How smart seals extend the service life of luminaires

July 8th, 2014, Published in Articles: Vector

 

“Intelligent seals” prevent the ingress of water and contaminants into luminaires’ housing while allowing pressure to equalise in both directions.

Standard neon tubes and energy-saving lamps are increasingly replaced by LED lamps. With a very long service life, these lamps are extremely reliable and environmentally friendly. To achieve this service life, housings must be sufficiently robust to protect the electronic components from damage.

This is why the housing is sealed against water and contaminants. However, changes in outdoor temperature cause the air pressure within the housing to fluctuate constantly, which in turn puts either positive or negative pressure on the seals and compromises their effectiveness.

Over time, seals begin to allow water and contaminants to enter the housing, which can lead to corrosion, electrical shorts and potential failure of the electronics. In addition, condensation on the inside of the luminaire can impact on the quality of its light.

Many causes, one effect

Changes in outdoor temperature are one of the most common causes of pressure differentials (see Fig. 1). These changes can be sudden, as in a thunderstorm on a hot summer’s day, or more gradual over the course of the day or of the year. Either way, they place significant stress on the seals.

Fig. 1: Impact of pressure on vented and unvented housings.

Fig. 1: Impact of pressure on vented and unvented housings.

Direct sunlight can often cause the air inside the LED luminaire to heat up rapidly, with the resulting higher pressure putting positive pressure on the seals. As temperatures drop again at night, the internal air contracts and creates a gentle vacuum which draws the seals inward. A quick drop in temperature can create a vacuum of up to 150 mbar inside the luminaire, while a 30°C change in temperature creates approximately 10% of volumetric flow of air in or out in a non-hermetically sealed enclosure (see Fig. 1).

Pressure differentials are also caused by temperature changes within LED luminaires. Although LEDs do not get as hot as incandescent lamps do, switching a luminaire on and off nonetheless results in significant temperature fluctuations. These are at their strongest immediately after switching, which means that switching luminaires on and off repeatedly places not only the electronics but also the seals under considerable strain.

The effects of altitude changes are also absolutely underestimated. Of course, it makes no difference whether LED luminaires are fitted on the roof of a building or in the garden, but the luminaire must first be shipped to where it will be used. Since production sites are spread around the world, manufacturers often ship their products to distributors by air – and this often involves several intermediate stops. This means that the LED luminaires are exposed to a difference in pressure of between a little over 1000 mbar at ground level and 800 – 850 mbar in the aircraft.

Thermal shock is another common cause of pressure differentials. This occurs when a hot LED luminaire is sprayed with cold water from a garden hose, for instance, or when a cold luminaire is washed with hot water. It can also arise when a luminaire encounters snowfall.

Equalising pressure while protecting against ingress

The challenge is to allow free air flow in and out of the luminaire while blocking water and contaminants. Labyrinth seals are out of the question as they are completely permeable to particles, insects, and water. However, hermetically sealing the device with more rugged seals, additional bolts, thicker housings or potting compounds requires the use of non-permeable materials and is unacceptably expensive. It also makes the device heavier, extremely difficult to open under negative pressure and repair almost impossible.

Another alternative is to install a felt element, sintered vent, or mechanical valve. The first two options address the pressure differentials, but they can become blocked by water and contaminants. The mechanical valve is a one-way solution from inside to outside, which means it cannot prevent a vacuum.

Fig. 2: Typical calculation of pressure differentials in a housing.

Fig. 2: Typical calculation of pressure differentials in a housing.

Gore’s solution is a vent made of expanded polytetrafluoroethylene (ePTFE). A two-way, breathable membrane continuously equalises pressure inside the luminaire housing while also preventing the ingress of water and contaminants. The microporous membrane can be coated to provide oleophobicity. ePTFE’s node-and-fibril microstructure is open enough to allow gas molecules and vapor to pass through it easily, but the openings are so small that liquid and other particulates are repelled.

An important consideration is how to deal with hydrogen sulphur. This is given off especially by inexpensive EPDM seals produced using sulphide vulcanisation, where the vulcanisation action did not bond all the sulphur atoms.

Sulphur vulcanised nitrile butadiene rubber (NBR) or other components containing sulphide can also give off hydrogen sulphide. This substance causes corrosion in luminaire components such as silver-plated lead frames, which impacts electrical contacts with the wire bond or die bond.

Comprehensive practical testing

Equalising pressure using an ePTFE vent reduces the potential for moisture vapor to condense on lenses and reflectors, and increases the service life of seals. This has been demonstrated by thorough testing to compare two commercially available LED luminaires, one conventionally sealed unit and one with an additional ePTFE vent. Although the on/off cycle of both luminaires caused temperatures to rise and fall, the amount of pressure placed on the seals is significantly different. In the sealed luminaire, the pressure spiked by 6,2 mbar when the light was switched on and dipped -6,9 mbar when switched off. However, the vented luminaire showed a change of only ±0,69 mbar (see Fig. 1).

Comparing the relative humidity inside the LED luminaires after a standard IPX5 water ingress test demonstrates the significance of pressure differentials. The relative humidity in the sealed luminaire was significantly higher than in the vented luminaire. Over the course of ten days, the relative humidity in the sealed luminaire almost always remained at around 100% (see Fig. 3).

Fig. 3: The unvented housing shows relative humidity of 100%, indicating condensation.

Fig. 3: The unvented housing shows relative humidity of 100%, indicating condensation.

This indicated condensation inside the luminaire caused by water entering during the test. Although the relative humidity in the vented luminaire rose immediately after the shock test, it decreased relatively quickly again and there was no evidence of condensation (see Fig. 4).

Fig. 4: Luminaire with condensation.

Fig. 4: Luminaire with condensation.

A further test, conducted outdoors over a period of five years south of Munich, demonstrated the longer service life of vented enclosures. Five units were tested: two with no vents; one with a side vent; one with a top vent, and one with two vents (one on each side). The testing showed that the pressure differential in the unvented units ranged from
-150 mbar in both to 131 and 147 mbar respectively (see Fig. 5).

Fig. 5: Long-term study of pressure differentials in electronics housings.

Fig. 5: Long-term study of pressure differentials in electronics housings.

A significant amount of condensation was also detected. In the vented units, the maximum pressure measurements fell to ±40 mbar with a top vent; ±30 mbar with a side vent, and just ±4 mbar with both vents. This is an impressive demonstration of how effective these venting systems are at equalising pressure. In addition, no condensation was detected, and neither was there ingress of water or dust. Further testing showed that the vents were fully functional, even after five years of outdoor operation (see Fig. 6).

Fig. 6: Long-term study of pressure differentials in electronics housings.

Fig. 6: Long-term study of pressure differentials in electronics housings.

Conclusion

Pressure differentials compromise housing seals. Not taking this into account when designing LED luminaires can reduce the service life of LEDs, power supply drivers and other electronics. The ingress of water through damaged seals also leads to condensation on lenses and reflectors which can decrease light efficiency and the aesthetic quality of the luminaire. As demonstrated through the IPX5 test, integrating an ePTFE vent into the housing equalizes pressure by allowing continuous airflow in both directions and preventing the ingress of water. The vent also reduces condensation because moisture vapor can escape from the luminaire before condensing.

Contact Lilian Thompson, WL Gore & Associates, Tel 011 894-2248, lithomps@wlgore.com

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