There are several pressure units and scales used across industry, physics, meteorology, and engineering. Below are the main ones:
1. Pascal (Pa) – International System (SI)
The Pascal is the official pressure unit of the International System of Units (SI).
1 Pa = 1 N/m²
It is used in physics, scientific research, and precise calculations.
2. Bar and mbar (bar / millibar)
Widely used in industry and technical applications.
1 bar = 100,000 Pa
1 mbar = 0.001 bar
Millibar is commonly used in meteorology.
3. Atmosphere (atm)
Represents the average atmospheric pressure at sea level.
1 atm = 101,325 Pa ≈ 1.013 bar
4. psi, psia, and psig (pounds per square inch)
Traditional U.S. pressure scale, widely used in hydraulic, pneumatic systems, and compressors.
- psi: pounds per square inch
- psia: absolute pressure
- psig: gauge pressure (relative pressure), excluding atmospheric pressure
Example:
0 psig means atmospheric pressure.
5. Torr and mmHg (millimeters of mercury)
Mainly used in vacuum systems, chemistry, and medicine.
1 Torr ≈ 1 mmHg
760 mmHg = 1 atm
6. inHg (inches of mercury)
Used in aviation and meteorology.
Example:
Atmospheric pressure ≈ 29.92 inHg
7. kgf/cm² (kilogram-force per square centimeter)
Very common in industrial applications, hydraulics, pumps, and boilers.
1 kgf/cm² ≈ 14.22 psi ≈ 0.98 bar
8. inH₂O and mmH₂O (water column)
Used to measure small pressures, mainly in:
- Ventilation systems
- HVAC systems
- Low-pressure systems
- U-tube manometers
🔍 Pressure Classification by Reference Type
In addition to units, pressure can be measured according to three reference types:
A) Absolute Pressure (abs or “a”)
Reference: absolute vacuum
Example:
1 atm = 1.013 bar abs
B) Gauge Pressure (g or “gauge”)
Reference: atmospheric pressure
Example:
0 bar(g) = atmospheric pressure
C) Differential Pressure (ΔP)
Difference between two pressure points.
Used in filters, ducts, pumps, and similar applications.
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