What leak rate should a vacuum valve have, and when do I need an all-metal one?

Read two numbers, not one: the leak rate across the closed seat, and the leak rate of the valve body to atmosphere. They are different specifications and a valve can be excellent at one and ordinary at the other.

Body leak rate is what limits the ultimate pressure of the system the valve sits in — it is an external leak like any other. Seat leak rate is what limits how well the valve isolates one part of the system from another; a chamber vented to atmosphere behind a closed valve will always push a little gas through the seat. Which number matters depends on what you are using the valve for, and asking for a tight seat when you actually needed a tight body is a common way to overspend.

Elastomer-sealed valves seal well and cost far less, and for anything down to the 10⁻⁶ mbar decade they are usually the right answer. Their limit is not sealing — it is permeation and outgassing. Helium and hydrogen diffuse through elastomers continuously, and any elastomer outgasses water and plasticiser; both set a floor under the pressure you can reach and both get worse the longer the seal has been exposed.

All-metal valves exist for the cases where that floor is too high: ultra-high and extreme-high vacuum work, and any system that must be baked. Bakeout is usually the decisive criterion — elastomers degrade above their rated temperature, so a chamber you intend to bake at 150 °C or more needs metal seals throughout, valves included. All-metal valves cost several times more, have a limited number of full-seal cycles before the seat needs attention, and require a defined closing torque.

A practical rule that saves money: use all-metal only where the pressure or bakeout requirement forces it, and use elastomer-sealed valves everywhere the requirement does not reach. Mixing the two in one system is normal, not a compromise.

When you enquire, the numbers that let us pick correctly the first time are: working pressure range, whether the system is baked and to what temperature, the process gas, the differential pressure across the closed valve and in which direction, and the cycle rate. Send those and the shortlist is usually two or three models rather than a catalogue.