Pressure compensating plugs: role and limitations
Pressure compensating plugs: what are we talking about?
In the trade we find pressure compensating plugs, also called pressure balancing plugs. They are mainly used in waterproof enclosures, such as electrical boxes, electronic boxes or connection boxes, in order to limit the effects of temperature-related pressure variations.
What is a pressure compensating plug?
A pressure compensating plug is a connector fixed directly into the wall of a watertight housing.
It is composed of a microporous membrane that:
- allows air (and therefore water vapor) to pass through
- blocks the passage of liquid water (rain, projections, runoff)
These devices are commonly used in waterproof electrical boxes, inside and out.
How does a pressure compensating plug work?
When the temperature rises
- The air inside the case heats up
- Its pressure increases
- Thanks to the plug, air can escape through the membrane
- Internal and external pressures are in balance
When the temperature decreases
- Internal pressure drops
- The outside air enters the housing via the membrane
- Again, the pressures equalize
- Liquid water does not penetrate, thanks to the hydrophobic membrane
The plug therefore acts like a passive pressure control valve.
What are pressure compensating plugs really for?
Without a plug, the repeated pressure variations in a waterproof electrical box can:
- excessively stress the gaskets
- accelerate their aging
- cause a long-term loss of tightness
The main function of pressure compensating plugs is therefore to preserve the mechanical tightness of the boxes, preventing liquid water from penetrating due to damage to the gaskets.
They are particularly used in:
- outdoor electrical boxes
- waterproof connection boxes
- boxes exposed to strong temperature variations
Pressure compensating plugs and condensation: pay attention to preconceived ideas
Can they eliminate condensation in a case?
The answer is no.
Pressure compensating plugs do not prevent water vapor from entering a case, since the membrane allows air to pass through… and therefore moisture in the form of vapor.
Why does condensation appear despite the plug?
When the outside temperature drops rapidly, the water vapor present in the air in the housing reaches its dew point and condenses on the inner walls.
Result:
- Condensation in the electrical box
- Water droplets
- Risks of corrosion and malfunction
The plug protects against external liquid water, but not against internal humidity.
How do you avoid condensation in a waterproof case?
To treat condensation, you must act on the water vapor present inside the case.
AS-B tape: a complementary solution to compensating plugs
The product AS-B was designed to address precisely this problem. It works by adsorption of water vapor contained in the case, which allows you to:
- reduce the internal ambient humidity
- avoid reaching the dew point
- prevent condensation formation
Unlike compensating plugs:
- AS-B acts directly on the humidity
- operates without electricity
- does not saturate permanently
- does not require any maintenance
Results validated in the laboratory
Our laboratory tests show that:
- Pressure compensating plugs are not enough to avoid condensation
- Only the use of an adsorbent such as AS-B makes it possible to maintain a dry environment inside the waterproof cases
Conclusion
Pressure compensating plugs are essential to maintain the mechanical tightness of the cases in the face of temperature variations. On the other hand, they do not treat internal condensation, which is still one of the main causes of corrosion and breakdowns.
For complete protection of a waterproof electrical box, the combination of pressure compensating plug + AS-B is a coherent, sustainable and effective approach.
Comparison table: breather vent vs AS-B sticker
The two devices address different problems in the same waterproof enclosure:
| Criterion | Pressure compensator | AS-B sticker | Both combined |
|---|---|---|---|
| Regulates internal pressure | ✅ | ❌ | ✅ |
| Regulates internal humidity | ❌ | ✅ | ✅ |
| Prevents condensation | ❌ | ✅ | ✅ |
| Lets water vapor through | ✅ (by design) | n/a (captures it) | Vapor enters and is adsorbed |
| Installation | Wall drilling | Adhesive inside | Both |
| Lifespan | 3-5 years | Unlimited | Depends on plug |
| Maintenance | Periodic replacement | None | Partial |
For an outdoor IP65+ enclosure subject to strong thermal amplitudes, the combination vent + AS-B is the most robust solution:
- The vent preserves mechanical waterproofness (seals)
- AS-B eliminates internal condensation (electronics)
Proof: IP66 climate chamber test
A comparative test in a climate chamber on 3 IP66 enclosures subjected to a cooling ramp (30°C → 0°C in 1h 20) showed:
- Empty enclosure: condensation
- Enclosure with breather vent alone: persistent condensation
- Enclosure with AS-B sticker: zero condensation
→ Read the full protocol and results
