Difference Between Gauge Pressure and Absolute Pressure – Definitions, Formulas & Examples | Query Point Official
.png)
Difference Between Gauge Pressure and Absolute Pressure
Understanding different ways to measure pressure is essential in physics, engineering, and fluid mechanics. Two commonly used pressure measurements are gauge pressure and absolute pressure. While both measure force per unit area, the key difference lies in their reference points — which determines how pressure values are interpreted and used in calculations.
What Is Gauge Pressure?
Gauge pressure is the pressure measured relative to the current atmospheric pressure. That means gauge readings ignore atmospheric pressure and show values above or below it. For example, if a gauge reads 50 kPa, this indicates the pressure is 50 kPa above atmospheric pressure at that location.
Gauge Pressure Formula:
`\(P_{gauge} = P_{abs} - P_{atm}\)`
A gauge pressure of zero means the measured system pressure equals atmospheric pressure. Gauge pressure can be positive (above atmospheric) or negative (below atmospheric), also called vacuum pressure.
What Is Absolute Pressure?
Absolute pressure is measured relative to a perfect vacuum — the lowest possible pressure (zero). It represents the total pressure exerted by a fluid or gas, including atmospheric pressure. Since it uses a fixed zero reference, absolute pressure values do not change with weather or altitude.
Absolute Pressure Formula:
`\(P_{abs} = P_{gauge} + P_{atm}\)`
Absolute pressure is always a non‑negative value because it uses vacuum as its zero reference. This makes it important in scientific calculations, thermodynamics, and vacuum systems where a consistent baseline is needed.
Key Differences (at a Glance)
| Feature | Gauge Pressure | Absolute Pressure |
|---|---|---|
| Reference Point | Atmospheric pressure | Perfect vacuum (zero) |
| Includes Atmospheric Pressure? | No | Yes |
| Common Uses | Tire pressure, hydraulics, pumps | Vacuum systems, thermodynamics |
| Can Be Negative? | Yes (below atmospheric) | No (always ≥ 0) |
Example
If a tire gauge shows a pressure of 200 kPa and atmospheric pressure is 101 kPa, then the absolute pressure inside the tire is:
`\(P_{abs} = 200 + 101 = 301 \, \text{kPa}\)`
This means the tire contains 301 kPa of pressure relative to a perfect vacuum.
Frequently Asked Questions (FAQs)
Q1: Why do most pressure gauges measure gauge pressure?
Gauge pressure is used because most systems of interest (like tires, water systems, and engines) operate relative to atmospheric pressure. It is convenient and practical for everyday use.
Q2: Is atmospheric pressure included in gauge pressure measurements?
No. Gauge pressure readings do not include atmospheric pressure. They are measured relative to the local atmospheric reference.
Q3: In what situations do we use absolute pressure?
Absolute pressure is required in scientific applications like gas laws calculations, vacuum systems, and high‑precision fluid dynamics where a consistent reference (vacuum) is essential.
Explore more about FLUID MECHANICS in Mathematics Notes & MCQs.
No comments