Pressure measurement fundamentals: an aneroid barometer (without liquid) is designed to measure which kind of pressure directly?

Introduction / Context:
Pressure terminology—absolute, gauge, atmospheric, and differential—is foundational for instrumentation. An aneroid barometer replaces the mercury column with a sealed, flexible capsule (aneroid cell) and is widely used for weather observations and altitude estimation.

Given Data / Assumptions:

• Instrument: aneroid barometer with an evacuated elastic capsule.
• Environment: open to ambient air, not connected to a process tap.
• Output: dial indicating atmospheric pressure.

Concept / Approach:
The evacuated capsule expands or contracts with changes in ambient atmospheric pressure. The mechanism thus reads the absolute atmospheric pressure—there is no reference to another pressure except the vacuum inside the capsule. Gauge pressure is pressure measured relative to ambient; a barometer does not read gauge pressure of a process line. Differential pressure transmitters compare two process taps; again, not applicable to a barometer.

Step-by-Step Solution:
Identify the reference: internal near-vacuum vs. ambient.Recognize that the ambient acts as the measured variable.Conclude the instrument directly indicates atmospheric (absolute) pressure.

Verification / Alternative check:
Textbooks define barometric pressure as absolute; altimeters are essentially calibrated aneroid barometers adjusted for standard atmospheres.

Why Other Options Are Wrong:
Absolute pressure of a sealed process: Requires a sealed absolute sensor connected to the process, not an aneroid barometer open to air.Vacuum only: A barometer measures full atmospheric range, not just vacuum.Gauge or differential pressure: Those require separate references and connections.

Common Pitfalls:
Equating “barometer” with “gauge”; remember barometer = atmospheric absolute, pressure gauge = typically gauge pressure relative to atmosphere.

Final Answer:
Atmospheric pressure (absolute atmospheric pressure)