Good vents make good housings

Cindy S. Marra
Senior Applications Engineer
W. L. Gore & Associates Inc.
Elkton, Md.

Edited by Stephen J. Mraz

Watertight enclosures are no guarantee that sensitive electronics will not have problems with dust, dirt, water, and other liquids, especially if the enclosures are in harsh environments. Damaged electronics inside sealed housings are common across many industries. And as electronics get more complex, these problems will get worse.

Most engineers address these problems by making the enclosures as watertight as possible. They seal the enclosure with more rugged O-rings or gaskets, make the enclosures thicker to minimize movement around the seal, and install more bolts around O-rings and gaskets to maintain a proper seal. But these fixes don't address the real cause: pressure changes inside enclosures. Pressure differentials are usually caused by rapid temperature changes, internal gas buildup, or both. Changes in altitude during shipping, along with external pressure fluctuations, also cause problems.

PRESSURE DROPS
Temperature changes, which cause pressure changes, are caused by a variety of factors. During normal operation, electronics generate heat, raising temperatures inside the housing. A quick drop in outside temperature can then create a 12-psig vacuum inside the enclosure. This leads to the housing drawing in air, which puts significant pressure on seals. (Typical seals on NEMA-4X enclosures draw in air and moisture at about 1.0 psig.) Once water gets inside a "sealed" box, it cannot escape and begins corroding sensitive electronics.

Another problem, especially for portable devices such as radios, mobile phones, and inventory tracking units, is the buildup of hazardous gases, such as hydrogen,-from the normal operation of some batteries, such as N:MHd cells. For example, a battery-operated toothbrush emits gases inside the housing at concentrations, which, if not released, can cause an explosion and hurt someone. And when the toothbrush cools as it is rinsed under a faucet, temperatures inside the enclosure fall rapidly. So now there are two factors that can damage a toothbrush's seal: temperature variations and gas buildup.

Altitude change is another factor that changes pressure within housings. Most devices shipped by air are stored in nonpressurized cargo bins, so pressure inside the package changes significantly and quickly when the plane takes off and lands. If the altitude change isn't compensated for, the resulting vacuum makes it hard, if not impossible, to open the package. For example, when the military drops containers from an airplane, it creates a significant vacuum pressure inside the containers as they fall from 32,000 ft. A vent in the containers prevents the internal vacuum and lets containers be opened immediately after touching down.

Although permanently installed housings do not have to deal with altitude changes or moving indoors and outdoors, many are often exposed to liquids other than water. For example, high-pressure sprays that clean sealed enclosures put significant pressures on seals and generate pressure drops greater than 2.0 psi inside the enclosure. These wash downs may include high-pressure sprays of water and strong detergents, so the potential for detergents to contaminate the electronics must be considered.

PRESSURE STABILITY
Pressure within enclosures is affected by diverse and complicated variables: temperature, battery-gas buildup, altitude, water, and other contaminants. The obvious solution is to equalize pressure-between the inside and outsideof the enclosure to eliminate stress on the enclosure's seals. But how? Simply drilling a hole in the enclosure lets air flow freely and equalize internal pressure but a hole in an airtight enclosure defeats the purpose of sealing the housing in the first place. Holes also let in a variety of contaminants — water, dirt, insects, and salt, to name a few. The challenge is to create an enclosure that lets air and gases flow freely, without letting in contaminants.

Installing a vent made with expanded polytetrafluoroethylene (ePTFE) provides constant airflow and prevents contamination. The porous microstructure of ePTFE lets air freely enter and leave the enclosure. At the same time, ePTFE's low surface tension makes it inherently hydrophobic, or water shedding. This means ePTFE will keep out water and most environmental contaminants. Although ePTFE lets some water vapor into the enclosure, the porous membrane also makes it easy for the vapor to escape, minimizing moisture and reducing condensation. For enclosures exposed to liquids other than water, there are oleophobic ePTFE membranes. These membranes repel liquids with low surface tension such as oil, soaps, and alcohol.

SELECTING THE RIGHT VENT
Vents come in a variety of shapes and sizes, and there are many variables that determine a vent's effectiveness, including the amount of free space inside the enclosure, the rate and range of temperature changes the enclosure will encounter, and IP requirements. IP ratings determine how much water pressure enclosures can withstand, and therefore, the strength of the membrane.

The amount of free space — 20, 50, or 80%, for example — dictates how much air flows inside the enclosure. And the more free space, the greater the vacuum inside the housing when there's a pressure differential. With regards to temperature, the most significant issue is its rate of change. The faster temperature changes, the greater the pressure on the seals. So rapid temperature drops are much more significant than gradual changes. For example, a 30°C change in 3 min creates a much stronger vacuum than a 60°C change over an hour.

Recently, my company got a call from an engineer who bought enclosures to protect sensitive animal-tracking devices. Before buying them, he tested them to ensure they met the required IP ratings. The boxes were mounted on trees. In less than three months, they began leaking and cracking and the electronics inside began losing accuracy. The engineer wanted to stop the leaks without taking the boxes down, so we recommended a vent that could be installed by drilling a hole and screwing it in like a bolt.

Another call came from a company that makes industrial computers that are routinely carried from outdoor loading docks directly into freezers. The extreme temperature changes build up a significant partial vacuum inside the computer case which pulled in frigid, moist air. To compensate for pressure changes, the manufacturer snapped a bladder into two holes on the underside of the keyboard. The company wanted to eliminate the bladders and cut costs. Holes under the keyboard still let pressure inside equalize with pressure outside, but they also let in contaminants and condensation.

Computer designers are famous for using every cubic millimeter of space inside of housings, so the manufacturer did not have enough free space to install a threaded vent in each hole. After evaluating the computers' operating environment, we recommended an adhesive vent retrofitted to the outside of the housing. The vents now protect the electronics from contamination, and provide free airflow that eliminate interior pressures fluctuations that generate vacuums. As an unexpected bonus the vents prevent water condensation inside the case, so the issue of fog on the screen was avoided.