Understanding pressure measurements can be confusing, especially when terms like absolute, atmospheric, and gauge pressure are involved. If you are new to the field, you might be asking, “what is gauge pressure?” This guide provides a complete, beginner-friendly explanation, breaking down the concept step by step.

What Is Gauge Pressure? Understanding the Core Concept

At its simplest, gauge pressure is the pressure measured relative to the ambient atmospheric pressure. It is the most common type of pressure reading we encounter in daily life and industrial applications. When you check your car tire pressure with a gauge, the reading you see (e.g., 32 PSI) is the gauge pressure. It tells you how much higher the pressure inside the tire is compared to the air pressure outside the tire.

Gauge pressure excludes the atmospheric pressure, which at sea level is approximately 14.7 PSI (101.325 kPa). Therefore, the formula for gauge pressure is:
Gauge Pressure = Absolute Pressure – Atmospheric Pressure.
This zero-based reference point makes it practical and convenient for most day-to-day measurements, as we normally live and operate under constant atmospheric pressure. If a gauge reads 0 PSI, it simply means the pressure inside is equal to the surrounding air pressure.

The Science Behind Pressure: Absolute vs. Gauge vs. Differential

To truly grasp what does gauge pressure measure, it helps to see how it fits into the broader family of pressure types:

Absolute Pressure (psia)

Absolute pressure measures pressure relative to a perfect vacuum. It includes the weight of the atmosphere. For example, standard atmospheric pressure at sea level is 14.7 psia. This reference is used in scientific calculations where a consistent starting point (zero absolute) is required. Vacuum packaging and meteorology frequently rely on absolute pressure readings.

Gauge Pressure (psig)

Gauge pressure (psig) is the pressure relative to the current atmospheric pressure. This is what most standard pressure gauges display. When you see the label “psi,” it almost always implies readings are in psig unless otherwise specified. Tires, boiler systems, and water pumps all use gauge pressure because the changes in atmospheric pressure are negligible for these applications. It simplifies the reading so you only see the process pressure relative to your environment.

Differential Pressure and Vacuum Pressure

Two other related terms include differential pressure, which measures the difference between two points, and vacuum pressure, which measures pressure below atmospheric. A vacuum gauge reading -10 psig means the system has been evacuated to 10 psi below ambient air pressure, a crucial measurement for processes like food sealing or filtration.

Common Questions About Gauge Pressure

Why does a closed pressure tank always read positive gauge pressure?

In a sealed system like a hydraulic tank or a compressed air line, the internal pressure is significantly higher than atmospheric pressure. The gauge reads this difference. If the reading is zero, the internal pressure equals the external pressure but does not exceed it. If the reading drops below zero, the system is under a vacuum relative to atmosphere.