Gauge pressure – basic definition Which expression correctly defines gauge pressure in terms of absolute pressure and atmospheric pressure?

Introduction / Context:
Pressure measurements in engineering commonly use gauge readings that exclude atmospheric pressure. Converting among absolute, gauge, and vacuum pressures is routine for pump sizing, vessel design, and HVAC systems.

Given Data / Assumptions:

• Absolute pressure p_abs is measured relative to absolute vacuum.
• Atmospheric pressure p_atm is the local ambient pressure.
• Gauge pressure p_g is what a typical Bourdon gauge reports (relative to p_atm).

Concept / Approach:

By definition, p_g = p_abs − p_atm. If p_abs < p_atm, p_g becomes negative and is often reported as a vacuum pressure p_vac = p_atm − p_abs.

Step-by-Step Solution:

Verification / Alternative check:

Instrumentation datasheets and standards consistently present p_abs = p_atm + p_g. Sample calculation: at sea level p_atm ≈ 101.3 kPa; if p_abs = 250 kPa, then p_g ≈ 149 kPa.

Why Other Options Are Wrong:

(b) Sum is incorrect; would double-count atmosphere. (c) This is vacuum pressure, not gauge pressure. (d) A correct relationship exists. (e) Mixing terms; vacuum pressure plus atmospheric equals absolute, not gauge.

Common Pitfalls:

Confusing negative gauge pressure with negative absolute pressure (which is impossible physically); forgetting that most gauges read zero at atmospheric conditions.

Final Answer:

absolute pressure - atmospheric pressure