Instrumentation – What a Barometer Measures A barometer is an instrument that measures the atmospheric pressure (the pressure exerted by the weight of the air column above the measurement point).

Introduction:
Understanding pressure measurement devices prevents misapplication. Barometers quantify ambient atmospheric pressure, which serves as a reference for gauge instruments and weather analysis.

Given Data / Assumptions:

• Standard mercury or aneroid barometer exposed to the atmosphere.
• Static conditions (no dynamic head components).
• Calibration to units such as Pa, kPa, or mm Hg.

Concept / Approach:

Atmospheric pressure is the force per unit area due to the air column. Barometers either balance this pressure with a mercury column (mercury barometer) or translate it via elastic elements (aneroid). Pipe pressure or differential pressure in internal flows require other devices like manometers or gauges.

Step-by-Step Solution:

Verification / Alternative check:

A mercury barometer reading plus local gravity corrections gives standard atmospheric pressure values comparable to weather-station data.

Why Other Options Are Wrong:

Velocity and pipe/channel pressure require pitot tubes or pressure gauges. Differential pressure between two points uses a manometer or differential transmitter. Absolute vacuum is a limiting concept, not directly read by a standard barometer.

Common Pitfalls:

Confusing atmospheric (absolute) pressure with gauge pressure; ignoring altitude and temperature corrections for precise measurements.

Final Answer:

atmospheric pressure