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Breather Valves

Control the development of pressure differentials in sealed enclosures and containers.

Breather valves, sometimes referred to as pressure relief valves, come in a variety of configurations and are used extensively across a wide swath of industries to serve a central purpose: the protection of a container or enclosure through pressure equalization.

Low-pressure, high-flow breather valves automatically adjust a container’s pressure with respect to the outside environmental pressure changes and thus prevent excessive pressure differentials during air or high-altitude truck or rail transport (Fig. 1). As a result, the use of a breather valve can save on the initial cost of building the container, as well as throughout the container’s life by reducing the cost of transport and storage.
Illustrated graph of pressure change with altitude increase from seal level.
Fig. 1
AGM breather valves are used primarily in the aerospace & defense, space, and logistics industries – although they are applicable for use in other sectors.

For more information about AGM breather valve uses, contact AGM Engineering.

As such, AGM breather valves protect containers and enclosures against over pressure and vacuum in a variety of circumstances, including flight (airlift), wide temperature swings, and liquid water. This is useful for aerial cameras, electrical boxes, electric vehicle battery packs, launch payloads, and much more. Below, we’ll discuss details of how breather valves are used and operate in these and similar industries.

Why use breather valves?

Relieving Pressure Differentials

Two main reasons:
1) To relieve pressure differentials
2) To protect against moisture attempting to enter an enclosure
The difference in air pressure between the inside of a sealed space and that of the outside surrounding atmosphere is known as a pressure differential.

When a pressure differential is zero or within the tolerance of an enclosure, such as between the inside of a vehicle and the outside air, things operate as normal. However, when an object is sealed tightly at one air pressure and is then moved to an area of lower air pressure, the air inside attempts to equalize with that of the outside and pushes against the walls of the container. For example, if a thin plastic bottle, like a water bottle, is sealed at sea level and then driven to the top of a mountain, the bottle “swells.”

If great enough, this force can cause the walls of the sealed enclosure to deform, rupture, tear, or even pop (explode). Such possibilities are very real in cases of airlift, where pressure differentials can generate rapidly and with significant magnitude. In such an event, it is very important to relieve the differential in time to prevent damage to an enclosure and thus protect the contents within.

Read our blog for a flow-rate comparison between spring and magnet breather valves.
How Breather Valves Regulate Pressure Differentials

Protecting Against Moisture

Why seal a container or enclosure in the first place? In the aerospace & defense industry, it’s often done to protect content from moisture, though there are many reasons. Nevertheless, breather valves must provide protection against liquid water entering an enclosure when they open to relieve pressure.

However, air carries with it some percentage of water vapor depending on the humidity of the area at that time. When a vacuum is generated in an enclosure and a breather valve opens (“inhales”) to equalize the pressure, that water vapor will enter the enclosure with the incoming air. Eventually, the inside air will hold enough moisture to damage contents. Depending on the conditions, the moisture could condense out of the air and form liquid water inside of the container. This is known to happen frequently in sea transport and causes a phenomenon known as container rain, which is responsible for approximately 10% of all lost goods during sea transport each year.

However, a breather valve is not enough on its own to fully protect against incoming water vapor, so desiccants are used.

Desiccants, which adsorb moisture, are placed either directly into the enclosure itself or inside a cartridge attached to the back of a breather valve so that incoming air must flow through the desiccant before contacting items in the enclosure. These desiccant-cartridge plus breather valve assemblies are known as breathing desiccators or just desiccators and are one type of breather valve.

Breather Valves: A Brief History Lesson

In essence, to protect against moisture, shipping and/or storage containers must be tightly sealed and desiccated. However, such containers may be exposed to pressure and vacuum differentials of as much as 5.7 psid (pounds per square inch differential) due to temperature and/or altitude changes. To resist pressures of this magnitude, a container must be constructed from a very strong material, making it bulky, heavy and costly to store and ship.

At one time, “free-breathing” containers were considered an alternative solution to this problem. The theory, based on Ficke’s Law, was that moisture would not pass freely through tubes with lengths ten or more times their diameters. However, this principle applies only when there is no pressure differential between the two ends of the tube. The method was found to be unsatisfactory in actual practice. In fact, in tests conducted by the U.S. Army Tank Automotive Command, the average water gain in three free-breathing containers was over six times greater than a controlled breathing container with an installed breather valve, which sealed at 0.5 psid pressure and 1.0 psid vacuum (see the McDermit Report).

How Breather Valves Work

Different types of breather valves function differently but at AGM, we offer breather valves in two main functionalities:

Spring Valves

A more traditional style of breather valve, spring actuated breather valves are ubiquitous across many industries.

An internal spring holds a poppet and seal in the closed position. The valve is set for some cracking and reseal pressures as necessary and when the pressure exceeds the spring force, the valve cracks and opens. Once pressure decreases to a safe level, the spring again pushes the valve closed.

Magnet Valves

A magnet valve is AGM’s own creation and leverages the power of magnets to instantly provide full-flow capability.

Because the force between two magnets decreases as a square of the distance between them, AGM magnet valves may be set in such a way as to provide full-flow once a small cracking pressure is achieved. This situation is ideal for situations when a large volume enclosure requires protection or if rapid pressure-change rates are expected.

Breather Valve Settings and Operational Characteristics

When looking for a breather valve, it’s important to have a full understanding of how a breather valve is set and what factors affect its functionality and performance. Below are five points to understand about breather valves:

1. Pressure and Vacuum Settings

Breather valves are made in a variety of pressure settings, ranging from 0.2 psid to 5.0 psid or more. These settings, which represent the differential values at which the valves seal, must be at least 1.0 psi to 1.5 psi below the pressure or vacuum which a given container can safely withstand without leaking or deforming (see Selection below).

Generally speaking, the lower the valve’s setting, the more often the valve will open, admitting outside atmosphere. Outside atmosphere will hold water vapor. If the contents of the container are sensitive to moisture, then it is likely the container also holds desiccant and the more often a breather opens and takes in atmosphere, the shorter the life of the desiccant. For this reason, it’s important to choose breather valves that feature optimal cracking and reseal pressure settings to get the most out of the desiccant and thus ensure a project’s longevity.
Understanding Breather Valve Pressure Settings

Breather valves are rated for two different pressures: Cracking pressure and full-flow pressure. The difference between these two measurements is sometimes a source of misunderstanding.
Cracking Pressure
Cracking pressure or opening pressure is, of course, the pressure setting at which a breather valve will begin to “crack”, or open. As defined in the SAE AS27166, the cracking pressure for a breather valve is the point at which the valve enables 1 cubic centimeter per minute of air flow – which is a very low flow rate.

Sometimes we are asked to supply a valve that will crack at a specified pressure, within a tolerance. While this requirement can be met, it involves additional testing and, therefore, increased cost, and – except under extraordinary circumstances – would appear to be unnecessary, since the settings for seal and design flow provide the desired protection for both container and contents. In addition, SAE Specification AS27166 sets maximum cracking pressure offsets from the reseal pressure.

Often, an engineer will request a valve with cracking pressure equivalent to that of the maximum pressure rating for their product or container. However, such a request demonstrates misunderstanding of the difference between cracking and full-flow pressure.
Full-Flow Pressure
Full-flow pressure is defined as the pressure at which a breather valve is fully open. At this pressure, the breather valve must meet or exceed SAE AS27166, which recommends 12% of an enclosure’s air volume per minute before reaching that container’s pressure or vacuum rating.

For example, a container with 8 cubic feet of air volume and a 2-psi pressure rating will require a breather valve that can flow at least 1 cubic foot per minute of air by 2 psi. Such a valve would typically crack at approximately 0.5 psi.

A word of caution: It may seem tempting to specify a breather valve with very low-pressure ratings in an effort to ensure pressure safety. However, doing so will lead to the selection of valves that open more frequently, which in turn will allow more moisture into the enclosure.

2. Temperature Variations

The number of times a breather valve will open during storage depends not only on the valve setting, but also on the magnitude and frequency of temperature variations which may occur in a particular storage area. In sealed containers there is a pressure change ranging from 1.0 to 1.5 psi for each 30°F temperature change (Fig. 2).

Long-term tests, which have been run on containers at AGM’s plant in Tucson, Arizona, indicate that valves with sealing pressures of 0.25 psid will open almost every day, while valves set to reseal at 0.5 psid may open up to 150 times a year, and valves set for 1.0 psid rarely open during storage. (It should be noted that these tests were run on rigid wall containers, and that low-setting valves on plastic containers with flexible walls will probably not open as often under the same conditions.)

There are only a few locations in the world other than Tucson where greater diurnal temperature variations occur. Therefore, under worldwide storage conditions, valves with a 0.5 psid reseal in both directions will open no more than 200 times a year, and valves set for a 1.0 psid reseal in both directions will probably open less than a dozen times.
Graphic of pressure change as a function of temperature change
Fig. 2

3. Temperature and Humidity

In addition to the number of times a breather valve opens, the amount of moisture taken into the container at each opening (or “gulp”) will in part determine desiccant life, and this is dependent on the climatic conditions of the storage area.

There are places in the world where as much as 0.015 grams of water per container cubic foot could be taken in at each “gulp.” (Reference NavWeps Report 8374, Table XII). However, high humidity tends to limit temperature variations (Fig. 3), so that even breather valves with very low settings will probably not open more than 2 or 3 times per year in these locations.
Illustrated graph of daily temperature variations as they diverge from a median as a function of relative humidity
Fig. 3

4. The Number of Airlifts

In aerospace or air transport, for each descent from 10.7 psia (normal pressurization level in an aircraft cargo compartment) to 14.7 psia (sea level), a breather valve set for 0.5 psid reseal in both directions will take in approximately 0.055 grams of water per cubic foot of container volume. Higher or lower valve settings will not substantially vary the amount of moisture gain per descent. Therefore, the amount of desiccant needed will, in part, depend on the number of airlifts anticipated.

5. The Amount of Desiccant

It has been noted above that in ground storage, each time a container must breathe it will take in as much as 0.015 grams of water per cubic foot, and during each air descent in a pressurized cargo compartment it will take in as much as 0.055 grams of water per cubic foot. Since MIL-STD-2073-I requires 1.2 units of desiccant per cubic foot in a sealed, rigid metal container (plus additional amounts for dunnage, if any) and one unit of desiccant will hold 6.0 grams of water at 40% relative humidity (RH) at 77°F, this amount of desiccant will protect the container for a total of 480 “gulps” in ground storage, or a total of 550 airlifts, or some combination of the two.

Keeping the above factors in mind, we see that a Breather Valve, properly selected and used in conjunction with adequate desiccant, can provide years of moisture protection in a lightweight, low cost container.


Before making a definitive choice, you’ll need to know:
  • How much pressure or vacuum your enclosure or container can withstand
  • The internal air volume of your container or enclosure
  • The expected rate of change of pressure
  • The tolerance of the contents for moisture and particulate
To choose an appropriate breather valve, it’s important to understand that, fundamentally, a breather valve must perform two functions: Protect a container from excessive pressure or vacuum differentials, while limiting the amount of moisture that enters the container. Therefore, an ideal valve should remain sealed except during altitude change or under extreme temperature changes, but when open should provide sufficient flow to relieve air pressure as fast as it builds up.

As has been described above, valves set as low as +0.5 (pressure) and -0.5 (vacuum) psid will protect against excessive moisture intrusion for years. Therefore, in order to select a valve which will adequately protect the container against excessive pressure or vacuum, we must know the following:


This figure (Fig. 4) will establish the pressure at which the valve must achieve its rated flow, which is measured at 1.5 psi above the reseal setting. If possible, a safety factor should be utilized by setting the required flow pressure slightly below the container’s deformation point.
IMPORTANT: Most containers can withstand more pressure than vacuum. For instance, the container pictured in Fig. 4 is normally pressurized to 5 psi and can probably withstand an internal pressure of up to 50 psi without deformation. However, it took less than 3 psi of vacuum, resulting from a temperature drop of 90°F during surface transport of this empty unpressurized container to cause the deformation shown.

For this reason, a pressure setting somewhat higher than the vacuum setting is often specified to provide a sufficient differential without overstressing the container. However, too great a differential (more than 3 psi) between the pressure and vacuum settings can cause valve design problems and increased cost. If a differential greater than 3 psi is required, multiple one-way valves should be used. It should be remembered that a total differential in sealing pressures of 1 to 2 psi will provide more than adequate moisture protection worldwide.

What is the Internal Air Volume of the Container

It is essential to know how much air, in cubic feet, will be inside the container. This may be calculated by subtracting the volume of the contents (engine, missile, etc.) from the inside volume of the container.

What is the Rate of Pressure Change

The highest rate of pressure change will usually occur during the depressurizing and repressurizing of an aircraft’s cargo compartment during air transport. According to the International Air Transport Association (IATA) Standard Specification 80/2, “Pressure Equalization Requirements for Aircraft and Shipping Containers (Par. 3.2),” the cargo compartment pressure decreases from standard sea level (14.7 lbs./in.²) to minimum cruise altitude equivalent of 8,500 feet (10.7 lbs./in²) at a maximum climb rate of 2,500 feet per minute and increases back to sea level at a maximum descent rate of 1,500 feet per minute. IATA also specifies (Par. 6.2.3) that Breather Valves shall ensure a minimum air flow of 12% per minute of the internal container volume. Society of Automotive Engineers (SAE) Specification AS27166 (which replaces cancelled military specification MIL-V-27166) “Valve, Pressure Equalizing, Gaseous Products,” also specifies 12% as a minimum flow rate, but subtracts the volume of the material in the container, resulting in the following formula (Par.

Minimum Flow Rate (ft.³/min.) = (Vc-Vm)*0.12″
Where Vc = Volume of container (ft.³)
and Vm = Volume (min.) of material in container (ft.³)

It should be emphasized that this is a maximum, and would tend to be reduced by other factors, such as temperature change and elasticity of the container, so no additional safety factors need to be added. However, where there is a possibility that an empty container could be transported by air, it might be wise to disregard the displacement of the contents and use the internal volume of the empty container as the basis for calculating flow requirements.

Particulate Protection: Sand and Dust Ingress

SAE Specification AS27166 requires that breather valves must still reseal after being tested for sand and dust protection per MIL-STD-810. The sand used in the test is similar to talcum powder, too fine to affect the sealing surfaces of the valve. However, the specification does not require the valve to keep sand and dust out of the container if it opens during a dust storm. This technicality has made it possible for valves with stamped or wire mesh screens to certify as meeting the specification.

For total sand and dust protection, you need to specify AGM Breather Valve series TA330, TA333-R or TA770-R, as these valves have dust baffles and covers that will protect the contents of the container from not only sand and dust, but also wind-driven rain and water from high-pressure decontamination hoses.

Which Breather VAlve is Right?

Now that you have established the flow rate and the reseal pressure (which is 1.0 to 1.5 psi below the flow rate pressure), you may proceed to actual selection of the breather valve you will need.

The Breather Valve Selection Chart (Fig. 5) will give you the flow rates of the various Breather Valves listed on this website, and indicate maximum net volumes of containers they may be used on.
A chart listing AGM's breather valves along with each valve's flow rate and max net container volume
Fig. 5
Each Breather Valve data sheet includes a Part Number Designation Chart which shows how to designate the desired resealing pressures in the part number of the valve. Dimensions, performance characteristics and optional features, such as a manual release button (essential for breaking a vacuum seal) and RFI/EMI shielding, are also indicated.

Specifications, Requirements, and Standards

SAE Specification AS27166, entitled “Valve: Pressure Equalizing, Gaseous Products,” details environmental requirements and the procedures for vibration, temperature, salt, fog, sand and dust, rough handling, etc., as well as settings and flow. (Note: Many design activities have found it impractical to use the settings designated in this specification and have called out other settings more suitable for their particular container design.) AGM Breather Valve Series TA238, TA240-R, TA330, TA333-R and TA770-R will meet all requirements of this specification (see Environmental Protection, above). AS27166 is also referenced in Department of Defense MIL-STD-648C.

In addition, AGM’s Breather Valves are specified on more than 300 Army, Navy and Air Force drawings.

If you require identification of AGM Breather Valves with a part number other than one shown on this website, please contact AGM for part number verification. You should also contact AGM if you plan to put one of AGM’s catalog numbers on a document requiring MIL-STD-130 identification.

If you have special requirements which cannot be met by using a standard valve, please contact AGM’s design engineering team regarding possible modifications or a special valve design to meet your needs.

Mounting (Installing)

Unless otherwise specified, all valves are supplied with a nut, washer and gasket for mounting through a hole on a flat, smooth surface. Valve gaskets may not provide a proper seal if the mounting surface is curved or rough. Valves can also be installed in a mounting flange or a threaded boss when a suitable counter bore has been provided for thread relief.

IMPORTANT: Every AGM Breather Valve is individually tested and certified for compliance to performance requirements.

Breather Valve Applications

AGM breather valves are used widely throughout many industries and to protect many different products. For example, AGM is proud to be a supplier to the electric vehicle industry to protect EV battery packs against pressure, vacuum, and liquid water. Additionally, AGM supplies breather valves to the aerial surveillance and remote sensing industry to protect specialized and sensitive aerial optics systems.

There are hundreds of other examples. While some can be discussed in detail, others must remain obscure for security purposes. However, shown below are just a few of the many containers currently in use that incorporate AGM Breather Valves, as well as other AGM products (Humidity Indicators, Records Holders, Desiccators and Tie Downs).

AGM Breather Valves have permitted many new and radical container designs using a variety of materials. Substantial weight and cube reductions have been achieved, resulting in impressive savings in fabrication and shipping costs.


Uses TA333-R Valve; also Humidity Indicator.
(Photo courtesy of Hardigg Industries Inc.)


Uses TA333-R Valve.
(Photo courtesy of U.S. Army)


Uses TA330 Valve; also Desiccant Port and Humidity Indicator.
(Photo courtesy of U.S. Army)


Uses TA333-R Valve.
(Photo courtesy of Zero Corporation)


Uses TA770-R Valves; Also Desiccant Port, Records Holder and Humidity Indicator.
(Photo courtesy of Plastics Research Corporation)


Uses TA440-R Valve; also Desiccant Port, Records Holder and Tie Down Straps.
(Photo courtesy of Raytheon and U.S. Navy)



Yes, but valves with a manual release button can also be opened manually to equalize pressure.


No, not unless you push the manual release button (only on those valves that have a manual release button). In automatic mode, they will equalize only to the level of the reseal pressure. That is, a TA333-05-05-R breather valve will only equalize to within 0.5 psid because below that the valve will be closed.


It depends. There are three common ways that liquid water can enter through a breather valve: submersion in water, directed streams of water (washdowns), and wind-driven rain.

For submersion, the valve will not open if the reseal pressure of the valve exceeds the hydrostatic pressure of the water plus any vacuum that has built up inside the container. For instance, at a water depth of one meter, the hydrostatic pressure is approximately 1.4 psi. Therefore, a valve with a reseal pressure of at least 1.5 psid will prevent water intrusion at a depth of one meter, subject to the following qualification. If the container has developed a vacuum due to a temperature change, or due to a pressure change from land or air transport, the valve could still inhale water. For example, a container with a vacuum of 0.5 psi that is submerged in water until its breather valve is one meter deep will need a valve with a reseal pressure of at least 2.0 psid, because it needs to withstand 0.5 psi vacuum plus 1.4 psi hydrostatic pressure.

Note that submerging a warm container in cold water will develop a vacuum inside the container. If your container is subject to this condition, contact an AGM engineer for assistance in calculating its effect on your container.

For a valve subjected to a washdown, the breather valve’s cover configuration and vacuum setting determines whether it is susceptible to water ingress. An AGM breather valve with a solid cover and a nominal cracking pressure in the vacuum direction (i.e., the inward flow direction) of at least 1.0 psid will repel water entry from a directed stream of water. AGM valves with a solid cover include the following: TA333-R, TA330, TA770-R, and TA225. On the other hand, valves without a solid cover, such as TA238-R, TA240-R, TA292-R, and TA294-R valves, as well as competitors’ valves with only a screen over the valve opening, could allow water from a directed stream of water to enter during a container washdown.

Finally, for the condition of wind-driven rain, an AGM breather valve with a solid cover will repel wind-driven rain even when the valve is inhaling. AGM’s unique cover design forces the airstream through two 180° turns, thereby causing most water droplets to fall out of the air stream. By contrast, a breather valve without a solid cover will repel wind-driven rain only when the valve is closed. This type of valve could allow rain to enter during the brief instances when the valve is inhaling.

Note that the passage of liquid water through a valve is only one of the two ways that moisture commonly enters a container. The more common way for moisture to enter a container is in the form of water vapor, which is entrained in the air that the valve inhales, and also permeates through a container’s seals. Contact an AGM engineer for assistance in determining how to protect your container’s contents against this type of water ingress.


No. AGM breather valves are small mechanical systems where a spring pushes a seal onto a seat. Due to the surface energy forces of the elastomeric seals, it will always require slightly more force to open the valve than to close it. Therefore, the opening pressure will be slightly higher than the reseal pressure. Also, the higher the setting of the valve, the greater the difference between the opening and closing pressures.


We have made valves with reseal pressures as high as 10 psid. However, these valves are difficult to work with and the settings may tend to shift after long periods of time and large temperature changes, but they will continue to perform their basic function.


May 11, 2023
Breather Valves and Pressure Differentials in the Aerospace Industry

Choosing the right breather valve to protect your project is no simple task. When it comes to pressure differentials and their interaction with sealed spaces during flight, aerospace engineers must take into account several aspects of pressure and environment to ensure the safety and reliability of their aircraft, spacecraft, and equipment. What are pressure differentials? […]

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July 8, 2021
Understanding Breather Valve Pressure Ratings

Breather valves are rated for two different pressures: Cracking pressure and full-flow pressure. The difference between these two measurements is another significant source of misunderstanding on part of many design engineers.

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September 24, 2020
Moisture Diffusion, Condensation, and the Trouble with Airflow in Sealed Enclosures

After moisture or water vapor enters a sealed enclosure, it diffuses through the air inside, where it can later condense during temperature changes. Condensation is a common obstacle in designing sealed enclosures intended to house electronics or other moisture sensitive equipment. To make things worse, moisture will enter a sealed enclosure in four different ways.

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AGM Container Controls, Inc.
3526 E. Fort Lowell Rd.
Tucson, AZ 85716

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