Alphatron low-pressure measurement range:\nThe Alphatron instrument (an ionisation-type vacuum gauge) measures absolute pressure most effectively in which microns of mercury range?

Introduction / Context:
Different vacuum gauges cover different orders of magnitude of pressure. The Alphatron (a form of ionisation/partial-pressure gauge) is used for extremely low absolute pressures in high-vacuum service. Knowing the valid measuring window prevents gross errors and instrument misuse.

Given Data / Assumptions:

• Pressure is stated in microns of mercury (1 micron = 0.001 mm Hg).
• The Alphatron is intended for high-vacuum ranges.
• We select the option that best reflects its practical operating window.

Concept / Approach:
Ionisation-type gauges infer pressure from electron/ion currents in rarefied gases and are effective at very low pressures (high vacuum). The Alphatron’s usable range is typically within a few microns of Hg and below; therefore, a limit such as “less than 3 microns” captures its normal application window more accurately than broader or higher ranges.

Step-by-Step Solution:

Verification / Alternative check:
Vacuum technology references list Alphatron-class instruments among gauges used in the 10^0 to 10^-1 micron region, consistent with “less than 3 microns.”

Why Other Options Are Wrong:

• > 3 or 10 to 100: These are too high for ionisation gauges; Pirani or capacitance manometers are better there.
• < 0.1: Possible for some gauges but overly restrictive relative to typical Alphatron use.
• < 1: Also feasible, but “< 3” better matches the commonly cited exam window.

Common Pitfalls:
Confusing microns with millimetres of Hg; mixing up gauge families (Pirani vs ionisation) across ranges.

Final Answer:
< 3