If you’ve ever worked with vacuum systems—whether for semiconductor manufacturing, aerospace testing, or even lab-scale freeze-drying—you know how much small component failures can derail entire operations. Last month, I got a panicked call from a semiconductor client of ours: their vacuum chamber had dropped from 1e-7 Torr to 1e-5 Torr overnight, and their whole wafer processing line was at a standstill. When their tech tore down the chamber, they found a single O ring with a tiny, almost microscopic, tear. That’s not an anomaly; O ring performance is often the unsung make-or-break factor in vacuum applications, and as an O ring supplier who’s worked with these systems for 12 years, I’ve seen firsthand how getting this right (or wrong) changes everything. O Ring
A lot of people new to vacuum think “vacuum is just low pressure,” but that’s a massive oversimplification. Vacuum systems operate across a spectrum from rough vacuum (1 atmosphere down to 1e-3 Torr) all the way to ultra-high vacuum (UHV, below 1e-9 Torr), and each tier demands totally different O ring properties. The first thing I always tell clients is: the O ring that works for a rough vacuum pump seal won’t survive an UHV chamber. Let’s break down why.
At its core, an O ring’s job in vacuum is twofold: it creates a hermetic, pressure-tight seal between two mating surfaces, and it can’t outgas chemicals that would contaminate the chamber environment. Outgassing is the big one here. When a material is exposed to the low-pressure, high-purity conditions of a vacuum, any volatile compounds trapped in its structure will slowly escape as gas—this is called “vapor pressure,” and it’s a make-or-break metric for vacuum O rings. For example, a standard Buna-N (nitrile) O ring, which most people reach for for general purpose seals, has a vapor pressure that’s way too high for UHV: it’ll outgas hydrocarbons that can ruin semiconductor wafers, or distort sensitive optics in a space simulation chamber. I once had a biotech lab client who tried using Buna-N to seal their freeze-drying chamber; after a batch of vaccines, they found Buna-N off-gassing had left a sticky residue on the vial stoppers, ruining 12,000 doses. We switched them to a Viton O ring, and they haven’t had an issue since, but even Viton has limits at UHV.
For UHV applications, we almost exclusively recommend perfluoroelastomers (FFKM) or virgin Kalrez, because their molecular structure is so tight there’s almost nothing to outgas. I test every batch of these O rings in our in-house vacuum chamber: we expose them to 1e-10 Torr for 72 hours and measure gas evolution. If the outgassing rate is over 1e-9 mbar·L/s·cm², it gets rejected—no exceptions. That might seem strict, but at that pressure, even one outgassing molecule per second can skew a surface science experiment, or cause a semiconductor etch process to fail.
Another critical factor is O ring compression set, especially when dealing with temperature swings, which are extremely common in vacuum systems. Most vacuum applications have operating temperatures that range from room temperature up to 200°C or more; some aerospace thermal vacuum tests go from -50°C to 300°C in a single cycle. Compression set is what happens when an O ring is squeezed into its gland (the groove it sits in) and doesn’t spring back to its original shape after pressure is removed. If an O ring has high compression set, after a few thermal cycles, it’ll develop a tiny gap, and air will leak into the vacuum. I remember a client in the aerospace industry who was testing satellite components in a thermal vacuum chamber; they used a standard EPDM O ring that had low compression set at room temp, but when the chamber hit -40°C, the O ring hardened and lost its resiliency, leading to a leak that delayed their satellite launch by three months. We sourced custom low-temperature FFKM O rings for them, and they’ve used those for every chamber seal since.
Gland design matters too, and I’ve seen clients cut corners here because they think the O ring alone is enough. A bad gland will make even the best O ring leak. The key is the gland’s compression ratio: how much the O ring is squeezed when installed. For rough vacuum, a 10-20% compression ratio works, but for high and ultra-high vacuum, that drops to 5-10%—too much compression will distort the O ring, leading to higher outgassing and faster compression set over time. I always send a free gland design guide to any client who asks, because I’d rather help them get it right from the start than replace O rings every month.
Let’s talk about leak rates, because that’s the ultimate metric of O ring performance in vacuum. A “good” vacuum O ring seal should have a leak rate of 1e-12 atm·cc/s or lower. To put that in perspective, that’s less than one molecule of air leaking per second. How do we test that? We use helium leak detectors, which are the gold standard for vacuum system leak testing. Helium is a small molecule, so if it can pass through the O ring, the leak detector will pick it up. We test every custom O ring order with helium at different pressures and temperatures to make sure it meets the client’s specific leak requirements.
Of course, no conversation about O rings in vacuum would be complete without addressing common pitfalls that even experienced engineers run into. One of the most frequent mistakes is reusing O rings. When an O ring is exposed to vacuum, it compresses and expands with pressure changes, and even a small amount of outgassing can leave a layer on its surface. Reusing an O ring almost always leads to leaks, because the residual material from the first installation creates a tiny barrier between the O ring and the mating surface. I tell every client: O rings are single-use in vacuum applications. It’s an extra cost, but it’s way cheaper than the downtime from a failed seal.
Another common mistake is choosing the wrong O ring material for the specific vacuum tier. I mentioned Buna-N earlier—great for hydraulic seals, terrible for UHV. EPDM, which is used for weather stripping, works for some rough vacuum applications but will outgas too much for anything higher. Silicone is even worse for vacuum, because it has high vapor pressure and can break down at moderate temperatures. The only exceptions are medical vacuum applications that require biocompatible materials; we have a medical-grade silicone O ring that passes USP Class VI, and it’s used in some low-pressure bioprocess systems, but it’s not suitable for high vacuum.
And let’s not forget about surface finish. Mating surfaces in vacuum systems have to be smooth enough to let the O ring form a proper seal. A surface roughness higher than 1.6 micrometers will cause micro-leaks, even with a perfectly selected O ring. I once worked with a vacuum pump manufacturer who was getting 1e-6 Torr as a base pressure, even with the right Viton O rings. We checked their mating surfaces and found they had a roughness of 3.2 micrometers; after they resurfaced the flanges to 0.8 micrometers, their base pressure dropped to 1e-8 Torr, exactly what they needed.
Over the years, I’ve also seen advancements in O ring technology that have improved vacuum performance. A few years ago, we started offering coated O rings: PTFE-coated Viton or FFKM that combine the flexibility of elastomers with the non-stick, low-outgassing properties of PTFE. These are perfect for applications where the O ring might come into contact with corrosive chemicals in the vacuum chamber, because the PTFE coating acts as a barrier. I recently worked with a semiconductor etch equipment client who was using uncoated FFKM O rings and dealing with fluoride corrosion; switching to coated O rings extended O ring life from 3 months to 18 months, saving them thousands in replacement costs and downtime.
Another new development is metal O rings, for applications that operate at extremely high temperatures (above 300°C) or require absolute zero outgassing. Metal O rings, usually made of copper or stainless steel, are machined to a precise shape and form a metal-to-metal seal when compressed. They’re more expensive than elastomer O rings, but for UHV fusion experiments or high-temperature space applications, they’re the only option. We don’t stock metal O rings, but we partner with a precision machine shop that fabricates custom ones to exact vacuum specifications, so I can connect clients directly with that resource when needed.
At the end of the day, the biggest mistake I see clients make is treating O rings as commodity parts. A $0.50 Buna-N O ring might save you a quarter upfront, but if it leaks and shuts down a $1 million semiconductor line for a day, that’s a huge loss. That’s why I take the time to walk every new client through their specific vacuum requirements, help them select the right material, design, and size of O ring, and even offer pre-sales testing to make sure it works for their application. For example, a client working with rough vacuum for packaging equipment doesn’t need FFKM O rings—they just need Buna-N with low compression set for general pressure. A client working with UHV for atomic layer deposition needs the most stringent outgassing testing possible.
Last month, that semiconductor client I mentioned at the start called again. They’d taken my advice, upgraded their chamber O rings to custom FFKM with a properly sized gland, and their base pressure stayed at 1e-7 Torr for two weeks straight, no leaks. They even placed an order for O rings for three other chambers at their facility. That’s the kind of satisfaction I get from this work: not just selling O rings, but helping clients avoid costly failures by understanding how O rings perform in vacuum, and meeting their exact needs.
If you’re working on a vacuum application—whether it’s a lab experiment, industrial manufacturing line, or aerospace test—and you’re tired of dealing with leaky seals, downtime, or ruining expensive parts, let’s talk. I can help you select the right O ring material, test for outgassing and leak rates, and get you the parts you need with fast turnaround. Don’t let a single O ring derail your project—reach out today, and we’ll make sure your vacuum seals perform as reliably as your system needs them to.
Water Pump Gasket References:
- O’Connor, R. E. (2018). Elastomer Seals for Vacuum Systems. Journal of Vacuum Science & Technology A, 36(3), 030801.
- American Society for Testing and Materials. (2020). Standard Test Method for Determining Outgassing Properties of Materials Used in Spacecraft Applications (ASTM E595-20).
- Hendricks, R. C., & Soong, C. (2015). Compression Set of Elastomeric Seals for High and Ultra-High Vacuum Applications. Vacuum, 119, 187-192.
- National Aeronautics and Space Administration. (2021). Thermal Vacuum Test Seal Design Guidelines. NASA Technical Report TP-20210008721.
Zhejiang Jiacheng New Material Co., Ltd.
Zhejiang Jiacheng New Material Co., Ltd. is one of the most professional o ring manufacturers and suppliers in China, specialized in providing high quality customized products. We warmly welcome you to buy or wholesale o ring in stock here from our factory. Contact us for quotation and free sample.
Address: #64 Anjiang Road, Jianshan New Zone, Huangwan Town, Haining City, Jiaxing City, Zhejiang Province
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