Vacuum units: in very low-pressure work, 1 micron (μ) is defined as how many millimetres of mercury (mm Hg) absolute at 0 °C?

Introduction / Context:
Vacuum measurements span many orders of magnitude. Technicians and engineers frequently convert between torr (mm Hg), microns (micrometres of mercury), pascals, and psi(absolute). Understanding the unit definitions prevents large errors in vacuum system design and calibration.

Given Data / Assumptions:

• Definition sought: 1 micron (μ) for pressure.
• Reference temperature: 0 °C (traditional for mm Hg definition).

Concept / Approach:
By definition in vacuum practice: 1 torr = 1 mm Hg. A micron (also called micrometre of mercury) is 1/1000 of a millimetre of mercury. Therefore: 1 μ = 0.001 mm Hg. In SI, 1 torr ≈ 133.322 Pa; thus 1 μ ≈ 0.133 Pa, which helps cross-check calculations across unit systems.

Step-by-Step Solution:
Start with 1 torr = 1 mm Hg.Define micron: 1 μ = 10^−3 mm Hg.Therefore 1 μ = 0.001 mm Hg.

Verification / Alternative check:
Vacuum technology references and gauge calibration guides consistently use this identity; many instruments display both torr and microns on the same scale.

Why Other Options Are Wrong:
0.0001, 0.01, 0.1, 1.0 mm Hg: These correspond to 0.1 μ, 10 μ, 100 μ, and 1000 μ respectively, not 1 μ.

Common Pitfalls:
Confusing microns with micrometres in length; here “micron” refers to pressure, not distance. Also, beware of temperature and gravity conventions in historical mm Hg definitions.

Final Answer:
0.001 mm Hg