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Drawing from previous studies published in SAE, a software program describes physical progress of water intrusion into or released from a lamp assembly. This program, based on test data, calculates condensation quantity and clearing time. It calculates moisture exchange between a lamp assembly and ambient, considering the moisture contained in the air inside of the lamp and moisture stored on the wall surface or in the materials. This program is applicable to different scenarios, such as initial humidity conditions of a lamp assembly, lamp size, type, venting type and test performance specifications. The significance of the program application in guiding test performance will also be addressed.

Certain conditions promote water vapor to condense on available surfaces within an automotive lighting assembly. Certain surfaces are more susceptible to water condensation than others. This is due partially to temperature gradients in the lamp and possibly water vapor concentration gradients. This study demonstrates that humidity gradients exist in an automotive lamp. How humidity gradients affect the likelihood of surface condensation for a particular interior surface of a lamp will also be addressed.

The effect of water vapor transfer from plastic materials commonly used in automotive lighting assemblies can be experimentally measured. This is accomplished by isolating this phenomenon from other mechanisms effecting water vapor concentration in the lamp. Quantifying this effect helps to address design considerations for liquid water and water vapor egress in the lighting assembly. The relevance of the current moisture clearing type test required in the industry is discussed in light of the empirical results for this effect.

The FMVSS humidity test, as described in 571.108 S8.7, for forward automotive lamps permits environmental conditions to vary within a given tolerance. Pre-soak conditions of the lamp for the test may vary. The tolerances during the soak portion of the test allow for a +4°C variation (from the initial 38°C environment) and a +10% variation from the initial 90% environment relative humidity. Tolerances during the wind portion of the test allow for a +4°C variation (from the initial 23°C environment), a +10% variation from the initial 30% environment relative humidity, and a -0.153 m/s (-30 ft/min) variation from the initial 1.68 m/s (330 ft/min) airspeed setting. Utilizing a climate control wind tunnel, this study demonstrates that the tolerances on the environmental test conditions will produce substantial test result variation. The discussion emphasizes the best environmental condition to minimize time to clear condensation for a particular automotive lamp.

Vented automotive lamps exchange moisture with the surroundings during steady state and transient conditions. An application of Fick's Law of Diffusion gives the mass transfer rate of a vapor A through a stagnant column of gas B for steady-state conditions. Automotive lamp ventilation is similar to this simplified problem given certain assumptions and where the vapor A represents moisture in the air at one end of the vent tube and the gas B is the air in or outside the lamp. This paper shows that the equimolar counter-diffusion problem, a solution for Fick's Law, has potential in reliably predicting humidity changes within an automotive lamp for static conditions.