Automotive Design
that condensate can form at the coldest point in the housing, which can lead to corrosion or cause a short circuit.
For large battery housings, this problem can be so extensive that it is difficult to solve without effective measures to equalise temperature and pressure. Given the housing’s size, even minor temperature differentials can put enough pressure on the housing to cause deformation.
In certain circumstances, driving a car out of a warm garage into the cold winter air can produce an interior vacuum that exerts a negative pressure of 500 kilograms per square metre. Lightweight housings are scarcely able to withstand such pressure.
Membrane technology OEMs generally deal with these problems in one of three ways. The first option is to pot the electronic components. While this solution creates a perfectly sealed system, the unit ends up significantly heavier and cannot be reopened and repaired if it fails.
Another way to achieve a hermetically sealed system is to use high-quality seals and thicker housing walls. The drawback of this system, however, is that it makes components more expensive and unnecessarily heavy.
A common and much more sensible solution is to incorporate a membrane that equalises the air inside the housing while at the same time preventing the ingress of liquids and dirt particles.
Airflow and water entry pressure Airflow and water entry pressure are the two fundamental characteristics that determine a membrane’s performance. Airflow describes how much air can pass through the membrane in a given period, at a given differential pressure. This defines how long it would take to equalise a pressure differential. Water
entry pressure is the minimum hydrostatic pressure that the membrane must be able to withstand before it leaks. Both parameters are influenced by the pore size of the membrane, among other factors. It is the membrane supplier’s job to provide the ideal combination of airflow and water entry pressure for each individual application.
A major challenge is the trend toward increasingly compact electronic components. This means venting components must also become smaller if they are to be integrated into smaller housings as effectively as possible. This in turn requires greater airflow per membrane surface area, resulting in a lower water entry pressure.
Typically, a system’s imperviousness is determined by ascertaining its IP protection rating (according to DIN 40050- 9). The IP test determines the electronics housing’s protection level against solid objects and liquids. The IP protection rating is defined by two digits: IPXY. The first digit (X) indicates the protection rating against ingress of solid foreign objects; the second digit (Y) indicates the level of protection against ingress of liquids. IPX9K shows how well the housing with integrated membrane is able to remain watertight when exposed to steam jets.
The IPX9K test is carried out in a testing chamber in which the housing, including its integrated membrane, is exposed to a steam jet from a distance of 100 to 150mm, at angles of 0, 30, 60 and 90 degrees. The airflow rate is kept between 14 and 16l/min, water pressure maintained at between 8,000 to 10,000kPa and temperature at 80°C.
ePTFE membranes A material that is ideally suited to venting applications because of its unique microstructure is PTFE (polytetrafluoroethylene). The PTFE raw material is stretched in a specially
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