The range from roughly 1 mbar down to 10⁻³ mbar, called fine vacuum in much of Europe. The mean free path grows to the same order as pipe and chamber dimensions, so flow is transitional, and heat conduction through the gas depends strongly on pressure, which is exactly the effect a Pirani gauge exploits. Freeze drying, many plasma etch and sputter processes, and vacuum heat treatment run in this band.
Etch runs at 50 millitorr, that's medium vacuum, so the baratron runs the throttle and the ion gauge stays off.
Also heard as
- fine vacuum
Related terms
Rough vacuum
The pressure range from atmosphere down to about 1 mbar, where gas still flows like a fluid.
High vacuum
The range from about 10⁻³ down to 10⁻⁷ mbar, where molecules mostly hit the walls rather than each other.
Transitional flow
Intermediate flow regime between viscous and molecular, where wall and intermolecular collisions both matter.
Pirani gauge
Thermal conductivity gauge that reads pressure from how quickly gas carries heat away from a hot wire.