Condensation has been observed in the head of an early luminaire fixture installed in the field at the Manitouwadge Airport. A sample luminaire was repurposed for R&D evaluation and subjected to condensation and immersion testing. No evidence of condensation was observed after multiple warm-up and cold water immersion cycles, indicating that the fixture is well sealed at least when manufactured according to assembly instructions. However, some concern remains as to the long-term effects of pressure cycling the luminaire head with repeated warm-up/cool-down cycles. It is recommended that long-term warm-up cool down testing be carried out on the luminaire to further assess lens susceptibility to fatigue failure brought on by pressure cycling.
Fixture
A white 347VAC sample luminaire was returned from initial field evaluation in Lethbridge, AB, and subsequently donated to R&D by Sam. The HV power supply was swapped out for a MeanWell HLG-240H-36A supply that was tuned to 6A output. The head was opened up and the lens flanges re-installed at 25-in.lb torque. The cable glands were left as originally manufactured.
Testing
The fixture was put through several heat-up/cool-down cycles. On the first two cycles the fixture was turned on in a horizontal output position and allowed to reach thermal equilibrium. In both cases the case temperature stabilized between 50°C and 51°C. While hot, the fixture was powered off and the head was fully immersed in 10°C to 11°C water. Once the fixture cooled and came to thermal equilibrium with the water it was removed and inspected for evidence of water ingress and condensation. No condensation or water was observed on the interior of the fixture in either test.
After the second round of immersion testing, the fixture was placed in the freezer, and checked after 10 minutes and again after 4 hours for signs of condensation as the lens and case cooled relative to the air inside the luminaire head. No condensation was observed. The fixture was then dunked in warm water to inspect for the presence of air bubbles as the air inside the fixture expands. No air bubbles were observed.
The fixture was then put through one more warm-up/cool-down test where the head of the unit was placed inside a box to limit heat transfer. In this case the head reached thermal equilibrium at 67°C. The fixture was powered off and the head immersed in ice water at 4°C. After testing the fixture was dried and inspected for condensation. No condensation was observed.
Evaluation
Given the lack of condensation evident after repeated quench cooling during full immersion of the luminaire head, it is clear that the fixture is well sealed against water ingress. However, this is not the only entry route for water molecules to enter the fixture. Over time, even with a fully sealed fixture there will be some diffusion of water vapour through the lens, gasket, and even metal case materials. Eventually the fixture will come to equilibrium with the same vapour pressure inside and outside the fixture. In a high humidity environment with a fast decrease in temperature, it is possible for some temporary condensation to occur on the lens as it will cool off faster than the air inside of the fixture. This condensation would be a harmlessly small quantity of water, and would dissipate over time. This will occur for all fixtures, not just a CRS Electronic’s product. If concern remains about the electrical safety of limited amounts of condensation within the fixture head, an easy fix would be to ensure that all boards are conformal coated as part of the manufacturing process.
The remaining point of concern from a product reliability standpoint is what the long term impact of pressure cycling the cavity during warm-up/cool-down cycles will have on the lens. Eventually, severe pressure cycling can lead to fatigue failure of the lens. Almost all amorphous thermoplastics exhibit poor fatigue resistance, and will eventually fail when placed under cyclic stress conditions. The risk of long term lens failures in the luminaire is considered to be moderately low, but cannot be completely ruled out without venting for pressure equalization of the fixture. It’s also worth noting that fatigue resistance can be significantly accelerated by the presence of certain cutting fluids or other chemicals in the install environment, in what is known as environmental stress cracking, as seen in the recently returned luminaire lens. It is recommended that we perform long-term temperature cycling on the luminaire units in order to reduce the uncertainty on lens fatigue life.
