FKM, EPDM or copper — which vacuum seal material should I specify?

FKM (Viton) is the default and covers most industrial vacuum work. Move off it for a specific reason: EPDM when your process fluid attacks FKM, and metal when pressure or bakeout puts elastomers out of range. Specifying by habit rather than by process is how seals fail early and mysteriously.

FKM earns the default position because it combines low outgassing, wide chemical resistance and usable temperature range at a reasonable price. Its known weaknesses are specific rather than general: it swells in some polar solvents such as ketones and esters, and it is attacked by hot amines and by some steam service. If your process includes those, the seal will fail regardless of how well the joint is made.

EPDM is the answer for exactly the cases FKM is weak in — steam, hot water, ketones, and many polar solvents — and it is worse than FKM almost everywhere else, notably in hydrocarbons and mineral oil, where it swells badly. It is a targeted substitution, never a general upgrade.

Copper (CF) removes the question entirely by removing the elastomer. Use it when you need to bake, or when permeation and elastomer outgassing set a floor above the pressure you must reach. The practical constraint is that a copper gasket is single-use: every time the joint is opened, a new gasket goes in. Budget for that in maintenance-heavy systems rather than discovering it during a shutdown.

Two things that matter as much as the material and get specified less often: the O-ring's condition and its history. An O-ring that has been compressed in one joint has taken a set; reusing it in a different joint is a leak waiting to appear. And an elastomer that has been exposed to helium during a leak test will bleed helium back out for a long time — which is a real cause of a leak detector background that will not come down.

Tell us the process fluid, the temperature range including any bakeout, and whether the joint is opened routinely, and we will name the material and the standard together — they are one decision, not two.