Breather valves are rated for two different pressures: Cracking pressure and full-flow pressure. However, many design engineers often misunderstand the difference between these two measurements.
Cracking pressure, or opening pressure, is the pressure setting at which a breather valve will begin to “crack”, or open. As defined in the SAE AS27166 Aerospace Standard, the cracking pressure for a breather valve is the point at which the valve enables 1 cubic centimeter per minute of air flow.
Engineers often request valves with cracking pressures equivalent to that of the maximum pressure ratings for their products or containers. However, such requests are representative of a general misunderstanding between cracking and full-flow pressures.
Full-flow pressure is defined as the pressure at which a breather valve is fully open. At this pressure, the valve must meet or exceed SAE AS27166 Aerospace Standard, 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 valve with lower pressure ratings to ensure safety. However, these ratings will see that the valve opens more frequently, which in turn will allow more moisture into the enclosure.
AGM Container Controls, Inc.
3526 E. Fort Lowell Rd.
Tucson, AZ 85716