association (NEMA) standards. However, once the sensor is installed in the field, it may begin to show evidence of water and particulates inside the housing. Using watertight enclosures does not necessarily guarantee long-lasting protection and reliable performance because pressure differentials — which over time can cause leak paths — have not been addressed. Fig. 2: Gore’s ePTFE vent enables pressure equalization to maintain sealed enclosures protected. Understanding the cause of leaks Designers often overlook the natural phenomenon of gas expansion and contraction. As internal and external temperatures fluctuate during the electronics power cycles and weather changes, the internal air expands or contracts in response, generating pressure changes. The enclosure tries to equalize the internal pressure by drawing air in or forcing it out – often referred to as breathing. In ambient conditions, the Ideal Gas Law describes the relationship of temperature, pressure, and volume as PV= nRT, where P is pressure, V is volume, n is the amount of the material, R is the universal gas constant, and T is the temperature. If the housing is completely airtight, the internal pressure changes in the form of a positive or negative buildup. Positive buildups cause the housing to bloat, while negative buildups create a vacuum. Either type of buildup puts stress on the seals, joints or gaskets, which in turn compromises and damages their effectiveness. The compromised seals begin to allow moisture and contaminants to enter the housing during vacuum part of Fig. 1: Pressure can cause seals in non-vented enclosures to fail; however, the vented enclosure equalizes pressure before reaching the point where its seals are compromised. www.electronics-eetimes.com Electronic Engineering Times Europe June 2015 41


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