Leakage rates on seal datasheets are often overlooked until a cryogenic vacuum loses hold of its vacuum or a clean-room system fails certification. In fact, the leakage rate of seals is one of the most easily misunderstood seal specifications. This is partly because the units for leakage are not very intuitive and partly because engineers may not realize that zero leakage is a technically meaningless goal.
This blog post examines what the seal leakage rate for metal seals measures, how it is tested, and why correctly interpreting it gives engineers an excellent tool for system design and accurate seal selection.
What Does Leakage Rate Measure?
The leakage rate quantifies the mass flow of gas passing through (or past) a seal per unit time. The units are expressed as a pressure-volume product per second (Pa·m³/s). The reason that
Pa·m³/s, not ml/min or cc/s, is used by engineers; this is simple: including pressure makes it more physically meaningful for compressible fluids (all gases and vapors are considered compressible fluids).
Here is an analogy: 1×10⁻¹⁰ Pa·m³/s is roughly equivalent to one cubic centimeter of gas at atmospheric pressure leaking every ~10,000 seconds, a leakage rate that is essentially imperceptible in most industrial settings.
There is a distinct difference between leak rate and permeation. Permeation is the process of molecular diffusion through the seal material itself, which is only relevant for polymers and elastomers. Metal seals leak through the interface gap, while polymers and elastomers can potentially link through both the gap and by permeation.
How Leakage Rate is Measured
The primary approach to measuring the leakage rate of a metal seal is helium mass spectrometry, in which a mass spectrometer “sniffs” for the presence of helium. Note that helium is used because of its inertness, small atomic size, and high detector sensitivity. In fact, it leaks about 2.7 times faster than nitrogen due to its molecular weight. If a heavier gas is involved, the test results will have to be corrected. Leakage rate tests are also usually performed at ambient temperatures. Still, real-world temperatures may involve expansion and contraction that will impact contract stress and need to be accounted for in the final design.
There are two standard test configurations used: the bombing method and the vacuum method.
In the bombing/pressurization method, the seal assembly is pressurized with helium while the mass spectrometer detector measures (sniffs) the downstream side. The vacuum method is the opposite: the seal assembly is placed under vacuum, helium is applied upstream, and the spectrometer measures the breakthrough.
Several standards apply, including
- ASTM F2132 (metallic gasket testing)
- ISO 3530 / MS-LNCE (aerospace leak test standards)
- SEMI standards for semiconductor process equipment
The Leakage Rate Spectrum
Let’s look at the leakage rates for some of the most common types of seals. As you can see, metal seals outperform in terms of leakage rates.
How does this compare to the thresholds in real-world applications? Here is an approximate visualization assuming well-prepared, precision-machined seals with properly prepared joints.
What Drives Leakage at the Seal Interface?
Five common issues lead to leakage at the interface of a metal seal: contact stress, surface finish, hardness differential, bolt load and seating stress, and thermal cycling.
Contact stress
No machined surface is perfectly flat: microscopically, such surfaces are full of jagged peaks and valleys. A high contact stress physically crushes the seal into these imperfections. Whereas rubber seals just stretch into the gaps, metal seals require a great deal of pressure to permanently yield (i.e., plastically deform) to create a custom-fit, leak-proof barrier.
Surface finish (Ra)
The Roughness Average (Ra) of the flange measures the depth of microscopic scratches on it. If the surface is too rough, the scratches will act as tiny leak tunnels under the seal. Even a highly compressed, coated metal O-ring cannot fill deep canyons, which is why tight leak specifications demand a finely polished finish (usually Ra ≤ 0.8 µm) to minimize gaps.
Hardness differential
The seal must always be softer than the mating flange. These requirements are so that it deforms instead of gouging the expensive hardware. High-performance metal seals often use a stiff, spring-like core (e.g., Inconel) wrapped in a softer, ductile coating (e.g., silver or PTFE) that smears into the harder stainless steel flange without causing damage.
Bolt load and seating stress
In a metal seal, bolts act like stiff springs that clamp the flanges together and crush the seal. If a joint is under-torqued or the load is not evenly distributed, portions of the seal will simply not reach the required pressure to deform properly. This leaves microscopic gaps, making improper bolting a leading cause of field failures for metal seals.
Thermal cycling effects
As temperatures fluctuate, different metals expand and contract at different rates. This causes the bolts to slowly lose their clamping force over time in a process known as stress relaxation. To prevent leaks due to thermal expansion and contraction, engineers use spring-energized seals. These specialized metal seals act like mechanical batteries, automatically expanding outward to maintain unbroken contact even if the flanges slightly pull apart.
Leakage Rate Check List for Metal Seals
Finally, here is a checklist to help engineers as they start specifying a metal seal for a design. For more information on how leakage rates are calculated, check out this article from Pumps & Systems: Calculate the Leakage Rate of a Mechanical Seal.
Conclusion
Remember that 1×10⁻¹⁰ Pa·m³/s isn’t just a number on a datasheet. It represents a specific, measurable physical limit with direct consequences for system performance and safety. And achieving good system performance requires the right seal profile, coating, and groove preparation to work together. To learn more about metal seals for leak-sensitive applications, we encourage you to contact the sealing experts at Advanced EMC.
