What is a Leak?
A leak is the unintended or uncontrolled movement of substances—such as liquids or gases—through a defect or inadequacy in a containment barrier. Leaks occur due to factors like material failure, manufacturing imperfections, joint or seal degradation, overpressure, corrosion, erosion, stress, or quality defects. They can manifest in a variety of environments and systems, from industrial machinery and pipelines to consumer products and medical devices. The consequences of leaks can range from minor inconveniences to significant safety hazards or financial losses, depending on the application and severity.
Leaks are characterized by their leak rate (flow rate), the properties of the escaping substance, and environmental dynamics, such as pressure differentials and temperature gradients. These parameters help in assessing the impact of a leak and determining appropriate measures for detection and containment. Fundamentally, a leak represents a failure of a barrier to maintain containment, creating an unintended path for substances to escape. It is inherently a quality defect that undermines the integrity and performance of a system.
In its most fundamental terms, a leak is when:
1. Something that was supposed to stay inside does not stay inside
2. Something that was supposed to stay outside, does not stay outside
3. Something that was supposed to act as a barrier does not adequately perform as a barrier
Types of Leaks
Leaks can be categorized into three primary types, each with distinct characteristics and implications for system performance. Keep in mind that a porosity leak could also be described as a combination of bunch of smaller pinhole and stringer leaks. For simplicity reason, we have divided these leak types into it’s most basic distinctions which are: pinhole, porosity, and stringer leaks.
Pinhole Leaks
A direct and localized path connecting the inside and outside of the barrier. These leaks often result from small defects such as tiny holes or cracks in the material. The leak rate decreases with increasing barrier thickness, as escaping molecules adhere to the walls, reducing flow. The interaction of molecules with the surface creates resistance, slowing the rate of leakage through narrow pathways. Pinhole leaks are common in systems where manufacturing defects or material degradation occur, and their detection often requires precise and sensitive methods.
Porosity Leaks
Result from small, interconnected voids or pores in the material that allow fluids or gases to pass through. Think of a sponge or net or a fence where there is a barrier but smaller items can pass through. These voids may arise during the manufacturing process or from inherent material properties. Porosity leaks are particularly significant in applications involving porous materials, such as ceramics, polymers, or sintered metals. Their impact depends on the size, distribution, and connectivity of the pores. Effective mitigation often requires material selection and quality control during production to minimize the risk of porosity-related leaks.
Stringer Leaks (Complex Leaks)
Characterized by a non-linear, twisted path through the barrier. Think of it as a cave with its random and interesting intricacies but on a much smaller scale. Unlike pinhole leaks, these paths are complex and meandering, making them harder to detect and quantify. Leak rate decreases due to the increased surface interaction of escaping molecules along the complex path. This resistance to flow can make stringer leaks less apparent in short-term testing. Stringer leaks often result from manufacturing defects, such as improperly fused seams or delamination, and require advanced testing methods to identify and address.
Non-Quality Defects and Virtual Leaks
In addition to leaks caused by quality defects, some phenomena can mimic leaks due to system design or material properties. These non-quality defect leaks can be challenging to distinguish from actual leaks and often require specialized techniques, knowledge, and experience to recognize and address.
Virtual Leaks
Air trapped within components that escapes slowly, creating the illusion of a leak. These are common in systems with complex geometries or insufficient evacuation during manufacturing or assembly. Virtual leaks do not represent a physical breach but can still affect performance. An example of a virtual leak would be having a bolt inside of a vacuum chamber and when pulling a high vacuum, the air that is trapped between the bolt and vacuum chamber will be released into vacuum chamber making it look like there is a leak.
Outgassing
Molecules released as gas or vapor from a material’s surface or structure. Outgassing can occur in various materials, particularly under vacuum or elevated temperatures. Managing outgassing often involves pre-conditioning materials or implementing coatings to minimize release.
Degassing
Dissolved or trapped gases released from liquids, solids, or mixtures due to changes in temperature or pressure. Degassing is frequently encountered in processes involving rapid pressure changes. An example of outgassing or degassing would be a material or liquid that is releasing molecules into the vacuum chamber from inside the chamber causing an increase in absolute pressure making it look like a leak.
Slow Permeation
A gradual escape of substances through minor defects over long periods. While not a quality defect, it is influenced by system design and material selection. Slow permeation is often acceptable in applications where long-term containment or high vacuum applications are not critical. An example of slow permeation would be acrylic chamber walls which slowly pulls air and water molecules from the outside towards the inside of the chamber through its walls.
Leak Rate Definition
Everything leaks, as far as we know there is no such thing as a perfect leak tightness. Below is a table defining various leak rates and correlating these to the hole size at 1cm of material thickness. The table below has been derived from various sources, modelling calculation, and industry consensus. When designing a part to have specific leak tightness, you can always be assured that if your application is leak tight at a lower leak rate, it will be leak tight at a higher leak rate. For example, something that is bacteria tight is proven to also be water and oil tight.
Table 1. Leak Rate Definition
Leak Testing Methods use for Various Leaks
Various leak testing methods offer differing levels of sensitivity. Understanding the strengths and limitations of each method is critical for selecting the most appropriate approach for a given application. The table below summarizes their capabilities. Keep in mind that this is not to go into too much detail, we have another write up which talks about each testing method, this table is merely meant to give you a general idea of corresponding leak testing sensitivity ranges.
Vacuum Bubble Leak and Pressure Decay tests have sensitivities of 0.1 sccm, making them suitable for detecting moderate leaks in simple systems such as medical, food, and pharmaceutical packaging. Vacuum Decay tests improve sensitivity by 100x to 0.001 sccm, offering a more precise option for tighter systems. Helium-based methods are significantly more sensitive, with Helium Vacuum Accumulation achieving up to 10⁻¹² sccm sensitivity, making it a billion times more precise than Pressure Decay testing. These methods are ideal for applications requiring the highest levels of containment integrity.
