The key difference between bar and barg is that bar represents absolute pressure, whereas barg represents gauge pressure measured relative to atmospheric pressure. In simple terms, bar = absolute pressure and barg = pressure above atmospheric pressure. Understanding this difference is crucial in engineering, instrumentation, gas systems, and industrial applications.
What Are Absolute, Gauge, and Differential Pressures?
Before comparing bar and barg, it is important to understand the three basic types of pressure measurements:
1. Absolute Pressure
- Measured relative to a perfect vacuum
- Includes atmospheric pressure + system pressure
- Always positive
2. Gauge Pressure
- Measured relative to atmospheric pressure
- Can be positive or negative (vacuum conditions)
3. Differential Pressure
- Pressure difference between two points
- Used in filters, flow measurements, level transmitters
Bar and barg are simply units used to express absolute and gauge pressures respectively.
What is Bar?
Bar is a metric unit of absolute pressure. While not an SI unit, it is widely adopted in scientific and engineering applications due to its convenience.
Key Facts About Bar
- 1 bar = 100,000 Pascal (Pa)
- Slightly less than standard atmospheric pressure at sea level
- Atmospheric pressure at sea level ≈ 1.01325 bar (or 1013.25 millibar)
Units Derived From Bar
- Megabar (Mbar)
- Kilobar (kbar)
- Decibar (dbar)
- Centibar (cbar)
- Millibar (mbar)
Common Conversions
- 1 bar ≈ 0.987 atm
- 1 bar ≈ 14.50 psi (absolute)
- 1 bar ≈ 750.06 mmHg
Where Bar is Used
- Meteorology
- Calibration of instruments
- High-vacuum measurements
- Scientific research
Engineers often prefer bar over Pascal because Pascal values become too large for practical use.
What is Barg?
Barg refers to gauge pressure expressed in bars.
It indicates the pressure relative to the current atmospheric pressure.
Formula for Gauge Pressure
Barg=Absolute Pressure (bar)−Atmospheric Pressure (1.01325 bar)
Why Barg Is Used
- Essential for industrial pipelines and pressure vessels
- Used in compressors, boilers, pumps and gas distribution systems
- Indicates the actual working pressure above atmospheric pressure
For example, if a compressor shows 6 barg, it means the pressure is 6 bar above atmospheric pressure, not total absolute pressure.
Difference Between Bar and Barg (Tabular Form)
| Feature | Bar | Barg |
| Type of Pressure | Absolute pressure | Gauge pressure |
| Reference Point | Perfect vacuum | Atmospheric pressure |
| Formula | Bar = absolute pressure | Barg = absolute pressure – atmospheric pressure |
| Usage | Scientific measurements, vacuum, calibration | Industrial systems, boilers, compressors |
| Always Positive? | Yes | Can be positive or negative |
| Example | 1 bar absolute in vacuum chamber | 6 barg in compressor outlet |
Numerical Example: Bar vs Barg
Suppose a pressure vessel shows an absolute pressure of 7 bar(a).
We know:
- Atmospheric pressure ≈ 1 bar
- Gauge pressure (barg) = Absolute pressure − Atmospheric pressure
Calculation
Gauge Pressure (barg)=7 bar(a)−1 bar=6 barg
Final Answer
- Absolute Pressure = 7 bar(a)
- Gauge Pressure = 6 bar(g)
This means the vessel pressure is 6 bar above atmospheric pressure.
Summary – Bar vs Barg
The main difference between bar and barg is simple: bar measures pressure relative to a vacuum, while barg measures pressure relative to atmospheric pressure. Bar gives the total absolute pressure, while barg indicates how much higher the pressure is compared to the surrounding environment. Understanding this distinction is essential for accurate pressure calculations, especially in engineering and industrial operations.
Using these terms correctly helps ensure safety, accuracy, and proper functioning of equipment in industries like gas processing, fluid mechanics, HVAC systems, and pneumatic systems.
Reference:
1. “Pressure Measurement.” Wikipedia, Wikimedia Foundation, 12 June 2019,
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