What Are Vacuum Pumps and How Do You Choose the Right Type for Your System?

A vacuum pump is a device that removes gas molecules from a sealed volume to create and maintain a pressure below atmospheric. Choosing the right type comes down to four things: the base pressure you need, the pumping speed required for your gas load, the gases you're handling, and how much contamination or maintenance you can tolerate. Roughing pumps (rotary vane, scroll, diaphragm) cover the range from atmosphere down to roughly 10⁻³ mbar; high-vacuum pumps (turbomolecular, diffusion, cryogenic) take over below that and almost always need a backing pump behind them. This guide explains how each family works and how to match one to your system.

The pressure ranges a pump must cover

Vacuum work is usually divided into regimes, and no single pump spans all of them efficiently:

The practical consequence: high-vacuum pumps cannot exhaust directly to atmosphere. They compress gas from a low inlet pressure to a higher outlet pressure, and that outlet must itself be pumped by a roughing or backing pump. This is why pumps are almost always staged.

The main pump families

Roughing pumps (backing pumps)

These start a system from atmospheric pressure and hold the exhaust side of a high-vacuum pump.

• Rotary vane pumps — oil-sealed, widely used, reach roughly 10⁻³ mbar. Oil lubricates and seals the vanes, but oil can backstream into the chamber if not trapped. Good general-purpose choice for many PVD and coating systems.
• Dry scroll pumps — oil-free, clean, quiet, reach roughly 10⁻² mbar. Preferred where hydrocarbon contamination matters, such as semiconductor or analytical applications.
• Diaphragm pumps — oil-free and compact, typically used as backing for smaller turbopumps or as roughing for clean, low-throughput systems.

High-vacuum pumps

• Turbomolecular pumps — spinning rotor blades impart momentum to gas molecules, reaching 10⁻⁷ mbar and below. Clean, fast to start, but require a backing pump and tolerate only limited particulate or corrosive gas unless specially configured.
• Diffusion pumps — use a boiling oil jet to direct gas downward to a backing pump. Robust and inexpensive for high throughput, but oil backstreaming is a real concern and they need careful warm-up and cooling.
• Cryogenic pumps — capture gas by condensing or adsorbing it on cold surfaces (typically ~10–20 K). Very clean and fast for high and ultra-high vacuum, but they are capture pumps: they must be regenerated periodically and cannot pump certain gases indefinitely.

How pumps are staged in a real system

A typical high-vacuum system works in two stages:

1. Roughing phase — the roughing pump pulls the chamber from atmosphere down to a pressure where the high-vacuum pump can operate (often ~10⁻² to 10⁻³ mbar).
2. High-vacuum phase — a valve isolates the roughing pump from the chamber and connects it as the backing pump for the turbopump, diffusion pump, or cryopump, which then pulls the chamber to base pressure.

The roughing pump therefore does double duty: it evacuates the chamber initially, then continuously removes the gas the high-vacuum pump compresses and expels. If the backing pump is undersized or fails, the high-vacuum pump loses its compression ratio and base pressure suffers.

Selection criteria

Work through these in order — each one narrows the field:

• Target base pressure. If you only need rough vacuum (e.g., for load-lock or backing duty), a rotary vane or dry pump is enough. For high or ultra-high vacuum, you need a high-vacuum pump plus a suitable backing pump.
• Pumping speed at the operating pressure. Pumping speed is not constant across the pressure range; check the manufacturer's speed-vs-pressure curve, not just the peak number. Match speed to your gas load (outgassing, process gas flow, leaks).
• Gas load and gas type. Heavy gases, hydrogen, and corrosive or condensable gases behave differently. Hydrogen is hard for diffusion and cryo pumps; corrosive gases demand corrosion-resistant materials and appropriate purge or trap arrangements.
• Contamination tolerance. If hydrocarbon or oil contamination would ruin your process, choose dry (scroll/diaphragm) roughing and a turbopump rather than oil-sealed rotary vane or diffusion pumps.
• Maintenance and duty cycle. Oil-sealed pumps need oil changes and filter service; dry pumps avoid oil but have tip-seal wear; cryopumps need regeneration. Match the maintenance burden to your uptime requirements.

Common failure points and troubleshooting

• Oil backstreaming — oil vapor migrates from a rotary vane or diffusion pump into the chamber. Symptoms include rising base pressure and contamination. Fixes: use a cold trap or foreline trap, or switch to dry pumps.
• Seal and vane wear — rotary vane pumps lose ultimate pressure as vanes and seals wear. A pump that once reached 10⁻³ mbar but now stalls higher often needs a rebuild.
• Loss of ultimate pressure — usually not the pump alone. Check for leaks, outgassing from chamber materials, a saturated trap, or a backing pump that can no longer reach its required foreline pressure.
• Cryopump saturation — capacity for a given gas is finite; base pressure rises as the cold surfaces load up, signaling time to regenerate.
• Turbopump backing loss — if the backing pump fails or the foreline valve is mis-set, the turbopump can overheat or lose compression. Interlocks and foreline pressure monitoring protect against this.

Where to source pumps and support

Kurt J. Lesker Company supplies vacuum pumps alongside flanges, feedthroughs, valves, traps, fluids, and leak detectors, and offers pump repair and service. If you're specifying a complete system, the pump choice should be made together with the chamber, flanges, and pressure measurement so the stages match. You can browse the vacuum pump portfolio, request a quotation, or place an order through their site; regional sites are available for the US, Europe, UK, India, China, Japan, South Korea, and other regions.