Ultra-low absolute pressure measurement: a pressure of 0.01 psi(abs) (about 0.517 torr) is best measured using which vacuum gauge type?

Introduction / Context:
Different vacuum gauges cover different ranges. Picking the correct device depends on expected absolute pressure, gas type, and whether continuous reading is needed. Converting units helps map the problem to a gauge’s effective range.

Given Data / Assumptions:

• Target pressure: 0.01 psi(abs).
• Unit conversion: 1 atm = 14.696 psi = 760 torr.
• Process gas is non-condensable for McLeod applicability.

Concept / Approach:
Compute torr: P = 0.01/14.696 * 760 ≈ 0.517 torr (≈ 0.517 mmHg). The McLeod gauge is a primary, compressing manometer typically used from about 10^−3 to several torr, overlapping the 0.5 torr region with good accuracy, provided the gas is not condensable. Ionisation gauges are intended for much lower pressures (≈ 10^−3 torr and below), while Pirani gauges span roughly 10^−3 to 10 torr but are secondary and gas-dependent; McLeod is the textbook choice for accurate measurement near 0.5 torr.

Step-by-Step Solution:
Convert 0.01 psi to torr → ~0.517 torr.Match to gauge ranges → McLeod comfortably covers ~0.5 torr.Select “McLeod gauge.”

Verification / Alternative check:
Handbooks list McLeod ranges overlapping 10^−3–10 torr. A Pirani could indicate this pressure, but it is gas-dependent and typically used for continuous indication rather than reference accuracy.

Why Other Options Are Wrong:
Ionisation: Better suited for < 10^−3 torr.Capacitance manometer/Bourdon: Either high-precision but different range selection, or inadequate sensitivity near 0.5 torr for standard types.

Common Pitfalls:
Using McLeod on condensable vapors introduces large errors; dry the gas or choose a thermal conductivity/Capacitance diaphragm gauge accordingly.

Final Answer:
McLeod gauge