Moisture Vent

A Moisture Vent (also called a breather vent or desiccant vent) is a passive component installed in sealed AGL enclosures to allow the escape of internal moisture while preventing liquid water ingress. Unlike a pressure valve, which primarily equalises pressure, a moisture vent focuses on removing water vapour that has entered the fixture (e.g., during maintenance or through imperfect seals) or has condensed from temperature cycling. The vent typically incorporates a hydrophobic membrane that allows water vapour (gas) to pass but blocks liquid water. Some vents also include a desiccant material to absorb residual moisture. By preventing condensation on lenses, prisms, and internal optics, the moisture vent ensures that the light output and beam pattern remain unaffected by fogging or water droplets, which is critical for maintaining photometric performance and meeting ICAO/FAA specifications.

The moisture vent exists because even a well-sealed AGL fixture will eventually accumulate internal water vapour — from temperature cycling that draws in humid air, or from tiny imperfections in seals over years of service — and if that vapour has nowhere to go, it condenses on the internal optics exactly where it will do the most damage to the fixture's light output. Left unaddressed, that fogging or droplet formation distorts the beam pattern in a way that's invisible from the ground but very real to an approaching pilot relying on that light.

By letting water vapour escape while still blocking liquid water intrusion, the vent solves this without compromising the fixture's overall environmental sealing — a passive, low-maintenance way to protect the photometric performance that ICAO and FAA certification depends on, without requiring active drying equipment or frequent internal inspection.

Moisture vents are used in all sealed AGL fixtures and enclosures where condensation could compromise performance. Key applications include:

• Runway & Taxiway Light Fixtures: Inset and elevated lights with sealed optical chambers.
• Threshold & End Light Housings: Where lens clarity is vital for approach guidance.
• Approach Lighting Fixtures: Barrettes, ALS units, and strobe housings.
• PAPI & VASI Optical Units: Precision visual glide slope indicators where condensation would cause false indications.
• Obstruction Light Housings: Sealed beacons and LED units.
• Isolating Transformer Housings: To prevent internal corrosion and insulation degradation.
• Junction Boxes & Control Panels: To protect terminals and electronic components from moisture.

The vent fits into a threaded or gasketed port in the fixture housing wall, sized and positioned so it sits above any water ingress point but low enough that the membrane doesn't block the optical path or interfere with the lamp/LED assembly. Orientation matters more than for most fixture hardware: the vent is fitted facing downward or to the side rather than directly upward wherever the fixture geometry allows, so standing water on top of the housing cannot pool against the membrane. Because the vent is a passive, unwired component, no cabling is involved; installation is limited to correct torque on the threaded fitting and a check that the surrounding gasket seats fully so the vent doesn't become an alternate leak path.

The moisture vent works on the principle of vapour pressure differential and membrane technology:

• Hydrophobic Membrane: A microporous PTFE (or similar) membrane allows water vapour molecules to pass through while blocking liquid water due to surface tension and pore size.
• Vapour Pressure Gradient: When internal humidity exceeds external humidity, water vapour migrates outward through the membrane, reducing internal moisture content.
• Condensation Prevention: By keeping internal humidity below the dew point (relative to the internal temperature), the vent prevents condensation from forming on cool surfaces like lenses and prisms.
• Desiccant Option: Some vents incorporate a desiccant (e.g., silica gel) that actively absorbs moisture, providing an additional layer of protection.
• IP Rating Maintenance: The vent is designed to maintain the enclosure's IP67/IP68 rating, allowing vapour escape without compromising watertight integrity.

The vent is typically installed in a threaded port on the fixture housing, often at the highest point to allow moisture vapour to rise and escape. It is a passive, maintenance‑free device, though some designs include a replaceable desiccant cartridge.

Unlike the networked components elsewhere in the AGL system, the moisture vent has no electronic control logic of its own — it is a passive part of a fixture or enclosure's mechanical architecture rather than a node on the ALCMS network. Its place in the system is defined by where it sits in the sealed housing: it forms one boundary of the enclosure, alongside the gasket, cable glands, and (in a fixture) the optical window, and it is the only one of those boundaries deliberately designed to let vapour pass while blocking liquid water and dust. When the enclosure heats and cools through its duty cycle, internal air expands and contracts; the vent equalises that pressure difference through the enclosure wall so the seals elsewhere aren't forced to do it, and it lets any moisture that does condense inside migrate back out as vapour. There is no signal or status associated with it — its condition (clear membrane versus clogged or degraded) is checked only through physical or visual inspection during maintenance, not through the monitoring architecture that covers the electronic components in the same enclosure.

The vent body is machined from stainless steel (316L), nickel-plated brass, or UV-stabilised polyamide, threaded to match the fixture's mounting port (metric or NPT) and fitted with an O-ring or bonded gasket at the seating face to maintain the enclosure's IP67/IP68 rating once installed. The hydrophobic PTFE membrane is heat-staked or ultrasonically welded across the internal vent channel, backed by a perforated support disc that protects the membrane from mechanical damage during installation while still allowing free vapour passage. Where a desiccant cartridge is included, it sits in a small chamber behind the membrane, accessible for replacement on models designed for periodic servicing, and sealed on non-serviceable models. The external face includes a wrench-flat or knurled grip for hand-tightening into the housing, and the thread sealing relies on either a captive gasket or PTFE tape applied during installation rather than adhesive, so the vent can be removed and replaced without damaging the enclosure port.