Diaphragm, scroll or rotary vane — which should I choose?

Choose on two questions, in this order: what pressure do you actually need, and can your process tolerate oil? Everything else — price, noise, service interval — follows from those two, and answering them in the wrong order is how people end up with a pump that technically works and practically does not.

Pressure first, because it eliminates options outright. Two-stage diaphragm pumps reach around 10 mbar and four-stage models a few mbar. Dry scroll pumps reach the 10⁻² mbar decade. Two-stage oil-sealed rotary vane pumps reach the 10⁻³ mbar decade. If you need 10⁻³ mbar, a diaphragm pump cannot get you there regardless of how many you buy.

Then oil tolerance. In a diaphragm pump the gas path touches only the diaphragm and the valve plates — which can be PTFE — so there is no oil to contaminate your process and nothing to backstream. That is why they dominate in chemistry, in analytical instruments and as the backing pump for small turbos on clean systems. A dry scroll pump gives you the same oil-free path with a decade more vacuum, at higher cost and with tip seals as a wear item. A rotary vane pump gives you the best ultimate pressure and the lowest cost per m³/h, but it puts an oil film in the gas path.

Service pattern differs more than the spec sheets suggest, and it is what people actually live with. Diaphragms typically last 8,000 to 15,000 running hours on clean duty, shorter with aggressive solvents, and replacement is a straightforward user-level job. Scroll tip seals are a scheduled replacement that produces dust and takes longer. Rotary vane pumps need oil changes on the interval your process dictates, plus vanes and seals at overhaul.

Corrosive vapour is a separate axis, not a footnote: chemistry-resistant diaphragm pumps with PTFE diaphragms, fluoropolymer-coated heads and inlet/outlet condensers exist precisely because standard versions do not survive solvent duty. Specify the version, not just the family — the difference between a standard and a chemistry-resistant model of the same size is the difference between two years and two months.

If you tell us the target pressure, the gas or vapour involved, and whether oil in the gas path is acceptable, we will name the family first and the model second — and say plainly when the cheaper family will not reach your number.