Pressure Scales – What a Gauge Indicates The pressure read directly on a mechanical or electronic pressure gauge equals gauge pressure, which is pressure above or below the local atmospheric reference.

Introduction:
Many industrial instruments report pressure relative to ambient air. Correctly interpreting gauge, absolute, and vacuum readings avoids errors in design calculations and safety assessments.

Given Data / Assumptions:

• Standard pressure gauge calibrated to read zero when exposed to local atmosphere.
• Local atmospheric pressure serves as reference.
• Positive readings indicate above atmospheric; negative readings indicate vacuum relative to atmosphere.

Concept / Approach:

Absolute pressure p_abs relates to gauge pressure p_g and atmospheric pressure p_atm by p_abs = p_g + p_atm. Gauges therefore do not directly provide absolute values unless specifically designed as absolute transducers with a sealed reference vacuum.

Step-by-Step Solution:

Verification / Alternative check:

When a gauge is disconnected and vented, it reads zero. However, the absolute pressure at that moment equals p_atm, confirming the relationship p_abs = p_g + p_atm.

Why Other Options Are Wrong:

Atmospheric pressure: This is the reference, not the measurement. Absolute pressure: Requires vacuum reference sensor. Mean pressure and vapor pressure: Not what a general pressure gauge indicates.

Common Pitfalls:

Using gauge readings in gas law calculations without adding p_atm; misreading compound gauges that can show vacuum in negative units.

Final Answer:

gauge pressure