Every vacuum system is only as good as its worst joint. The three dominant flange standards — KF (QF/NW), ISO, and CF — each exist for a reason, and mixing up their use cases costs you either money or base pressure. Here is the practical field guide.
The three standards at a glance
| KF (DN10–50) | ISO-K / ISO-F (DN63–500+) | CF (DN16–250+) | |
|---|---|---|---|
| Seal | Elastomer O-ring on centring ring | Elastomer O-ring on centring ring | Copper gasket, knife-edge |
| Closure | Quick-release clamp | Claw clamps or bolted | Bolted (high force) |
| Vacuum range | Rough to high vacuum | Rough to high vacuum, large bore | UHV — to 10-11 mbar class |
| Bakeable? | Limited by elastomer (~150°C FKM) | Limited by elastomer | Yes — typically to 250–450°C |
| Reusable seal? | Yes, inspect each use | Yes, inspect each use | No — new gasket every closure |
| Assembly speed | Seconds, no tools | Minutes | Slow — torque sequence |
Choosing in practice
- Foreline and roughing plumbing → KF. Fast, cheap, endlessly reconfigurable — the standard for everything between chamber and backing pump.
- Large-bore connections — turbo inlets, chamber ports → ISO. KF geometry tops out around DN50; ISO carries the same elastomer-seal convenience to large diameters.
- UHV, bakeout, or anything below ~10-7 mbar → CF. Elastomers permeate and outgas; the CF knife-edge biting into an OFHC copper gasket is the only one of the three that holds UHV and survives bakeout.
The mistakes that cause 90% of leaks
- Reusing a visibly nicked or flattened O-ring — elastomer seals are cheap; leak-hunting time is not.
- Greasing O-rings heavily. A thin film of the correct vacuum grease at most — excess grease traps gas and collects debris. Many labs run KF joints dry and clean, successfully.
- Reusing a CF copper gasket, or scratching the knife-edge — one careless screwdriver across a knife-edge writes off the flange.
- Uneven CF bolt tightening — always work in a star/cross pattern in stages; a canted flange leaks at one side and deforms the gasket.
- Trapped volumes and virtual leaks: unvented screws and blind holes inside the vacuum outgas for weeks and mimic a real leak on the gauge.
Elastomer choice matters
Standard FKM (Viton-type) suits most lab duty. Move to FFKM for aggressive chemistry, EPDM where FKM is attacked, and consider metal seals or CF where permeation dominates the gas load — an FKM-sealed system’s achievable base pressure is ultimately permeation-limited, no matter how good the pump.
Buying fittings used — what to inspect
Surplus fittings are one of the best-value categories in vacuum — stainless hardware does not age — with two exceptions: inspect every sealing surface (KF/ISO grooves for scratches, CF knife-edges for nicks) and treat included elastomers as suspect; budget fresh O-rings as a matter of course.
PLC North carries surplus KF, ISO, and CF fittings, adaptors, valves, and centring rings — sealing faces photographed up close in every listing, because that is the only part of a fitting that matters. Browse our vacuum fittings and flanges.