Protect Sealed Enclosures From Pressure Differentials and Humidity Intrusion
Sealed cases and enclosures are designed to protect sensitive equipment from the outside environment. But even a well-sealed system still has to deal with pressure changes.
Temperature cycling, altitude changes, air transport, ground shipment, and long-term storage can all create pressure or vacuum differentials between the inside of an enclosure and the surrounding atmosphere. If such differentials are not equalized, an enclosure may become difficult to open, seals may be stressed, or the case may experience structural strain to the point of deformation. In the worst case, an enclosure could be destroyed and its contents ruined.
To that end, breather valves are used to equalize pressure differentials. However, for moisture-sensitive equipment, pressure relief alone may not be enough and may even create a new problem.
Every time outside air enters an enclosure, that air carries humidity along with it. Accordingly, repeated breathing cycles can increase internal moisture over time, contributing to condensation, corrosion, mold, electronics issues, optical fogging, or degradation of stored components.
So, how is it possible to solve both problems? A breathing desiccator. Breathing desiccators allow the enclosure to equalize pressure while drying incoming air before it reaches the protected interior.
Why Sealed Systems Need Pressure Equalization
A sealed enclosure is constantly exposed to changing external conditions. As the temperature outside the case rises, the air inside the enclosure expands. Conversely, as the external temperature drops, the internal air contracts. Similar effects can occur when a case is shipped by air, moved between elevations, or stored in an environment with large day-night temperature swings.
The Effects of Diurnal Temperature-Cycling
Diurnal temperature-cycling is the fluctuation in temperature that occurs throughout a day; or, more specifically, throughout a 24-hour period. As the day wears on, temperatures rise steadily, beginning just after dawn, and then cool, starting in the late afternoon and continuing through the night.
Generally speaking, diurnal temperature-cycling causes the air inside of sealed enclosures, such as containers used for shipping and storage, to expand and contract, creating pressure and vacuum respectively. As a result, the expanding air pushes out against an enclosure’s walls (pressure) during the day when the atmospheric temperatures are warmer, and pulls against those walls (vacuum) during the night when temperatures are cooler.
These changes can create two common pressure differential problems:
Positive Pressure Buildup
This occurs when the external temperature of an enclosure increases and the air inside expands. When the air inside the enclosure expands, the internal pressure increases, pushing out against the enclosure walls. This may stress seals, gaskets, fasteners, or enclosure walls themselves.
Vacuum Formation
This occurs when the external temperature of an enclosure decreases and the air inside contracts, which can cause a vacuum. This can make the case difficult to open, deform flexible surfaces, or pull outside air through unintended leak paths – which will, in turn, bring in moisture and increase the internal humidity. Generally speaking, most enclosures are weaker against an internal vacuum as compared to an over pressure.
In both of the above cases, breather valves are often used to relieve these pressure and vacuum differentials. When the pressure difference reaches the valve’s set point, the valve opens and allows air to move in or out of the enclosure. But when air moves into the enclosure, moisture can come with it.
The Moisture Problem with Ordinary Pressure Relief
While a pressure-relief or breather valve helps a sealed system breathe, it does not control the humidity of the air entering the enclosure. When the contents of the case are sensitive to moisture, a breathing desiccator provides a more complete solution. Accordingly, sealed systems must be designed to protect equipment such as:
- Electronics and circuit assemblies
- Aerospace and defense components
- Optical devices and sensors
- Medical or laboratory equipment
- Industrial controls and instrumentation
- Metal parts, tools, or assemblies susceptible to corrosion
- Long-term storage or transport containers
The mounting issue is that with each inward breathing cycle, more humid air is introduced. If the enclosure later cools, that moisture may condense on internal surfaces. Further, even when condensation is not visible, elevated relative humidity can still create reliability risks over time.
What is a Breathing Desiccator?
A breathing desiccator is a compact, non-powered device that combines a breather valve with a replaceable cartridge filled with desiccant.
More specifically, AGM cartridge-style breathing desiccators attach a replaceable desiccant cartridge to a compatible AGM breather valve. When outside air is drawn into the enclosure through the valve, it immediately flows through the desiccant cartridge. The desiccant removes moisture from the incoming air before it reaches the enclosure interior.
From a design standpoint, the cartridge provides a practical maintenance advantage. Rather than permanently installing desiccant inside the case, the cartridge can be replaced as needed. Additionally, with an optional mounting flange, the desiccant cartridge can be replaced without opening the lid of the enclosure, helping reduce maintenance time and limiting additional moisture exposure during service.
The breather valve controls air movement into and out of the enclosure, while the desiccant cartridge dries incoming air before it enters the enclosed space.
As a system:
- Outside air enters only when pressure conditions require it.
- That air passes through the desiccant cartridge.
- Moisture is adsorbed by the desiccant.
- Drier air enters the sealed enclosure.
This allows the enclosure to equalize pressure while reducing the amount of moisture introduced during inward breathing cycles.
Lastly, applications that need additional protection, an optional RFI/EMI shielded nut assembly is also available for use with the mounting flange.
When to Use a Breathing Desiccator
A breathing desiccator is a strong fit when a sealed enclosure must relieve pressure or vacuum differentials but cannot tolerate uncontrolled humidity ingress. Some such situations may include:
Aerospace and Defense Cases
Equipment cases may experience altitude changes, air shipment, field deployment, and long storage periods. As such, breathing desiccators help manage pressure changes while reducing moisture exposure.
Electronics Enclosures
Circuit boards, sensors, connectors, and electrical assemblies can be sensitive to condensation and elevated humidity.
Industrial Controls and Instrumentation
Outdoor or mobile systems may face repeated temperature cycling, washdowns, humidity, and changing environmental conditions.
Medical, Laboratory, and Precision Equipment
Sensitive instruments may require a drier internal environment during transport or storage.
Long-Term Storage Containers
Cases used for standby equipment, spares, tools, or mission-critical components may need protection from gradual moisture accumulation.
Optical and Sensor Systems
Humidity can contribute to fogging, corrosion, or degraded performance in optical, imaging, and sensing equipment.
Breathing Desiccator vs. Breather Valve
A breather valve opens to equalize pressure between the enclosure and the outside environment in order to prevent damage.
Conversely, a breathing desiccator is a breather valve fitted with a desiccating cartridge. The breather valve will still open to equalize pressure, but incoming air must first filter through the attached desiccator where it will be dried, before filling the enclosure volume.
For situations where humidity is not an issue, a breather valve alone is fine. However, for applications where the enclosure must remain dry, a breathing desiccator adds moisture control to the pressure-management system.
Breathing Desiccator vs. Loose Desiccant
Loose or packaged desiccant can help control moisture already inside an enclosure. However, if the case repeatedly breathes through a valve or leak path, humid air may continue entering the system.
In comparison, a breathing desiccator helps address moisture at the point of entry. Incoming air passes through the desiccant before it reaches the enclosure interior.
In many applications, breathing desiccators and internal desiccant can be used together. The breathing desiccator helps dry incoming air, while internal desiccant helps manage residual moisture inside the case.
Engineering Considerations when Selecting a Breathing Desiccator
Selecting the right breathing desiccator depends on both pressure-management and moisture-control requirements.
Relevant criteria include:
Enclosure Volume
Larger enclosures may exchange more air during temperature or altitude changes, increasing moisture-control requirements.
Expected Temperature Cycling
Frequent or severe day-night temperature swings can cause repeated breathing cycles.
Altitude or Transportation Profile
Air shipment, mountain transport, or rapid elevation changes can create significant pressure differentials.
Required Valve Settings
The breather valve should be selected based on the pressure and vacuum limits of the enclosure, seals, and protected equipment.
Airflow Requirements
Adding a desiccant cartridge affects airflow. Note that the typical airflow rate through a breathing desiccator is approximately 25% to 33% of the flow rate through the valve by itself, so the full assembly should be considered when evaluating pressure-relief performance.
Desiccant Type and Capacity
Cartridge length, desiccant type, and desiccant quantity affect moisture capacity and service interval.
Maintenance Access
Applications that require field replacement may benefit from a cartridge design that can be serviced without opening the enclosure.
Humidity Indication
Some applications benefit from a visual indication that the humidity level inside the enclosure may be too high.
RFI/EMI Protection
Electronic or defense-related applications may require shielded installation hardware.
AGM Cartridge-Style Breathing Desiccators
AGM cartridge-style breathing desiccators are designed to minimize moisture content and relieve pressure buildup inside an enclosure. They combine a replaceable desiccant cartridge with a compatible breather valve, allowing incoming air to be dried before entering the sealed space.
The desiccant cartridge uses porous plastic walls to enable fast moisture adsorption and moderate pressure drop. Standard cartridge lengths are available up to six inches.
Standard desiccant cartridges are available for multiple AGM breather valves, including TA225, TA225-R, TA330, TA333-R, TA340-R, and TA341-R. Specifically, the TA340-R and TA341-R valves are popular options as they include visual humidity indication via an HMI card that may be installed in the top of the valve, which can alert users when humidity inside the enclosure is dangerously high.
Benefits of Cartridge-Style Breathing Desiccators
Pressure Equalization and Moisture Control in One Device
A breathing desiccator helps relieve pressure and vacuum differentials while drying incoming air.
Replaceable Desiccant Cartridge
The cartridge can be easily replaced when the desiccant is spent, supporting ongoing moisture control in long-life systems.
Reduced Moisture Entry During Maintenance
With AGM’s optional mounting flange, the cartridge can be replaced without opening the enclosure lid.
Compact, Non-Powered Operation
No electrical power is required, the breather valve operates mechanically.
Compatible with Multiple AGM Breather Valves
Standard cartridges are available for several AGM valve families, enabling protection across a variety of application requirements.
Optional Humidity Indication
TA340-R and TA341-R valve options include visual humidity indication via humidity indicator cards.
Optional RFI/EMI Protection
Shielded mounting hardware is available for applications that require additional electromagnetic protection.
Talk to AGM About Pressure and Moisture Control
Choosing the right breathing desiccator depends on your enclosure design, operating environment, pressure-relief requirements, and moisture-control goals.
AGM’s Engineering Department can help evaluate valve settings, cartridge selection, and expected moisture ingress during air flight, ground transport, or storage.
Or
