Question

Vacuum pressure is usually measured using:

• A. Bourdon gauge
• B. U-tube manometer
• C. McLeod gauge
• D. Diaphragm gauge

Hint:

Logic Tip: The McLeod gauge solves the problem of "there is barely any gas here to measure" by trapping a huge sample of it and squishing it down until the pressure is actually high enough to see on a scale.

Answer 1

The Correct Option is C

Concept:
A vacuum is a space where the pressure is significantly lower than standard atmospheric pressure. Measuring high/deep vacuums requires specialized instruments, as standard mechanical gauges lack the necessary sensitivity.

Step 1: This uses a coiled metallic tube that uncoils under pressure. It is excellent for high positive pressures but generally lacks the extreme sensitivity required for deep vacuum measurement.

Step 2: This uses a liquid column. While it can measure slight, near-atmospheric vacuums (like duct drafts), it is physically impractical for high vacuums.

Step 3: Similar to Bourdon tubes, these are mechanical and are better suited for low-to-medium positive pressures or very mild vacuums.

Step 4: The McLeod gauge works by taking a large, known volume of the highly rarefied vacuum gas and using liquid mercury to physically compress it into a tiny, measurable capillary tube.

Step 5: By applying Boyle's Law ($P_1V_1 = P_2V_2$) to this compressed sample, it can accurately calculate the original, incredibly low vacuum pressure. It is the classic standard instrument for this specific task.