A pump connected to the outlet of a high vacuum or booster pump through the foreline, removing the gas those pumps compress. Turbo, diffusion and Roots pumps cannot exhaust to atmosphere, so the backing pump must hold the foreline below their critical backing pressure, from about 0.1 mbar to over 10 mbar for a turbo depending on its drag stage. If the backing pump stops, a diffusion pump's vapour jets collapse and send oil toward the chamber, and a turbo slows under the load and trips.
Foreline's at 2 mbar, backing pump's on its way out, swap it before the turbo starts tripping.
Keep the foreline at least a decade below the turbo's critical backing pressure so a process gas burst does not push it over the limit.
Also heard as
- forepump
- fore pump
Related terms
Foreline
Pipe that connects the exhaust of a high vacuum or booster pump to its backing pump.
Roughing pump
Pump that takes a chamber from atmosphere down to the pressure where the high vacuum pump can take over.
Turbomolecular pump
High vacuum pump whose rotor blades spin at tens of thousands of rpm and bat molecules toward the exhaust.
Diffusion pump
High vacuum pump with no moving parts that drives gas toward the exhaust with downward jets of hot oil vapour.
Throughput
How much gas flows past a point each second, counted as pressure times volume, in units such as mbar·L/s.