Enhancing Sensor Reliability through Venting

Sensors Insights by Gary Chan

As the Internet of Things continues to expand, it is estimated that by 2020 anywhere from 25 to 30 billion electronic devices will be connected wirelessly. The purpose of these devices is to exchange data with other connected devices or operators to perform tasks such as:

  • Rotating solar panels to maximize access to sunshine or measuring humidity and soil moisture for commercial and farming irrigation systems;
  • Increasing vehicle safety with active assist systems in automotive and heavy-duty equipment;
  • Controlling a manufacturing process to ensure product quality or managing inventory availability; and even
  • Creating Smart City networks for improving traffic flow, 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, i.e., 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, e.g., severe weather; ingress of moisture, particulates, and other contaminants. Also condensation, high-pressure sprays, shock, and vibration can cause problems.

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. For example, sensors inside that automatically control LED luminaires are increasingly vulnerable to the demands of this evolving technology such as:

  • 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 make warranty claims more numerous.
  • 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.

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 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 the vacuum part of 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°C and 120°C (200°F and 250°F). 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 then 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. 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.

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.
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.

The unique structure of ePTFE improves the reliability of 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, be it welding, adhesive, screw-in, or snap-in, as well as the vent size and construction, e.g., 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.

About the Author
Gary Chan works in the industrial products division of W. L. Gore & Associates, specializing in new product development for venting applications in challenging environments. Prior to joining Gore, he was a senior process engineer with Corning, Inc., where he focused on high-tech ceramics for catalytic converters. Gary has a Bachelor of Science and Masters of Science in Chemical Engineering from the University of Rochester, with post-graduate work at the University of Delaware's Lerner College of Business and Economics.

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