What backing pump does a turbomolecular pump need?

A turbo pump cannot start against atmosphere — it needs a backing pump to bring the foreline into the low mbar range first, and to keep it there while the turbo runs. Sizing the backing pump is about two different requirements, and most mistakes come from only checking one.

Why the turbo cannot do it alone: a turbomolecular pump moves gas by momentum transfer — fast-spinning blades knock molecules toward the exhaust. That only works when molecules travel further between collisions than the blade spacing, i.e. in molecular flow. At atmosphere the gas is viscous and the blades just stir it, so the pump makes no net pumping and heats up instead.

The first requirement is the critical foreline pressure (sometimes called maximum backing pressure) printed on the turbo's data sheet — typically in the low mbar range, lower for wide-blade high-compression models. Exceed it and compression collapses: the turbo keeps spinning, draws current, gets hot, and your chamber pressure goes the wrong way. This is the single most common cause of 'my turbo will not reach ultimate pressure'.

The second requirement is throughput at your working point. Backing-pump speed has to remove the gas load the turbo delivers, at the foreline pressure where you actually operate — not at atmosphere. A pump that is ample during pump-down can be marginal during a process that keeps gassing.

The choice between oil-sealed and dry matters more than people expect. A rotary vane backing pump is cheap and quiet but adds hydrocarbon backstreaming risk, which matters for clean or analytical work; a dry scroll or diaphragm pump removes that risk and costs more per m³/h. Diaphragm pumps only suit small turbos with low gas loads, because their ultimate pressure sits near the turbo's critical backing pressure with little margin left.

Two details that decide reliability: fit the foreline with a valve and vent arrangement so the turbo never sees atmosphere while spinning down, and keep the foreline short and wide — a throttled foreline raises the pressure the turbo sees even when the backing pump itself is adequate.