Pressure measurement fundamentals A McLeod (often misspelled “Meleoid”) gauge is designed to measure which class of pressure?

Introduction / Context:
Accurate pressure measurement spans many decades of absolute pressure, from ultrahigh vacuum in scientific apparatus to high-pressure chemical reactors. The McLeod gauge is a classical instrument specifically intended for very low absolute pressures (high vacuum). Recognizing its operating range prevents misuse and helps with instrument selection in laboratory and process environments.

Given Data / Assumptions:

• McLeod gauge compresses a known volume of gas and infers pressure from Boyle’s law.
• It is suited to clean, non-condensable gases.
• It is not used for continuous industrial monitoring due to manual operation and sensitivity to vapors.

Concept / Approach:
By sealing and compressing a trapped gas volume with mercury, the McLeod gauge converts a very small unknown pressure into a measurable height difference. Using P1 * V1 = P2 * V2 and the mercury column reading, absolute pressure in the high-vacuum region is determined. This method is unsuitable for high or even moderate positive pressures and cannot measure atmospheric pressure directly. It is also not a device for “very high pressure.”

Step-by-Step Solution:

Verification / Alternative check:
Vacuum technology references list McLeod gauges effective roughly from 10^-1 to 10^-6 bar (order-of-magnitude), depending on design.

Why Other Options Are Wrong:

• Positive/very high pressure: Outside intended range; structural and principle constraints.
• Atmospheric pressure: Not the direct purpose of the device.

Common Pitfalls:
Using McLeod gauges with condensable vapors leads to erroneous readings because the gas may partially condense during compression.

Final Answer:
Sub-atmospheric pressure (high vacuum, very low absolute pressure)