One thousandth of a millimetre of mercury, the unit used to measure deep vacuum.
A micron is 0.001 mm of mercury of absolute pressure. Atmospheric pressure is about 760,000 microns. Evacuation targets sit in the hundreds of microns, a range a manifold's compound gauge cannot resolve, so a dedicated micron gauge is used.
Read the micron gauge at the system rather than at the pump, and follow the equipment manufacturer's target. 500 microns is a common target. What the reading does after the pump is isolated matters as much as the lowest number reached: see vacuum decay test.
Exam work areas
B · Tools, piping and refrigerant handling