Vacuum instrumentation: the McLeod gauge is specifically used to measure which of the following quantities in a system?

Introduction / Context:
The McLeod gauge is a classic laboratory instrument for measuring low absolute pressures in the vacuum range. It compresses a known quantity of gas using a mercury column and infers the original pressure from Boyle’s law.

Given Data / Assumptions:

• Applicable range: typically from about 1 Torr down to 10^-3 Torr (order of magnitude; depends on design).
• Clean, non-condensing gases only due to gas compression.
• Manual, non-continuous indication.

Concept / Approach:
By sealing and compressing a trapped gas sample, the McLeod gauge creates a measurable pressure and volume change. Using P1V1 = P2V2, the unknown low pressure (vacuum) is calculated. This method directly measures vacuum (very low absolute pressure), not velocity or flow.

Step-by-Step Solution:

Verification / Alternative check:
Instrument manuals specify it as a primary standard-type gauge for vacuum calibration in the millitorr region.

Why Other Options Are Wrong:

Common Pitfalls:
Using McLeod on condensable vapors; readings become erroneous.

Final Answer:
Vacuum