ConFlat Flange
Two rings of stainless steel with matching knife-edges that bite into a soft copper washer and hold vacuum tighter than any elastomer ever could.
A ConFlat (CF) flange pair provides a detachable, rebakeable, ultra-high-vacuum-tight joint between two sections of a vacuum system. The seal is achieved by a thin, punched ring of oxygen-free high-conductivity (OFHC) copper compressed between matching knife-edges on both flanges. When the bolts are tightened, the knife-edges deform the copper plastically, extrude it into every surface imperfection, and work-harden it into a spring-like state that maintains contact even through repeated thermal cycles between −196 °C and 450 °C. The result is a joint that the vacuum industry has verified can hold leak rates below the detection limit of mass-spectrometer-based helium leak detectors — below 10⁻¹³ mbar·l/s — and that supports base pressures below 10⁻¹³ Torr in a baked-out chamber. Every ultra-high vacuum instrument in the world — surface-science chambers, particle accelerators, gravitational-wave interferometers, electron microscopes, quantum-computing cryostats, and space-simulation chambers — uses CF flanges at most or all of its joints.
Both mating flanges are machined identically ('sexless'), with a circular knife-edge ring recessed slightly below the flat face. The knife-edge geometry is precise: the edge angle and height are set so that when bolts are torqued in a star pattern, the edge penetrates ~0.1–0.2 mm into the copper gasket, causing controlled plastic deformation. The extruded copper fills microscopic machining marks and scratches on both mating surfaces. As the copper work-hardens it retains residual elastic stress — this is what keeps the seal tight through temperature cycling. Metal gaskets, unlike elastomers, have negligible gas permeability and negligible outgassing, which is why CF flanges uniquely support pressures well into the ultra-high vacuum range. The standard gasket material is OFHC copper; silver-plated copper is used for systems requiring frequent high-temperature bakeouts (to prevent copper oxide from flaking into the chamber). For systems requiring baking above 200 °C, stainless steel CF flanges are mandatory — aluminium CF flanges are available but limited to 200 °C bakeout. North-American standard flange sizes run from 1⅓-inch (mini CF, DN16) up to 16½-inch outer diameter; European/Asian sizes follow the DN (nominal diameter) system from DN16 to DN250.
In plain terms
Imagine pressing two cookie-cutters rim-to-rim with a piece of soft Play-Doh in between. The sharp edges of the cutters push into the dough and squeeze out every air gap, making a perfect seal. A ConFlat flange works exactly like that, but the 'cookie-cutters' are precision-machined stainless steel rings and the 'Play-Doh' is a copper ring that has been purified so carefully it contains almost no dissolved gas.
Also called: CF flange, CFF, knife-edge flange, UHV flange, copper-gasket flange.
Why it matters
- Uniquely achieves detachable, rebakeable joints with leak rates below 10⁻¹³ mbar·l/s — impossible with any elastomer O-ring, which permeates gas and cannot withstand bakeout above ~150 °C.
- The bakeout capability is critical: heating a vacuum chamber to 150–450 °C drives adsorbed water and hydrocarbons off the walls, reducing outgassing by orders of magnitude and allowing pressures otherwise unattainable.
- A de-facto global standard: CF flanges from different manufacturers are interchangeable for sizes up to 10-inch outer diameter, making modular vacuum construction as straightforward as plumbing.
- Enables 60+ years of ultra-high vacuum technology — without the CF flange, surface science, LEED, XPS, AES, molecular beam epitaxy, atom traps, and every modern UHV electron microscope would require all-welded (non-disassemblable) chambers.
Where it lives
- CERN LHC beam pipe (all CF-flanged sections, baked to 300 °C in-situ)
- LIGO and Virgo gravitational-wave interferometer beam tubes
- Scanning tunnelling microscopes and surface-science UHV chambers worldwide
- Quantum-computing cryostat wiring flanges (Bluefors, Oxford Instruments)
- IBM and Google quantum processor loading locks
- Molecular beam epitaxy (MBE) growth chambers
- Mass spectrometer differentially pumped source chambers
- Ion trap quantum-computing and atomic-clock vacuum vessels
Key numbers
- Ultimate pressure (baked, clean chamber): <10⁻¹³ Torr (<1.3 × 10⁻¹³ mbar) (Verified by Kurt J. Lesker technical notes)
- Operating pressure range: 760 Torr to <10⁻¹³ Torr (Full range from atmosphere to XHV)
- Temperature range: −196 °C to +450 °C (Stainless-steel flanges with OFHC copper gaskets; aluminium flanges limited to 200 °C bakeout)
- Standard bakeout temperature: 150–250 °C (48 h bakeout at 120 °C typically achieves 10⁻¹⁰ mbar; higher temperatures achieve lower ultimate pressures)
- Gasket material: OFHC copper (single use) (Silver-plated copper for frequent high-temperature bakeouts; aluminium for Al-flange variants)
- Standard sizes (NA): 1⅓" to 16½" OD (Interchangeable across manufacturers up to 10-inch OD; DN16–DN250 in European sizing)
Surprising facts
- A correctly assembled CF flange pair — copper gasket, clean surfaces, proper bolt torque — reliably holds vacuum to below 10⁻¹³ Torr, a pressure at which it would take several months for a single monolayer of gas molecules to land on a surface.
- The copper gasket is used only once: when the flange is opened, the work-hardened copper has deformed permanently and cannot re-seal. A new gasket is required every time the joint is broken.
- ConFlat flanges tolerate a temperature swing of 646 °C — from liquid-nitrogen cooling at −196 °C to bakeout at 450 °C — without leaking, thanks to the work-hardened copper maintaining its elastic preload throughout.
- The ConFlat design, invented in the early 1960s, has not required any fundamental revision in over sixty years; every major manufacturer's 2¾-inch CF flange today is interchangeable with one made in 1970.
A short history
Ultra-high vacuum equipment before the 1960s was built almost entirely from hand-blown borosilicate glass — a medium that allowed excellent vacuums but made access to the interior nearly impossible and precluded metal construction. Metal vacuum systems joined with rubber O-rings could not be baked above about 100–150 °C, limiting ultimate pressures to the high-vacuum range. The ConFlat flange was invented by Bill Wheeler at Varian Associates (Palo Alto, California) in the early 1960s — 'ConFlat' being a contraction of 'conflated flat' (referring to the flat face of the flange) — and was trademarked by Varian Associates. Wheeler's knife-edge copper-gasket design solved both problems simultaneously: it was metal-rebakeable and detachable. Varian Associates commercialised the design, and it rapidly displaced all alternatives for ultra-high vacuum systems. The vacuum equipment industry eventually adopted it as a de-facto standard without a formal standards body requiring it — a sign of how completely it solved the problem. 'ConFlat' remains a registered trademark (subsequently owned by Agilent Technologies after Varian's acquisition), though the generic term 'CF flange' is universal. Kimball Physics introduced Multi-CF hardware in the 1990s extending the system into complex multi-port chambers without fundamentally altering Wheeler's original seal geometry.
Sources
- Kurt J. Lesker Company — ConFlat (CF) UHV Flanges & Components Technical Notes (https://www.lesker.com/newweb/flanges/flanges_technicalnotes_conflat_1.cfm)
- Kimball Physics — MCF Design Theory and Part Numbering System (inventor attribution: Bill Wheeler, Varian Associates) (https://www.kimballphysics.com/resource/mcf-design-theory-and-part-numbering-system-mult-cf-hardware/)
- Ideal Vacuum — ConFlat (CF) Copper Gasket product page (pressure and temperature specifications) (https://www.idealvac.com/en-us/Conflat-Flange-(CF)-Copper-Gaskets-CF-Size-2-34-inch-1-each/pp/P102278)
- Pradeep Research Group (IIT) — ConFlat (CF) Flanges technical overview (seal geometry and operating range) (https://pradeepresearch.org/pdf-files/grouppresentations/instrumental_presentation2013/ConFlat%20(CF)%20Flanges.pdf)